ECOI..oGI0-i.L INTEPJl.CTIONS OF THREE LI'I'T9RINA (GASTROFODA, PROSOBRfu~CH:A) ALONG 'I'J::IE WEST COAST OF NORTH M!.ERlCA by EYINIA BEHRENS A DISSERTATION Presented to ,the f)epal.tment of Biology a.nd the Graduate School of the University of Oregon in partial fulfillment of the requirements for the degree of Doctor of Philosophy September 1974 APPROvr:D: I~ 0:,\ Iv"-/ ~ - )-"'~-..Jv~pe-t-er-W-.-F·ra.rinkk-------:~--·---- ii VITA NAME OF AUTHOR: Sylvia Behrens prACE OF BIRTH: Hamburg, Gennany DATE OF BIRTH: !¥!ay 7, 1946 tlNDERGRl'.DUATE AND GRADUATE SCHCiOI.S ,'\'l'TENlJED: University of British Columbia University of Oregon DEGREES A1fll1...RDED: Bachelor of Science, 1968, University of British Columbia ~laster of Science, 1971, University of B:citish Columbia AREAS OF SPECIAL nr:;::;::REST: Popula'!:.ion Ecology Marine Ecology PROFESSIONAL EXPERIENCE: Teaching Assistant, Department of Zoology, University of British Co1uwbia, Vancouver, B.C., 1968-1970 Teaching Assistant, Biology Deparr.ment, Uni'1ersity of Oregon, Eugene, 1970-1972, Spring 1974 Sessional Lecturer, Department of Zoology, University of British Columbia, Vancouver, B.C., 1972-1974 Instructor, Zoology Departi1.1ent, University of Hanitoba, Winnipeg, July-August 1973 iii AWARDS lu\lD HonORS: Society of the SigIT'a-Xi Grant-in-,z\id of Re.search, 1971-1972; 1973-1974 Ptf.dLlCATIONS: Buckland-Nicks, Chia altO. Behrens, 1973. Oviposition and Develop- ment of ;I'wo Intertidal 3nails, Li.t.1;:orina sitkana. cUld .!:. scutulata. Canadia.. Zoologist, Vol. 51, No.3! 359-365. Behrens, S., 1972. The Role of Wave Impact alid Desiccation on the Di.stribution ofr...ittorin~ sitka..,~ Philippi f 1845. The Veliger, Vol. 15: 2 I 129-132. Behrens, S., 1971. 'l'he Distl~ibutioIi. and Abundance of Intertidal Prosobranchs rJittorina si tkana and L. scut.ulata. M. Sc. thesis ZooloSTY Dept., U.B.C. Abstract in Americ~n Zoologist 10(4): 540. Behrens f S. I 1968. S:alanus. copepods phoresis. B.Sc. Protein Differences between Two Forms of as elucidated by micro-starch-ge1 electro- the~is, Zool. Dept., U.B.C. Harger, J. R. E., 1972 (used work done by Low and Behrens). CCillpetit.ive Co",:;ds tenet:: among In te:ctida1 LlVertebrates. America~ Scientist, Vol. 60, No.5, 600-607. PAPER PRESE~"'T.ED: The Role of Wave Exposure on the Interaction of Littorina sitkana ~,d L. scutu1ata. Pacific Division A.A.A.S., Ecological Society, Eugene, Oregon, 1972. iv ACKNUflLEDGMEHT3 Many people helped rr~ in various ways curing this project. Peter Frank and P~bin Harger provided much constructive criticism and encouragement for which I a~ very grateful. I v,,::..nt to tl',ank Russ Yamada for helping me set up th~ experimental cages at Hopkins Marine Station, Dale Straughn and Jim Rote for releasing Littorina sitkana in California and my friends who accompanied me on field trips. The rooperation of the faculty, staff and studenots of Friday Harbor I,aboratories, Hopkins Marine Station, and Oregon Institute of Mari~c Biology ~s greatly acknowledged. 1 ,,!ant to thank t,."'e Society of the Sigma-Xi for awarding me two Grants-in-Aid of Research for the years 71/72 and 73/74. '-I ~ I TABLE OF CON'l'ENTS LIST OF TABLES. LIST OF FIGUP£S • GENERAL INTRODUCTION. CHAPTER 1 GEOGRAPHIC RANGE LIMITATION OF LITTORINA SITKANA AND ------L. PLANAXIS. Materials and Methods. Results. Discussion • 2 THE El"FECT OF SHELTER ON THE INl'ERACTION OF LITTORINA SCtiTULATA A.."ID L. SITYANA Materials and Methods. Results. Discussion • •viii zi 1 5 8 9 10 13 21 27 33 3 INTERTIDAL ZONATION OF LITTORINA SCUTUL..'\'l'A Al\i"D L. PLANAXIS. 36 Possible Factors of L. scutulata. Desiccation Starvation. Competition Possible Factors of L. planaxi~ • Dro....'ning. Sta't"Vaticn. Wave Force. Competi-tion Predation • Limiting the Upper Distribution with L. olanaxis. _ -*'-------- Limiting the Lower Distribut:'on with L. scutulata • vi 37 37 40 40 40 40 41 41 41 41 TABLE OF CONTENTS continued CHAP'I'ER Feeding Experiment " Materials and Methods Resul-ts • Wave Force Experiment. Materials and Nethods Results Competi tion Experiment • Materials and Methods Results. Tide Level Effects • Species Interaction Effects. Density Effe'::ts. Predation Experiment • Materials and Methods . Results • Discussion . 42 42 43 43 43 44 44 44 48 48 49 50 51 51 52 57 4 THE ROLE OF TIDE LEVEL CO:r..1DITIONING iI-.LJ~ PRED7I.TION ON THE UPWpJm MOVEMENT OF LITTORINA PL~N&XIS. Materials and Methods. Re.G1.:'lts. Discussion • 59 59 60 64 5 NICHE RELATIONSHIPS IN LITTORI~~S. APPENDIX TABLES 1-1 TO 5-2 LITERATURE CITED. vii 66 73 • 108 TABLE 1-1 1-2 2-1 2-2 LIST OF TABLES Littorina planaxis and L. sitkana- tra....splantation experiments • • • • • • • • • • • Total prey o:r.ganisms eaten by shore crabs in 23 days. Densi'cy and species ratio of littorines found in three si,tes of varying wave exposure. • Position of L. sitkana egg masses fOlli1d in sixteen cages at Cantilever Pier beach. • • • • • • • "14 77 78 79 2-3 If> Number of animals of both species ~d in crevices of quadrats taken San Juan Island • • • • • • found on the surface at t\ro sites on 80 2-4 2-5 2-6 2-7 2-8 2-9 2-10 2-11 3-1 Behavioral Response of littorinei'i to crevices • Growth of animals caged at three sit,es I two substratum textures and two species combinations • Growth and survivo:r.ship responses of L. scutulata caged at various densities from Ap~il 16 to July 20, 1972 • Growth and survivorship responses of L. sitkana caged at various densi'ties from April 16 to July 20, 1972 • Percent of animals found in crevices at Cantilever Pier on December 5, 1972•••••• Laboratory aggregation experiment Percent of animals in crevices under various weather conditions. . .. . . . " . . . . . " . . . . . . _ . . Number of juvenile animals found inside experimental cages • • • • • • • • • • • • " • • . • • • • Fecal pellets produced by littorines collected from various levels at Hopkins Marine Station. • • • • • viii 81 82 83 84 85 86 87 88 89 LIST OF TABLES continu.ed T1>..BLE i t 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-10 3-11 Number of animals remaJ.ll:tng attached to substratum after cne high tide _ • • • • • • • • Proportion of animals aggregated in lower portion of cages during August 1972. • • • • • Comparisons of b,. scu·tulat~ survival rates at high an.d low shore. levels from spring to winter 1972 • Comparisons of IJ. planaxis survival ra.tes at high and low shore levels froITI spring to winter 1972 • • Percent survival of lm'1 density t single species cages in 1971 and 1972. • • • • • • • •• Comparisons of survival rates in single and mixed species cages • • • • • • • • • • • • • • • • • The effect of adding 20 animals of species B to 20 animals of species A from May 1972 to Januar}7 1973. • NUi."1'.'t2r 0:= rews of :cadula:c tee·th per microscopic field for littorines of equal size. • • • • • • • Comparisons of sUr\7ival rates in low and high density cages • " .. • • .. " • • • " .. • • • " Growt~ response of b,. planaxis caged at various densities in the splash zone above the high water mark from January to May 1972. • • • • • • • • • • • • 90 91 92 93 94 95 96 97 98 99 3-12 3·-13 3-14 Survey of hermit crab shells for Acanthina drill marks. 100 Number of L. scutulata drilled more and less than 8 rom in length -: " . . . " . A • • • • .. • • • "" • • ., 101 The effect of adding Acanb~ina on the distribution of littorines in experimental cages • • • • • • • • • • • • 102 4-1 Position in the enclosure of L. planaxis conditioned _ .....-====::.:::;. to the splash zone and to a lower level at 4:15 pm••• ~ • 103 4-2 Number of low and high conditioned b,. planaxis found in the enclosures and on rock face above tile enclosures after ~wo consecutive high tides ~ • • • • • • • 104 ix TABLE 4-3 5-1 5-2 LIST OF TFBLES continued Number of 1m.. and high conditioned L. p1aIliuds remaining on substrat.um • • • • • Comparisons of the three species of 1ittorines. Growth responses to var}'ing grazer densities. 105 106 107 FIGURE 1-1 2-1 2-2 LIST OF FIGURES Map showing the distribution of littorines. . Number of animals found en rocks of different roughness Seasonal mortality patterns of L. scutulata as a function of density • • . • • • • • • • 6 15 16 2-3 Seasonal mortality patterns function of density • • for L. sit~ana as a 17 2-4 2-5 2-6 Total number of animals fo~~d in exposed sites inside experimental cages • • • • • • • • • • • Stainless steel mesh cages. Experiment.al cage£' at False Bay 19 22 24 2-7 2-8 2-9 2-10 Blood sample tray used in aggregation experiment showing dispersion of littorines. .• •• • • 26 Growth of littorines under two substrat~~ treatments and two species combinations. • • • • . • • • • • • • 28 Survivorship and growth index as a function of density of anh~als caged at False Bay from April 16-July 20, 1972. 30 Growth response of littorines caged at various species ratios from February 2 to April 26, 1973. • • • • • • • 31 2-11 3-1 3-2 3-3 3-4 The effect of density on growth response. Zonation of Littorina at Hopkins Marine Station Size frequency distribution of ~. planaxis and ~. scutulata at Hopkins Marine Station. • • , • Upper distribution of Acanthina spirata in relation to experimental cages • • • • • • • • • • • • • • • Time taken by Acanthina to drill and eat littorine. 32 38 45 47 53 LIST OF FIGUP£S conti~ued FIGURE 3-5 3-6 . 3-7 4-1 Number and percen'c of L. scut1.l1ata shells drilled by Acanthina as a function oTsize class •••••• Number cf L. pla!]p.xis shells drille d by Acat".thina as a f':U1ctiC'n of size class • • • • • • • • • • • Percent of L. planaxis shells drilled by l\canthina as a function of size class • • • • • • • • • • • • Experimental enclosures aT, HopJdU3 Marin.e Station 54 55 56 61 4-2 Up\'lard migration of L• .El.~~~. under various treac.'nents.. 63 5-1 Changes in number of Ii ttorL1.c1J on Galiano mud flat from 1961-1973. • • • • • • • • • • 68 5-2 Relative niche widths ~ld overlap for the tree species of littorines . . .• . . . • • • • . • . • 72 xii GFNEPAL INTRODUCTION Littorines. or periwinkles, ca.. be found on barnacles, rocks and seaweed throughout the rocky intertj.dal regi.ons of the world (Stephenson and Stephenson, 1949). Since they are ubiquitous and easy to collect, they have been tne object of many studies. As early as 1911 Haseman attempted to describe the physical factors responsible for the oscillato~? movements of Littorina littorea which corresponded to tidal cycles. In 1916 Kanda looked at the negative geotropic response of littorines to a combination of factors such as light, angle of inclination, submergence and emergence, texture and moisture of substratum. He noted that animals were sensitive to desic- cation in that they moved down if ·the substratum was dry. Hertling and Ankel (1927) describe the mode of development of various Atlantic !,ac~ and Littorina species. Littorina littorea a::1Q ~. neritoides both have planktonic egg capsules fuAd veliger larvae. Littorina littoralis fasten their gelatinous egg masses to the fonds of fucoids. The veliger stage is passed inside the egg and the young snails hatch as miniature adults. Littorina saxatilis, on the other hand, is viviparous. Struhsaker and Costlo~,; (1968) reared the Hawaiian Littorina picta from egg capsules to miniature adults by feeding the veligers on phytoplankton.. Just before metamorphosis the veligers were observed to prefer substrata wi.th algal cover to s~.bstrata without algae. 1 & 2Much attention has been devoted tc the study of Atlantic a~d Hawaiian littorines but no long term studies havE: been carried out on the three easte1.il Pacific species, Littorina. scutu1ata, Littorina sitka.na and ~. planaxis. Littorina scu~u1at~ (Gould 1849) has the widest distribution being found on both exposed and sheltered beaches from Alaska to Eaja California (Oldroy, 1926 and Y...een, 1937). This species is rar2 or absent in habitats that are relatively isolated from the open sea, such as high tide pools and lagoons. Littorina sitkana (Philippi, 1845) is present from the Bering Sea to southern Oregon and is characteristic of wave and slli.-sheltered locations such as mud flats, tide pools, lagoons and rocky shores containing numerous crevices (Behrens, 1971). Littorina planaxis (Philippi, 1847) occurs from southern Oregon to Baja California and lives in the splash zone, above the high water mark. Both Littorina scutulata and L. pla.."1.axis develop by means of planktonic larvae, \'lhereas L. sitkana lays benthic egg masses from which juvenile snails hatch directly (Behrens, 1971). By transplanting animals beyond their ra.."1.ge, I have attempted in this study to define the factors limiting the southern distribution of L. sitkana and the northern distribution of L. planaxi,s. The Littorina scutulata-L. sitkana species pair co-exists at the same tidal level on most beaches near the city of Vancouver, British Columbia and in Puget Sound. Co-existence, however, is not the rule with the~. scutulata-L. planaxis species pair. Littorina scutulata is found in the high intertidal zone and L. planaxis lives in the spray zone above the high water mark (Bock and Johnson, 1968). 3The competitive exclusion principle, or Gause's principle states that two ecologically similar species using the same resource, be it food or shelter, cannot co-exist indefinitely for one species would be more effi.cient at utilizing that reso"uree and thus would increase in . numbers and displace L~e other species (Hardin, 1960). This was the Case ~1i'ith Paramecium caudatum and!:. ~~!~lia grown in culture vials (Gause, 1934). In single species cultures both species survived in- definitely but when gro,...m together, the smaller species P. caudatutll, with a greater rate of increase, could acq~ure food more efficiently than P. aUrelia and thus P. caudatum increased in numbers and displaced P. aurelia. The more complex the laboratory environment becomes and the more genetic variability b~ere exists within the competing species, the greater will be the chances of their co-existence. Ayala (1972) sum- mariz6s a number of such studies. Since in nature, environmental con- ditions are more variable in time and space than in laboratory cultures, co-existing species utilizing the same resource are not Ul..coItU-non (Harger, 1972). Usually two competing species are adapted to different extremes of an enviro~-nental gradient with only one species living at either end. Littorina scutulata does not live in high splash pools or lagoons, pre- sumably because its planktonic larvae cmUlot settle there. L. sitkana cannot live on exposed coasts without shelter from waves and sun, as adults tend to get dislodged by wave action and juveniles tend to die from desiccation (Behrens, 1972). In intermediate habitats us~ally both species ca.. co-exis"t. This (::o-existence does not necessarily imply a ~I lack of competition between the species, ~or in the case of L. s~ticana and~. scutulata caged for one year C.t vari.ous densities and species compositions in an intermediate habitat, evidence for competition 'lrlaS found (Behrens, 1971). This study a.ttempts to explain the great degree of overlap in the distribution of ~. sitkana and L. scutulata and tl1e lack of overlap bet'l.i'een L. sctttulata and .!:.. planaxis. The array of envirolli~ental gradients within \~lich a species can persist has been described as an lin-dimensional hypervolume" or "niche" of that species {Hl....tchinson, 1957). Since each environmental gradient , is measured in different units, it is ....ery difficult to quantify the niche size of a species. In the present study niche-size ffild overlap for the three species of littorines has been rated on a relative scale based on spatial distribution. An attempt is made to relate niche size to the degree of specialization of a species. / 4 I , CHAPTER 1 GEOGRP2HIC RANGE LIMITATION OF LITTORINA SITKANA AND LITTORINA PLANAXIS Among marine intertidal organisms, tolerance to such physical factors as temperature or salinity probably would not directly dictate geographic boundaries, for intertidal organisms experience great varia- tions in these factors within their latitudinal range. With species having a planktonic dispersal stage, cnrrents could playa large role in determining distribution. Likewise, wide ex-panses of unsuit.able habitats such as sandy beaches could hinder the dispersal of a species lacking planktonic larvae. Eastern Pacific littorines are suitable species for looking at such problems, since Littorina scutulata and L. planaxis possess plal1.ktonic larvae whereas L. sitkana develops directly from eggs attached to intertidal rocks. Littorina scutulata has t.1-te widest distribution, occurring from Alaska" to Baja California, (58°N to 19°N latitude). Oldroy, 1929, and Keen, 1937, state that L. sitkana is present from the Bering Sea to Puget Sound and that L. planaxis occurs from Puget SOund to Baja Cali- fornia. I find, however, that the meeting place of the latter two species is in Charleston, Oregon and not Puget Sound (Fig. 1-1). All references of Littorina planaxis occurring in Puget Sound date 5 6Baja Calif. Los Angeles .. 0 A 0 Monterey San Francisco .. [] .. 0 .t. [] D • a Lincoln City .. 0 III ~ Cha.rleston Ii. 0 A 0 o S1i .. 0 mLittorina sitkana A a .. a A 0 It,. Littorina planaxis [] Littorina scutulata Fig. 1-1 Distribution of Littorines I f t 7 back to naIl, 1921. Dr. J. Rosewater of the Smithsonian Institute kindly made available Dalles type specimens #3928. These specimens are of L. planaxis, hQ;\7ever, the collection site was not Puget Sound but San Fra."1cisco. The specimens labelled L. p!a'1a."{is in t.he Friday Harbor collection are actually misidentified L. sitkana with their grooves eroded away. It is therefore very unlikely that L. planaxis extends as far north as Puget Sound. Thomas (1966). found LH:torina sitkana i.n bays just north and south of Lincoln City, Oregon. Isolated populations of small individuals can also be found associated with the alga, Prasiola, and barnacle cover at Cape Arago nea.r Charleston. This species has direct deveJ.opment and is found only in favorable habitats in which both adults and juveniles can survive. Since adults a~e susce~tible to being dislodged by waves, mld egg masses and juveniles are susceptj~le to desiccation, this species is most abundant in sun- and wave-protected sites such as crevices and tide pools (Behrens, 1972). When such habitats are scarce, or isolated as are the rocky outcroppings along the sandy southern Washington and northern Oregon coasts, L. sitkana is absent. To determine tile factors limiting the geographic distribution of a species, one has to transplant a population of that species beyond its range. If the transplanted population persists, Le. if individuals survive, grow and reproduce, then some factor or factors must prevent that species from dispersing into such favorable habitats. If individ- uals survive, but fail to grow and reproduce, it may indicate that the new environment only allows them enough energy for maintenance. Low Bfood abundance and/or high energy expenditures'necessary for resisting the effects of a harsh physical enviro:nrnent may be active factors re- suIting in low growth and reproduction. If predators are excluding a species from a physically benign environment, one would eA~ect that species to thrive in predator free refuges. A program involving transplantation of Litt.orina populations was undertaken in an att~upt to define factors responsible for dete~ination of the southern limit in L. sitkana and nor"thern limit in L. planaxi.s on the west coast of North America. In conjlli~ctionwith the field experiments, interactions between shore crabs and littorines ..-lere also ir.vestigated in ehe laboratory. Materials and Methods For the transplantation experiments, littorines were collected from a given locality and the lips of their shells painted so that growth could be detected. Animals were either transported by car in plastic bags or mailed in cardboard boxes containing damp paper towels. Animals were mailed at the beginning of the week and received three days later. Jim Rate and Dale Straughn report that all the Littorina sitkana shipped in this fashion survived. Likewise, Littorina planaxis transported in plastic bags survived the two day car trip. Before animals were re- leased on the shore, they were immersed in salt water so that they could open their operculum and attach to the substratum. Sites were searched periodically for the presence of transplanted animals. Locations in which animals were collected and released, and their 9sU~vival in the new habitats are listed in Table 1-1. To investigate crab predation on littorines, two plastic dishpans "lere filled with 5 em of sea water and cooled by setting them in a water table with circulating tap water. A male ano. female pair of Hemigrapsus nUc1i~ 'IlaS introduced into one pan and a pair of H..o~3onensis into the ot-.her. A rock was placed into the center of each pan to provide shelter for the crabs. Six ~ittorina planaxis, six L. scutulata and six L. si.tkana were then introduced into the bottom 6f each pan. The number of crushed dead snails were noted and dead animals were replaced with Iiving ones in an attempt to keep the nlliwer of each prey species constant. Resu1.ts Both Littorina planaxis transplants to more northern habitats were quite successful. At least 17 individuals ~ur~ived at Friday Harbor, San Juan Island, for two years. Not only did some adults survbre the unusually cold winter of 1972/73, but they also grew ilnd producod egg capsules. In the laboratory these eggs gave rise to actively swimming ~~liger larvae in l~ days. The Littorina sitkana transplants were relatively unsuccessful. Only one out of five persisted for any length of time. From a cohort of 50 L. sitkana released into a very high splash pool at Monterey, six sUl:vived and grew from December 31, 1970 to May 27, 1971. These animals died as their pool d:ded up by June 14, 1971. The presence of empty cracked shells in May suggests that the beach crab Pachygrapses crassipes 10 may have been responsible for seme of their deaths. In a laboratory situation in which t.he beach crabs Hemigrapsus nudis and~. oregonesis were offered a choice among L. ~itk~~, ~. scutu1ata and L. planaxis, they only attacked and ate L. sitkana. Littorina planaxis, and!:.. scutulat~ seem to escape crab predation by crawling out of the water and by having thicker shells than L. sitkana. Discussion Since at least some Littorina pla~axis grew, reproduced, and sur- vivedthe 'cold winter at Friday Harbor temperature or other physical factors do not appear to prohibit this species from living further north. I postulate that either south flowing currents during planktonic development or decreased larval survival might limi·t t..'le northern dis- tribution of 1::. planaxis. Thorson (1950) showed that a greater proportion of northern proso- branch species pass the veliger stage inside egg capsules than do ~'leir southern counterparts. This trend holds for the littorines in that Littorina sitkana, the most northern of the three species has benthic egg masses, whereas the southern species L. scutulata and L. planaxis have planktonic larvae. Thorson suggests that direct development might be an adaptation to the unpredictability of phytoplankton food sources for veligers in northern waters. Whipple, 1966, estimated that 99% of the mortality of Hawaiian littorines I L. pintado and L. picta, occurs during plan..~tonic larval life. Any factor such as a decrease in temperature or food supply that 11 would prolong planktonic larval development would also increase the e~~osure time to such hazards as predation e~d seaward flowing currents. To compensate for possible increase in larval mortality at higher latitudes, L. pla~axis would have to be more fecund than at lower latitudes to maintain a breeding population. Drift bottle studies indicate that coastal surface currents along Washington and 01:"egon tend to follow the seasonal changes in prevailing winds that result in a southerly flow in the summer and a northerly flow in the winter (Barnes, Duxbury and Morse, 1972). A south flowing cur- rent during summer, when L. planaxis larvae are in t."e plankton would prevent the northward spreading of this species and the self-perpetua- tion of transplanted northern ~. E.!:.anaxi~ popclations. Ricketts rold Calvin (1968) state that the shore crab Pachygrapsus crassipes is abundant from the Gulf of California to Charleston, Oregon. This distribution pattel."Il suggests, that the presence of Pachygrapsus may contribute to limitation of the southern distribution of L. sitkana around Charleston, Oregon. These shore crabs move higher up the shore at low tide than do the smaller Hemigrapsus crabs which overlap geographically ,tlith L. sitkana. At low tide, Pachygrapsus can be found in damp roc~y cracks and crevices arid in pools. Since L. sitkana is also dependent on crevices and pools, it is conceivable that, in their search for these damp and wave protected sites, they fall prey to Pachygrapsus. I predicted that the site just below the sea'liater system over-flow pipe at Hopkins Ma~ine Station would be an ideal refuge from desicca- 12 ti.on, wave action and predation fer r.... sitk~. This site is consta.."1tly sprayed with seawater, and is about six feet above the high water mark. Unfortunately the seawater systen was turned off for four days right after the animals were released in June 1973 (Table 1-1). Upon closer 'observation; this site was also found to harbor about 20 Pachygrapsus under boulders and in rock crevices. I was m'lable to find a suitable site for L. sitkana in the ~lonte:r.ey area. f i CHAPTER 2 THE EFFECT OF SHELTER IN THE U...·TERACTION OF LITTORTNA SITK~~~ ~ID LITTORINA SCUTULATA Two species of intertidal snails, Littorina scutu1ata and Littorina sitkana, co-exist at the same tidal level on most beaches near the city of Va.n-:::ouver, British Columbia and in Puget Sound. The rela- tive abundance of these blO species, however, varies along a wave expo- sure gradient with only one species living at either extreme (Table 2-1). Littorina ~~ulata does not live in high splash pools or lagoons, prer.;u:mably bacaUS8 its plan};,tonic larvae do not settle there. Littorina sitkana does not live on sun or wave e>"'Posed shores without· shelter since adults tend to get dislodged by waves and eggs and juveniles tend to get desiccated (Behrens, 1972). It appears that Littorina sitkana compensates for its greater susceptibili.ty to adverse physical effects of waves and desiccation by keying' in on sheltered micro-habitats. The follovli.ng evidence was found ill a previous study (Behrens, 1971): 1. In a field experiment, 93% of L. sitkana egg masses were deposited in wave and sun sheltered places (Table 2-2). 2. As the tide recedes on a dry day, proportionately more L. sitkana than L. scutulata tend to be found in crevices (Table 2-3) . 13 14 3. When placed on clea.:l and barnacle covered oyster shells, both species tended to leave the smooth, cleaner shells, but tended to remai.r. on the rougher barnacle covered shells (Table 2-4). Tf.1hen animals were put into a sea~,1iater aquarium with roC'.kse-f··· different roughness, signific~1tlymore L. sitkana were found on the extremely jagged rocks tha..'1. L. scutulata (Fig. 2-1). This may suggest that L. sitkana is better able to find shelter thaT). I,. scutulata. Given habitats with tide pools, horizontal shelves or crevices that mitiga.te the effect,:s of wave action and desiccation, both species of littorines co-exist over a ';fIide range of irlaVe exposures. This co- existence between~. sc~ulata and~. sitkana does not necessarily imply a lack of competition between them. A field experiment in whid1 species composition and density were varied indicated ~~at at high densities both species survived better in single than in mixed species treatments (Behrens, 1971, Fig. 2-2, 2-3). At low and at medium density no such species interaction effects were detected. At high density, L. sitkana and L. scutulata probably did not compete for food since food abundance and groirl't.~ rates were always the same in single and zrJ.xed species cages. Shelter may have been an important factor in the species interaction at high densities. An indication that shelter may have been limiting in high density treatments comes from a survey of animals found in exposed sites inside experimental cages (Fig. 2-4). Proportiona"tely more animals of both species were found in exposed places as the density increased. 15 30 . •.. .. .. .. .. .. "It .. .. .. .. • " i 1 *** .i.+ .. .. .. ~ .. .. • .. .. ".. .. .. It It .. .. .. .. .. .. .. t .. .. .. .. .. .. .. It .. .. .. ,4 .. '" .. ... _" ,6,. --.- ......-~",.-- " ....- ...r--.... : L. scutulata .. L. sitkana .. $ .... ,," .".... ". ,," .." ,,"... ." ..'" .".... 8'" 20 - w ~~ z ~ ~~ 0 ~ ~ ~ ~ 10 .z smooth medium rough Fig. 2-1 Number of animals found on rocks of different . roughness. Note that significantly more L. sitkana were found on rough rocks than L. scutulata. LITTORINA SCUTULATA 3°f Summer Mortality 20 ....... """"",, .....~ .,..~. 2 10 L"",,,, ~:"..". ,...&..l.-_"".._"=".._".._.. _.-__-_-"""-_-_-_-_-_---J-O----;..;-.::;;::;::::~\JA~--""'"-:.'::::::.---·------· .4\:& .. • .,:--- saa=,;c;; 4biLa ~:-.::;: 20 40 80 DENSITY 16 f:l:.1 o ~ 60 ~~~ 50 10 .. Winter Mortality 20 40 Density 80 Fig. 2-2 Number of dead L. scutulata found in the summer and in the winter. Note that in the winter animals in more sheltered cages surviv~d better than those in more exposed cages (X2= 15.22***). In the winter animals in single specie3 cages survived better than animals in mixed species cages (Xl"" 21. 63***) • wave exposed cages A single species • ' mixed species sheltered cages A---single species o---mixed species Number of Dead L. sitkana found in the Summer - -- and in the Winter. During the winter at high density animals in single species cages survived better than animals in mixed species cages (X2= 21.63***). Animals in sheltered cages su2vived better than animals in exposed cageseX = 15.22***). 17 Fig. 2-3 wave exposed cages A single species • mixed species sheltered cages A single species o mixed species < I 18 LITTORINA SI'l'K.ANA 40 20 8040 DENSITY ...._ .........._ .... -JL "'•.,,_......."""""""'j,&,..""""".. """'''''__......... 20 Winter Mortality tr.l :j f:il' 120p:: tr.l ~ ~ ':00' ~ 0 p:j ~ ~ 80 Z .8 # •• .... .+ ••.~ 'fl. ". •+ •• •• .+ .... .' • .." •• ." ".. •• ."O· . ". •• ." .... •• .. ...- .. -"..- ..f.--A,""d 20 40 80 DENSITY 19 Fig. 2-4 Total Number of Animals found in Exposed Sites in Experimental Cages OIl two Occasions (March 3 and March 6 1970) us a Function of Density. Proportionately more animals were found in exposed sites as the density increased (X2 pooled species treatments, d.f.=2, L. sitkana= 9.93** L. scutulata= 15.76***). 40 30 Littorilla sitkana 20 40 80 20 Li.ttorilla scutulata 10 o /, I, ,/ /, I ... " . ••, . , .- " .. • + .I .-- ,I I· .- ---I .." / ' ...,--••• , --I .- I ."I _-, .. I ."... _e,_ ............. :-;..IIA A "1i;-;":•• tIlf!;0.''''' 20 40 J 80 NUMBER OF ANIMALS PER CAGE 21 Littorines viould especially need shelter during storms and in wave exposed sites. wnile di.ving in rough water on the west coast of Van- couver Island, I noticed t.hat all Iittorines uel."e wedged into crevit::es, whereas in quiet waters they tend to be out3ide of shelters. The greatest mortality in littorines occurs during the winter (Figs. 2-2, 2-3) 'linen food is most abundant. During this time significantly more animals of both species died in the more wave-exposed block of cages than in the more sheltered block (Figs. 2-2, 2-3). No such difference in mortality rates between the two blocks of cages occurred during L~e summer. This evidence may indicate a greater need for shelter during the winter storm season than during the summer. In order to elucidate the mechanism of the suspected co~petiticn for shelter, species interaction experiments were set up in three habi.tats, 'Nave exposed, wave sheltered and intexmediate. Shelters ",,'ere added to half of the experimental cages at each site. Other expE:riments were set up in the hope of detecting species ratio effects. Materials a~d Methods a) For a long term species interaction experiment 72 cages consisting of stainless steel mesh baskets (13 by 13 by 3 ern) were constructed by braiding the corners of a square piece of hardware cloth (3.2 meshesl em) • Two such cages were inverted and screwed to a cement slab (4 by 19.5 by 39 em) using plastic washers, plastic screw anchors and stain- less steel screws (Fig. 2-5). The surface of the cement slab under one of the two cages was perforated with "creyices," 12 holes 1 em deep and 1 em wide. Twelve such slabs ...'ere taken to each of the three experi- Fig. 2-5 Stainless Steel Mesh Cages 22 23 mental sites varying in "mve exposure. At False Bay and Cantilever Pier, ti1e sheltered and intermediate habitat, the slabs were placed on the gravel substratum (Fig. 2-6) but at McGinities, the most exposed habitat, they were cemented to t.he basaltic rock substratu..-n. Animals were collected from a site of' intermediate wave exposure next to Cantilever Pier and the lips were marked with a cellulose base paint. Twenty animals, either all L. sitkana, all L. scutulata or 10 of each species 'flere put into each cage. Twice as many replica.tes were run for mixed than for single species treatments in order to keep ~~e number of aTlimals the same in both treatments. The experiments were set up 'around December 8,1971 and monitored on February 21 and April 4, 1972. In the sum..-ner ~ when the storms subside I animals in 'i.'1'5;\t6 exposed and intermediate sites would not be expected to benefit from crevices as :much as in t..l)e ';,i'inter. Crevices, however, would pro'7ide damp mi.:::ro- habitats during mid-day lOTtl tides in the SUTIUner. Since the most ';,>lave protected habitat is also the most sun-exposed, one might expect dif- ferences in survivorship and growth rates in crevice and smooth treat- ments. A similar experiment, varying the density of snails was set up in the three sites on April 18, and monitored July 20, 1972. Ten, 20, or 40 L. scutulata or L. sitka'1a were put into each cage. Unfortunately, there were not enough cages to run mixed species treatments. b) Experiments to test for species ratio effects were set up at the intermediate habitat from Oct. 3, 1971 to December 5, 1971 and from February 2, 1973 to I"1arch 26, 1973. In the first experiment the treat- ments consisted of: Fig. 2-6 Experimental Cages at False Bay 24 25 40 L. sitkana per cage with crevices 30 L. sitkana and 10 J.... scutulata per cage with crevices 20 L. sitkana and 20 L. scutulata per cage with crevices 10 L. sitkana and 30 L. scutu1ata per cage with crevices 40 L. scutulata per cage with crevices Since no signifi.:::ant gro\o1th occurred during this time, the proportion of animals inside and out of crevices was noted. The treatments for the second species ratio e=weriment consisted of: 24 L. sitkana 18 L. sitkana and 6 L. scutulata 12 L. sitka~a and 12 L. scutulata 24 L. scutulata 12 L. sitkana 12 L. scutula.ta Half of the cages allotted for each treatment had "crevices" and half of them did not. The positions of each animal was noted February 22 and March 25, 1973. Growth rates were measured April 26, 1973. c) To test for recognition between species, individual Littorina scutulata and L. sitkana were placed into depressions of a blood sample tray so that each animal had the same chance of encountering individuals of both species (Fig. 2-7). Animals were wetted and allowed to move on the tray. After animals had attached to one another, the lower and upper member of each aggregation was noted. e ~ 0 0 ~ Ii!a 0 0 Ii B 0 0 Ii 9 0 0 0 0 SlI Iii 0 0 III II 0 0 e Ii 0 0 iii Ii 1I • 0 0 !:a ~ 0 0 Cl fiI 0 0 II Pi 0 0 0 0 II II 0 0 Il1 II 0 0 m Ird 0 0 ~ m m Ii 0 0 • \I 0 0 i 1\'1 IlB 0 0 Ii 51 0 0 I 0 0 • I!I 0 0 II C!JI • •r 0 0 0 0 III II! , scutulatai • L •;, 0 L. sitkana}i I t I Fig. 2-7 Blood Sample Tray used in Aggregation Experimentshowing Dispersion of Littorines. f~ f 26 27 d) ~~ attempt was made to relate weather conditions to the proportion of Littorina sitkana and L. scutulata found in sheltered sites. tion data from the previous experiments were used. Posi- e) In order to gain some information on the recruitment pattern of littorines, the number of animals less than 4 rnmin shell height inside experimental cages was noted. Comparisons were made among sites and between smooth and crevice cages. Results a} In the most sheltered habitat animals in all the species and sub- stratum treatments grew at the same rate (Table 2-5). As the habitat became mOJ:e exposed, animals in "crevice" cages tended to grow better than animals in srrDoth cages. This was especially true for mixed species Littorina sitkana. No species interaction effects were ever detected within smooth substratum cages (Fig. 2-8). The exposed site on February 21 and the intermediate site on April 4 showed similar trends. When crevices were provided, L. sitkana tended to grow more and L. scutulata tended to grow less in mixed species than in single species cages. This may indicate that in mixed species cages L. sitkana benefits from crevices at the expense of L. scutulata. The summer density experiment did not yield the desired info:t"ma- tion. Too many cages were damaged and too many animals escaped to make valid growth rate comparisons between smooth and crevice treatments. Food in the intermediate and exposed habitats was relatively fi g. 2-8 ~uwH of Lithriflu under Ii (lfiil.!~ Tr~ahullh 28 <'--a Slng!e 5pec,~ 0---0 mixed 3peCI..,S till smooth er crev.ces tOO L.suhlab 100 l.dUllllll .c ~ 80 80 ..0 ~--. ,,.-' thpGU!f ~0 ..' Cl 60 " ...Sill! • ,.. ,--0 60 ..- it~ ...- ~... -- ....:.: ~ -- +feU!l -- "0 0- ..'.c ~ .. . ,E __0 mt; oM~ lH 0 20 ~ Ls::Ql0$-IQlP-4 ""... .,:!' A. ",..*"'" ......- _.... ..,,-. /,~ L. scutulata / 0/" Ii>. ... ..~ "".+. +. •• '0.. L. sitkana .....4;0. ...... 0;• ••••• •••• ...... ·'0 4020 ........ O· .. ·.Sl _ _.. 0 0·1ll"· L. sitkana Density per Cage .., " L. scutulata " A __ -..... -- ---A 10 Fig.2-9Survivorship and Growth Index as a Function of Density of AnLmals caged at False Bay from April 16-July 20 '72 100 .c:: ~ ~ 0 ~ ;i tl.O { s:: '~ "S 600 .c:: CI.l m ..-f ! ~ 'H 0 20~ ~ 0 "..Q) Pi l r, t ! 31 .22 G..-""~ .20 __.__0 0 . $---- t ~~; *** ,,'"0__ J, ",..""" ........ 18 ~ ----.. O .12p..( \.tI) '" ~ :Xl ~.24 " 08~ " c:> .20 " .04,. 0 12 24 12 24 DENSITY DENSITY Fig.2-11 The Effect of Density on Growth Response of Littorines -caged at Cantilever Pier from Feb. 2 to April 26, 1973. o smooth substratum .. crevice substratum 33 under different weat.."l1er condi t.ions. Under bad ....,eather condi.tions of desiccation or during storms significantly mora L. sitkana than L. scutulata evidently are generally found in crevices. On sunny and calm days ",ith relatively high humidity ~. sitkana appear more comlnonly to be outside of crevices than L. scutulata (Table 2-10). e) Significantly greater recruitment of littorines occ~'red at the sheltered sites than at the intermediate and exposed sites (Table 2-11). In every case recruitment: was greater inside creyice cages than inside smooth cages. In these situations I,. si tkana ~ad a greater recruitment rate than L. scutulata. Discussion TYro resources, food and shelter, can become important factors in limiting the distribution and ablliidance of littorines. In the summer, when algal growL~ is inhibited due to desiccation, littorines at high densities can die from starvation. During the "linter, sto.:ms would sweep away any littorine not found in shelters. GroTtrth responses by experimental animals substantiate the importance of food in the SUi.'1Uner and shelter in the wi.nter. Littorina sitk&ia appears more sensitive to food levels than L. scutulata in that growth and survivorship appears to be inversely related to density (Behrens, 1971). Littorina scutulata grm'1 less, but survive better as the densities increase. Since for animals of a given size, L. scutulata have finer radular teeth than L. sitkana, they may be able to utilize a lower standing crop of food. Littorina scutulata often ,'.I; 34 decreased in siz~ (e.g. erosion is greater than growth of shell), where- as L. sitkana either maintained their size or else died. Higher growth rates together with a greater locomotor activity ~lOuld indicate that L. sitkana may have a. higher metabolic rate and may. require more food per individual than L. scutulata. The greatest. recruitment of littorines occurred in the most shelter- ed habit and inside crevice cages. Both species thus seem to benefit from shelter during their juvenile stages. Adult L. sitkana seem more sensitive to shelter thal1. L. scutulata, but also are better able to locate shelter than L. scutulata. Littorina sitkana's greater need for shel·ter is borne out by experiments in that L. sitkana tended to grow better .in crevice cages during the winter and spring than in smooth cages. This effect was not as consistently obserTed for L. scutulata. Species interaction between Littorina scutulata and L. sitkana appears to be very subtle. It cannot be detected over a short period, nor under relatively benign conditions. Since littorines do not dis- criminate against individuals of the opposite species, no antagonistic behavior seems to be involved in the interaction. Littorina sitkana appear to be more sensitiire to environmental changes than L. scutulata. They tend to be the first ones to sep.k out shelter when desiccation sets in or storms strike, but also t.~e first ones to come out of shelters and f·orage when conditions are favorable. It is probable that the greater utilization of cre~lices by L. sitk_~ under stormy conditions is responsible for the observed species in+.:.er- action effects. Under bad weather conditions L. sitkan~ appear to be ~. 35 better competitors for crevices than L. scutulata. One would eA~ect the 10 L. sitkana in rraxed species cages to benefit more from the 12 crevices than the 20 L. sitkana in the single species cages. Like\V'ise, single species L. scutulata would benefit more from crevices than L. scutulata. caged 'ir1i tb I~. si tkana. Littorina scutulata's advantage in its resistance to being dis- lodged by waves appears to be counterbala."1ced by ~. sitkana 's superior ability to seek out crevices. The greater survival rate of L. scutulata is counterbalanced by L. sit~ana's greater recruitment rate. Littorina scutulata can survive on very little food, but~. sitkana is better able to locate food. The balancing effects of these dif- ferential advantages may be part of the mechanism allowing these two species to co-exist over such a wide range of habitats. ( CHAPTER 3 INTERTIDAL ZONATION OF LITTORINA SCL~ULATA A~ID LITTORINA Ph~AXIS Two common periwinkles of the genus Littorina inhabit the upper intertidal and spray zones of t..~e California coast. Littorina scutulata (Gould, 1849) has the widest geographic distribution; occurring from Alaska to,Baja California whereas L. planaxis (Phi.lippi, 1847) occurs from southern Oregon to Baja California. Locally, L. scutulata is found on wa.ve exposed as well as on sheltered shores. It lives throughout the intertidal range, but its greatest aba~dance is in the barnacle zone. Littorina planaxis predominates in wave exposed places and seldom is found in harbors. It occurs below the high water mark in sheltered habitats, hut in exposed habitats it is found only in the splash and spray zones; above the high water mark. Subtidal marine animals live i.n a physically benign environment. The hi.gher an intertidal organism lives on the shore, the longer it is e>""Posed to air and the harsher the physical environment becomes. Connell (1961) suggests that as a rule, harsh physical factors such as desiccation and/temperature extremes set the upper limit to the distribu- tion of intertidal organisms and biological interaction such as preda- tior. and competition set the lower limits of distribution. 36 3'7 In the absence of heavy ~"e.Ye ,,,,ash and ~~pra.y" L. planaxis and ~. scutulata have similar upper limits to their vertical distribution. In e~posed habitats the two species, however, occupy discrete zones with very little overlap (Fig. 3-1). I decided to determine the factors responsible for this discrete zonation pattern wiL~ ~. planaxis living in t.~e spray zone and L. scutulata below the high '.'later mark. Possible Factors Limiting the Upper Distribution of Littorina scutulata Desiccation Mattox (1949) found that the higher an intertidal species is found on tr& shore, the ffior~ resistant it is to desiccation. Littorines are very well adapted for shore life, for under drying conditions they will close their ho~~y operculum and glue themselves to the substratlli~ by Ill8ans of mucus. Whipple (1966) claims that !-,ittorina pintado sur"ive at least two years without being submerged in water or being fed. Littorina planaxis can be kept out of water for at least 64 days without being ·submerged in sea water (Hewatt, 1937). Unpublished accounts set the record time for L. planaxis at 148 days desiccation without lethal effects. For equal sized animals, Littorina scutulata has a faster rate of \ water loss than L. Elanaxis (Bowlus, 1966). Unfortunately, no studies have been done on relative survival of the two littorine species exposed to desiccation. I kept 30 animals of each species out of water next to a water table for one month with no lethal effects. Just on the basis 38 Fig. 3-1 Zonation of Littorina at Hopkins l1arine Statioll. ----- Upper limit of L. planaxis .......,a-. Lower limit of 1•. planaxls ••• II! •••• Upper limit of L• scutulata \\\\\\\\ Region of overlap Aug .20 1972 Jan.31973 June 3 'j 973 I E () o LO .... 1 39 40 of size, since L. scutulata tend to be smaller th~l L. planaxis, one would not expect th~a to resist desiccation as well as L. planaxis. Starvation Bock and Johnson (1968) propose that the inability of .!=.. scu~ulata to eat microscopic algae and lichens may ey.plain the absence of this species in 'd1e spray zone. At Friday Harbor, Washington, L. scutulata ate almost exclusively microscopic algae (Behrens, 1971). Remnants of black lichens were found in the feces of I•• sClltulata collected from the lower spray zone at Hopkins Marine Station. Littorina scutulata definitely does not require ID3.croscopic algae I but it. is possible that food in the spray zone is too scarce for maintenance of L. scutulata. Competition with Littorina ~anaxis It is also possible that Littorina planaxis rrey keep L. scutulata from penetrating the spray zone. On the southern Oregon coast where L. planaxis is extremely scarce, large L. scutulata live in the spray zone above the high water mark and small L. scutulata live in the intertidal zone below the high water mark. It is postulated that in the absence of L. planaxis, L. scutulata is capable of exploiting the spray zone. Possible Factors Limiting the Lower Distribution of Littorina planaxis Drowning Ricketts and Calvin (1968) erroneously claim that littorines drown 41 ',yhen continually submerged. An obvious refutation of this theory is the existence of tide pool littorines of both species. The littorines in some large pools appear to be permanently submerged residents. Ralph Dykes (personal communication), caged L. plai1axis in a subma~ine canyon for 3 months with no detrimental effect. Starvation Bock and Johnson (1968) suggest that L. E,lanaxis. are unable to eat macroscopic algae and thus are not found low on the shore. If this h~lPothesis were true, L. planaxis should have reduced survhral and grO'/lth rates lO'tl on t.lJ.e shore in comparison to the spray zone. rlave Force Bigler (1964) found that wave shock was capable of removing a sizable number of L. planaxis from the shore. Differences in suscepti- bility to being dislodged by wave action may account for some aspects of the final disposition of both species. Competition with Littorina scutulata Littorina scutulata may be more efficient than L. planaxis in utilizing resources such as food supplies lrli thin the intertidal zone and thus may keep L. planaxis out of this zone. Species interaction experi- ments carried out in the field would test for this hypothesis. Predation Bigler (1964) tested various intertidal predatoI:s as well as ground 42 squirrels and birds with respect to their ability to eat L. planax_is. The only predators that would take L. planaxis were crabs (Cancer antennarius), starfish (Patiria miniata and Leptasterias pusilla), and the snail AcanL~ina ~pirata. The crab and starfish are low intertidal species and would present a threat to dislodgeo snails; but p,canthina ~Eirata penetrates the lower limits of L. planaxi~. distribution. Bigler observed frequent Acanthina attacks on L. planaxis in the field and describes how this predator catches and consumes its prey. One can get an estimate of the proportion of deaths due to Acanthina predation, since Acanthina leaves a characteristic drill mark on the prey's shell next to the columella. By looking at empty shells obtained froM hermit crabs, Bigler found a drilling frequency of 20% and concluded, that Acanthina must be an important predator on L. planaxis. Differential behavioral responses of the two littorine species to Acanthina may help explain the observed zonation of the littorines. Feeding Experiments Materials and Ivlethods To determine whether Littorina scutulata and L. planaxis specialize on different diets, five animals of each species were collected from different levels on the shore. Animals were isolated and put into individual petri dishes with a thin layer of sea water. After a day the dishes were examined for the presence of fecal pellets. Littorines of three species were observed feeding on a patch of the green macroscopic algae Enteromorpha sp. in the laboratory. Each 43 animal was isolated a""1d put into an individual petri dish 1.'lith a thin layer of sea ''later. After 4 hours these dishes 'flere examined for the presence of fecal pellets. Results Both Littorina ~~tulata and~. planaxis collected from higher shore levels contained remnants of black lichens in their feces (Table 3-1). Both species collected from the barnacle zone produced from light to dark brown fecal pellets of unidentifiable digested matter. All tne animals observed grazing on Enterornorpha produced fecal pellets. Some of the fecal pellets were of the same shade of green as Enteromo:r.pha and some of t.h.em were brown. Adding 1% HeL, to stimulate the digestive process, to the green pellets turned them brown. The fact that Littorina scutulata and L. planaxis produced identi- cal looking fecal pellets in the two experiments would indicate that these two species are capable of handling the macroscopic algae Enteromorpha and black lichens equally well. Wave Force Experiment Material and Methods One hundred littorines of each species were collected at Hopkins Marine Station, matched for size and marked ,..it.l-). cellulose base paint. These animals were allowed to attach to a sloping rocky intertidal protuberance which 'lIas facing the ocean. A gully fonned the base of this protuberance. After one high tide the snails remaining attached Ii 44 to the rocky protuberance were counted and all the animals four:.d in the gully \Vere assUt.'lled to have been dislodged by "\.-;raves. Resul~:.s - - For equal size ani:mals, Littorina planaxis are better able to re- main attached to the substrate t...'1an~. scutulata (Table 3-2). This infonnation appears contradicto~7, since L. scutul3ta in the intertidal zone are exposed more to the direc·t impact of the. waves than L. planaxis in 'Ule spray zone. The siz,~ frequency distribution of littorines from the experimental site, however, reveals that Li·ttorina scutulata are considerably smaller than L. planaxis (Fig. 3-2). The smaller I •. scutulat.a can persist in the intertidal exposed to heav'Y surf by seeking shelter in barnacle or rock crevices. Animals too large to fit into these crevices would have to migrate out of tile wave zone or be dislodged. Competition Experiment Materials and Method In order to test the hypotheses: a) Littorina planaxis prevents L. scutulata from exploiting the splash zone; h) L. scutulata prevents L. planaxis from exploiting the intertidal zone I attached 24 cages to the rocky shore at Hopkins Marine Station on December 28, 1970. The cages t inverted wire mesh baskets (13 by 13 by 45 10 JMLittorina86 ~ plan.axistj 4~ £2 ..CY~J... P baauob........" 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Littorina scutulata t; Z ~§ ~ r-l 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Size Class (mm) Fig. 3-2 Size Frequency Distribution of L. planaxis and L. scutula~ at Hopkins Harine Station. - Area sampled consisted of eight 20 by 20 cm quadrats taken in the high intertidal and in the spray zone. 46 3 em), were made frcm sq~are pieces of stainless steel hardware cloth with 3.2 meshes to the em (Figs. 2-5, 3-3). Twelve .636 C~ wide holes, 2~ em deep were drilled in "the slash zone and 12 in the high interdial using a Skil rctohaIluIler. Plastic screw anchors were inserted into holes and cages were attached using plastic washers and stainless steel scre.1S. "1:4arine-Tex" (epoxy putty, Travaco r..aboratories I Chelsea, Nass.) or "Life Calk" {Boat Life Division, Flo-Paint Inc., 545 49th Ave. Ie Long Island City, New York 11101) was run along the edges of eageR so that littorines could not escape L~rough the rock-cage interface. Littorines of both species were collected from surrounding areas and rr.atched foX; size. The lips of snails were marked using cellulose base paint (techpen paint 161 Coolide Ave., Mark-Tex Corp., Engelwood, N.J.) so that growth could be measured as lip increments. 'l'wenLy marked animals ,...ere introduced into each cage. 'l'wo of tlle cages at each level contained 20 L. Elanaxis, two contained 20 L. scutulata and four contained 10 animals of each species. The initial experiments were designed to test the hypothesis that species interaction results in sharp zonation. Later experiments were modified to detennine \'lhet..:.'1er increasing snail density would magnify any competition effects. Cages were nonitoi"ed in spring, late sununer and winter; new experiments ..,ere set up in spring and winter. Survivorshif:.' data ...,ere anaJ.yz8d using x2 tests and growth data using analysis of covariance. Acanthina , spirata 150 centimeters t , 47 Fig. 3-3 Upper Distribution of Acanthina ~irata in Relation to Experimental Cages. 48 Results Tidal Level Effects Desiccation does not appear to prevent Littorin~ scutulat~ fro~ living in the splash zone. At no time ciid this species survive more poorly in -t.he splash than it did in the intertide.l zone. The cages may have offered some shelter from desiccation, for in the summer all the L. scutu1ata tended to aggregate in the bottom corner of the upper cages (Table 3-3). Miyamoto (1964) found that aggregation behavior in L. planaxis increased with an increase in temperature and also with a decrease in moisture, and suggests that it may be an ada.ptive mechanism to mini~j.ze desiccation. Cage walls and corners would have the same sheltering quality that long vertical rock crevices have in allowing littorines to extend further up the shore than they are found on adjacent sun exposed sites. In two out of three years, Littorina planaxis survived just as well in the intertidal as they did in the spray zone. In August 1971 anima.ls of both species in the lower cages appeared sick and by December 1972 only 36% of the B. plana"'<:is at the low level survived as opposed to 88% at the high level. For L. scutulata the differential mortality at the two tidal levels was not as pronounced in that 10% survived at the low level as opposed to 18% at the high level (Tables 3-4, 3-5). An unusual high number of Pelagic Cormorants t.vere roosting on the experimental site in the winter of 1972. The upper rock was totally covered with bird droppings except inside the cages in which the 49 grazing action of the snails cleaned the surfaces. Comparing survivor- ship data of single species cages for 1971 with 1972 indicates that the upper cages shmrled L'1e same survival rates for the two years, whereas the lower L. planaxis cages showed a decrease in survival in 1972 . (Table 3-6). I suspect that deat.'1 due to parasitism might have been responsible for the decrease in su!.vival at the low tide level in 1972. The incidence of parasitism in snails appears to be directly related to the density of bird colonies. since there were so many birds in my study . area, tbe snails' chances of being inoculated with parasites, mainly fluke larvae, would have been quite great. Oyster culturists use a simple trick to rid oysters of their parasites before taking them to market. They simply expose them to the desiccating and heating effect of the sun. This treatment kills the parasites but not the oysters. Similarly, the littorines caged at the upper level may have been avoiding parasitism in this way. Surviving littorines were ex~~ined for signs of parasitism in December 1972 with negative results. This is to be expected, if selection for parasite susceptibility had already taken place. Species Interaction Effects At the low tidal level, both species survived equally well in mixed and single species treatments at all times (Table 3-7). This would indicate that Littorina scutulata does not prevent L. planaxis from living lower en the shore. 50 It would appear, however, that Littorina planaxis did inr.ibit the survival of L. scutulata in mixed species cages at the high tidal level in two out of three experiments. This inhibitory effect of ~. planaxis on L. scutulata at this level is probably linked to resourGe competi- tion, for in the winter of 1972 t..~e effect was increased with sI~.ail density (Table 3-7). No consistent effect of L. scutulata on L. planaxis was observed. Another way of looking at species interaction is to conpare single species low density cages with mixed species high density cages. In this case we are looking at the effect of adding 20 individuals of species B to a constant number of 20 individuals of species A. Table 3-8 shows that the addition of L. scutulata increases the survival of L. ~"1axi~. However, the addition of It. planaxi.~ decreased the survival of L. scutulata. Littorina planaxis have finer toothed radulae than L. scutulata (Table 3-9), and may be grazing the algae to such a low standing crop that L. scutulata cannot maintain themselves. Likewise the grazing action of L. scutulata with their coarser radulae way not affect the potential food for L. planaxis. Density Effects The survivorship data indicate that Littorina planaxis are rela- tively insensitive to increased density. Littorina scutulata, however, survived better at low density than at high density in three ou.t of four cases (Table 3-10). Data for the period winter to spring 1972 indicate that doubling 51 L~e density had no effect on L. planaxi~ growth response and that quadrupling the density decreased grm.,rth rate by only 20% (Table 3-11). ~nis relative insensitivity of L. planaxis to density may be a reflection of this species' ability to utilize lower food abundance levels than L. -scutulata • Predation Experiments Materials and Method Hermit crabs inhabiting Littorina shells were collected from various sites to deter,mine the percentage of littorine mortality &1e to Acanthina spirata predation. Each hermit crab shell was measured to the nearest 1/20 of a rom and examined for drill holes. In order to determine behavioral responses of littorines to Acanthina, the empty cages of the competition experiments were used. One Acanthina was introduced into 6 of the 12 lower cages on June 2, 1973. The nex"t day 10 L. scutulata and 10 L. planaxis \'\Tere added to each of the 12 lower cages. After each high tide the proportion of littorines in the lower one eighth of the cages \vas noted. Similar experiments were run in September using 4 adjacent stainless steel enclosures 20 by 40 em (Fig. 5-1). Six Acanthina were placed at the bottom of two of the fenced off areas. Six hours later 25 ~. planaxis and 25 L. scutulata were added to each enclosure. Three hours later, just after a high tide, the positions of the snails for each treatment were noted. 52 Results Significantly more ~. planaxis shells obtained from hermit crabs a.round Hopkins r.1.arine St.ation showed drill marks than L. scutulata shells (Table 3-12). Menge (unpublished manuscript) found that Acanthina pU.i"lctulata from southern California prefers L. plar~is over ~. scutulata. From an energetic standpoint, it would pay for ~canthin~ to choose L. planaxis over L. scutulata since for animals of a given bio- mass L. scutl1lata takes longer to drill and eat ·than L. planaxis (Fig. 3-4) .• size frequen~y data reveal, that larger ~ittorina scutulata may be preferred over smaller ones since a significantly higher proportion of L. scutulata '",Jere drilled above 8 rom in length than belo", 8 mm (Fig. 3-5, Table 3-13). This preference ",",-auld also be adaptive from an energetic standpoint since the energy expenditure in drilling a larger L. scutulata would not be much greater than drilling a smaller one, but the energy return from a larger snail would be so much greater. No size preference could be detected for the L. planaxis shells drilled by Acanthina. in that most of the al"limals were larger than 6.5 rom in length (equivalent in biomass to a 8 rom L. scutulata [Figs. 3-6, 3-7]). When no Acanthina were present, L. planaxis Were evenly distributed in the cages, e.g. 1/8 (24/190) of them were found L~ the bottom 1/8 of the cages (Table 3-l4). Littorina scutulata had a greater tendffilcy to aggregate in the bottom of the cage. Adding Acanthina to the cages had the effect of causing a.."1imals to spend more time in the upper parts ()f the cages. The percent I•• scutulata remaining in the bottom parts of 9scutulata Littorina8 planaxis Littorina ................... jt ........... ~ .................. ...., ..". ....... A............- ... ....... A_ .......... _... 7 6 p- i ,/ 10 ,," " ,,? 9 ,," ,,' ,," ,,' " . ,." 8 ,/ ,,"/' "tI' 7 ,,~ ,,/ "6 ,~ ., 5 ," •,,/ "4 18 4 '2 20 -~16 00 ',..j Q) )14 >. 1-1 "t:l 00 12 o o 010 .-I -.-I '-" 8 tI.l tI.l ;tj 6 o H):t:l 10 20 30 40 50 so 70 Time taken by ~canthina to drill and eat Littorine. (Minutes) Fig.3-4Time taken by Acanthina to extract a given biomass of L. planaxis and L. ~tt11ata. Numbers indicate corresponding shell height of 1ittorines in nun. Data adapted from Menge~ 1973. In W 80 , t tI) 60 ~ ~I u AIJq 40, E-! H r H H~ H~, p::\ , ;.:l Cl I ~ f-l 20z 0I ~ z,. ~'"' ~ t p(, ~f-!l ufx:l PH IJq H H 0 ...:I U . ~:::;" 20 tt:l P H EJ lJ:l tI) < c:Q 30 A fz:l H H H ~ A tf.l H H ~ ::Ll Cf.l 20 Cf.l H f ~H t ~ ~ ~ ~ "f 0 10t ~l • (1, ~, ~I ~ ~ f 0 F ~~ Zrxl ~ ~. ~~ 56 Ii o • •o o •o • = Ii •o o o • •o. iii ~ o Ii o -•o, : A =• •o ~-,.. IIIl1!t 1Il1ll. III •• III. ".lIIlQrlllllO'" Ill,. __ It ~ •• ""ltIIl III 'II • fUll :'" 11I11I.41 •• • '11. It,; III 1III .. :iIt •• e. all. QIII"IQ III ~ II!.III IIIMI!. 'II f3 $t .... A: ... o Ii o •o •o o o • •o t • = !It • ••o Ii • •o o o •Ii •lI. o o •t. :4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Size Class in length (rom) Fig. 3-7 Percent of Littorina planaxis shells drilled by Acanthina as a function of size class. (Pooled 1964 and 1973 data) 57 the cages was reduced by one half froID 33% to 17%. The relative reduc- tion for L. planaxis was much greater, from 12.6% to .08%. Discussion Both species of littorine have to escape the hazards of wave action and intertidal predation. Littorina planaxis is adapted to the harsh spray zone habitat above the high "7ater mark and thus escapes both these factors. Littorina scutulata's solution to avoiding these factors is in being small and hiding inside crevices. The predatory snail Acanthina spirat~ appears to be size selective in that it takes a higher propor- tion of snails above 8 rom in length than be low 8 rom. Snails inside crevices are less likely to be dislodged by ''laves than on smooth sur- faces. In wave sheltered and predator free sites, L. plfulaxis will live lower on the shore and L. scutulata will be larger. Inside the cages L. scutulata survive in the splash zone just as well as in the intertidal. The presence of L. ~lanaxis tended to de- creaSe the survival of L. scutulata in tr~ higher cages. Littorina planaxis are probably better competitors for limited a~ounts of food than L. scutulata. Conversely, the presence of L. scutulata had no effect on t.'le survival of L. planaxis at the lower level. Competition with L. scutulata thus is not preventing L. planaxis from living lower on the shore. In one year out of three, Littorina planaxis in the intertidal had a higher nortality rate than in the spray zone. It is believed that parasitism may have been responsible for this effect, since during this 58 year an unusual high number of pelagic cormorants roosted on the experi- mental rocks thus greatly increasing the snails' chances of ingesting fluke eggs from the bird feces. Animals living in the dry cages in the splash zone may have been immune to parasitism. Wave spray as well as the physical presence of the predatory snails Acanthina spirata induces ~. planaxis to migrate up the shore. This behavioral trait is an adaptation to being dislodged by the wave of the incoming tide and to being preyed upon. CHAPTER 4 THE ROLE OF 'I'IDE LEVEL CONDITIONING AND PREDATORS ON THE UPWARD MOVEME~1T 01" LIT'I'ORINA PL..~NA..,{IS Littorines are most a~tive during the rise and fall of the tides. Under continuously dry conditions littorines are quiescent, ~'lithdrawing into the shell and closing their operculum. As the moisture con"tent of air and substratum increases from the incoming tide, littorines open their operculu.'Il and initiate feeding excursions. On vertical surfaces these excursions are in an upward direction. As the tide recedes and desiccation sets in, littorines reverse the direction of their migration and seek out shelter in damp crevices. Littorina planaxis in wave exposed habitats linut their vertical migrations to the spray zone. Individuals transplanted from the spray zone to a level below the high water mark will migrate back into the spray zone. Bock and Johnson (1968) suggest that this negative geotaxis is an important factor lirnitiI!g the lower distribution of L. pla'1.axis. Any such behavior, how- ever, must have an evolutionary reason. This study seeks to investigate the effect of tide level conditioning as well as the presence of preda- tors on the negative geotactic behavior of L. planaxis. Materials and Methods Four adjacent enclosures (20 by 40 em) attached to a vertical ro~~ 59 60 face in the splash zone at Hopkins Marine Station were used for the up- ward migration experiment (Fig. 4-1). These enclosures consist of stainless steel mesh fences 3 em high glued to the substrate with Marine Tax (Truvaco Laboratories, Chelsea, Mass.). At low tide six individuals of the predatory snail Aca.."1thina spirata were placed on the lower wall of two enclosures and six individ- uals of the herbivorous snail Tegula funebralis into t.'he other two enclosures. Two hours later 50 Littorina planaxis were added to the lower section of each enclosure. Half of tile littorines in each enclosure had been previously caged for four months in the splash zone above the high "'ater mark and half below the high water mark. Initially both batches of littorines were collected from the same location. Animals conditioned to the lmver level were painted orange and those conclitioned to the splash zone blue. The position of animals was noted periodically as they moved up the rock with the incoming tide. For each 5 em an animal moved up the enclosure, it received a score of one point. In order to discount dis- lodged animals, the score for each treatment was divided by the number of animals remaining. Results The rate of upward migration of Littorina planaxis increased with the amount of spray and wave wash {Fig. 4-2}. Animals conditioned to the spray zone showed a greater tendency. to move up the sho't'e than animals conditioned to the high intertidal 61 Fig. 4-1 ~:perimenta1 Enclosures at Hopkins Marine Station 563 ,.." ~ Ql Ql ~ ...... 4 Eo- J:: m l> ~ J:: ~ $-4 ~ ..-fro $-I $-I ~ 'M ctl "" OJ . Q) OJ .... ..., C!I . OJ OJ .... -0 /)l) ~ Po ~ ,.0 ro J:: ~ Po ~ 1j ro J::"" 0 § .-I ;:l 0 r-l ;:l$-4 ~ ~ ~~ ~ ~ ~ ;:l ~~0 Poi Z Z Poi Z TABLE 2-7 Growth and Survivorship Responses of L. sitkana caged at various Densities from April 16 to July 20$ 1972. EXPOSED SITE 10 6 5/5 1 3 +t 9 9/9 0 1 + 40 3 3/3 0 37 ++ ~. INTERMEDIATE SITE ~ i 10 8 8/8 0 2 +t+ 4 3/3 1 5 +-H-~; 9 9/9 0 1 +++ 20 19 18/18 1 0 -H- 2 2/2 0 18 +1-1- 20 18/18 0 0 -H- 14 13/13 0 6 ++ . 40 28 28/28 0 12 -H- 9 8/8 0 31 -H~ 37 32/32 0 3 + 35 28/29 0 5 +( I SHELTERED SITE i' 10 5 4/5 5 0 + ~ 6 5/S 1 3 + 3 2/3 3 4 + 20 11 9/9 "7 2 + 3 3/3 7 10 +I 6 4/6 8 6 + 2 2/2 14 6 + 40 5 3/4 28 7 + 3 3/3 9 28 + 0 35 5 + ..c: ..c: ..... ,j,J ~ ~0 to $-I ~t!) t!> oM 00 eo rn 00 c: '" eo s:: "d s:: OJ .;i .;.t 0 OJ .;i oM 0OJ ~ "0 rn 0 !> '" ro 0A ~ tU ro ~ -rl ~ "" ro f%.l ..-I 0 OJ .;.t OJ M 0 Q.l 'r! OJ ..-I -< .r:: A ::<: Q.l tJ < .a A ::<: OJ tJCG tIJ :> ~ til > ;::: Cl $.4 $.4 H ..-1m $.4 $.4 $.4 -rim oM OJ . OJ 5*** . t l~l 108 4.2·~ 22*l.n .arm..1 - I very dl"y very dry Dec.II 1971 exposed 120 80 Sep .. 8 1972 Sheltered 140 121 Interm~ 132 106 Dec o 5 1972 intermo 230 184 i'1ar.25 1973 interm. 93 110 Dec .. 15 1971 interm. 120 120 Oct,,) 1972 sheltered 119 108 interm. 125 91 Feb.22 1973 interm. 90 112 90 := 93 49 >24** 35 ) 22* 58 := .54 .52 := 52 65 < 88*** 32 <53** 19 :::: 28 31 < 53** wincly windy very cold -SoC overcast 9°C ealm sunny sunny sunny * values are significantly different at 0\=.05 * «=.01 ** ~:=.OOl *** TABLE 2--11 Number of Juvenile Animals (less than 4 mm long) f01Uld inside Experimental Cages. FALSE BAY CANTILEVER McGINITIES cr.,. sm" cr. SID. cr. sm. Number of k" 5]'tkfl.lW. Feb c 19 1972 177 34 .5 2 4 0 Apr o .5 1972 221 98 12 6 1.5 0 Ju.ly 18 1974 134 33 45 22 16 6 • • 0 0 • • • • • • • .0. • • • • • • • • • • • • 0 • • • • • e _ • • • • • • • • TOTAL .532 16.5 62 30 3.5 0 Number of !!. scuBJ..la.ta Feb. 19 1972 28 10 1 0 0 Apr. .5 1972 130 36 1 2 10 0 JU1~ 18 197 27 12 1'1 10 26 8 • • • • • ~ • • • • • • 9 • • • • • • .0. • • 0 • • 0 • • • • • • • • • • • • • • TOTAL 18; .58 19 12 41 8 cr.= crevice substratum sm.= smooth subs-tratum 88 TABLE 3-1 89 Fecal Pellets produced by littorines· collected from various levels at Hopkins Marine Station. ZONE Next to black lichen zone ( grazed clean ) Under ledge in black lichen zone, damp brown sand stone shelf Littorina planaxis black to dark broif"m * black to dark brown *' black to dark brown * Caloth.rix sp.? L. scutulata black to dark brmffi * black to dark brOTtill * unidentified digested matter vex'tical wall mostly ullidentifiad digested ma.tt61'" * * Barnacle zone unidentified digested light brown light brovm dark brm'l'11 light a.71d dark broltc1:l no fecal pellets matter '* light bro¥'m light broirm light and dark bro~m light and dark brown light brown * denoted positive identification of black lichen remains 90 TfillLE 3-2 Number of Animals Remaining Attached to Substratum after one High Tide. L. planaxis L. scutulata Number Remaining Attached 77 36 Number \-lashed Into Gully 9 35 Number Missing 14 29 Disregarding the number of animals missing, a significantly gre~ter proportion of ~. planaxis remained attached to substratum than L. scutulata. X2 = 27.18 *** TABLE 3-3 Proportion of Pu.imals Aggregated in Lower Portion of Cages During August 1972. (sum of two runs). HIGH SHORE LEVEL 91 L.scut. L.pIan. x2 Low Density lower portion of cage 46 6 upper portion of cage 14 5lf 51.62 *** High Density lower portion of cage 140 34 upper porticn of cage 20 126 138.87*** LOW SHORE LEVEL Lo", Density lower portion of cage 45 9 upper portion of cage 35 71 34.24 *** High Density lower portion of cage 77 12 upper portion of cage 83 148 63.75 *** TAB1,E 3-4 92 Comparisons 01" r scutulata Survival Rates at High end Low.. -'-'e Shore Levels from Spring to Hinter 1972 .. Low High 2X 14evel Level SING·LE SPECIES Low Density ali.ve 5 12 deE.td 25 27 NeS., High Density alive 4 16 dead 74 64 7,,89*1!- Mixed species LOl'l Density alive 7 :3 dea.d 14 17 N.S. High Density alive 1 1 dead 37 38 N.S" Total Survival at Low l,eve1 = 17/167 = 10% Total Survival at High Level = 32/178 = 18,% X2 value = 4 .. )0 * TABT",E 3-5 93 Comparisons of L" plana~is Survival Rates at High and Low Shore levels from Spring to \'linter 1972" Low High x2Level level SINGLE SPECIES LottI Density a.live 17 28 dead 23 9 7",4-** High Density alive 24 63 dead 47 1.0 39" .31i :-*i.. MIXED SPEGIES 'Low Density alive 6 19 dead 12 1 13 .. 38** High Density alive 15 38 dead 29 0 3.5.92*** Total Survival at Low Level = 62/173 = 36% Total Survival at High Level = 148/168 = 88% x2 = 98.38*** TABLE 3-6 Proportion and Percent of Animals Surviving in Low Density, Single species Cages in 1971 and 1972. 1971 1972 X2 prop. % prop. % L. scutulata High Level 12/57 21 12/33 31 N.S. Low Level 16/46 35 5/30 17 N.S. L. planaxis 94 High Level Low Level 45/57 79 371"54 68 28/37 76 17/40 43 N.S. 6.41* TABLE 3-7 C~mparisons of Survival Rates in Single and Mixed Species Cages. t<.,. : 95 \oanter to Spring to Spring to Spring to Spring 71 vlint?r 71 Winter 72 Fall 73 s ill s m s m s m L. scutulata High Level 20 animals per cage 61* 36 21 8 31 1].. 40 animals per cage 20* Lmv I.evel 20 animals per cage 48 38 35 19 17 33 40 animals per cage 5 3 L. planaxis High Level 20 animals per cage 100 96 79* 57 76 95} 100 96 40 animals **per cage 86 100 100 100 LOvl Level 20 animals per cage 100 83 69 71 43 33 100 100 40 animals per cage 34 34 99 98 s = single species m= mixed species 96 TABl.E 3-8 The Effect of Adding 20 A.."1:i.mals of Species B to 20 Animals of Species A from May'72 to January 1973. HIGH SHORE LEVEL L. scutulata x2 If alive /I dead low density single species 12 27 high density mixed species 1 38 9.23 ** 1... p1anaxis low density single species 28 9 high density ndxed species 38 0 8.33 ** l.OW SHORE LEVEL L. scutulata low density .single species 5 25 high density mixed species 1 37 N.S. L. plallaxis low density single species 17 23 high density mixed species 15 29 N.S. TABLE 3-9 Number of Rows of Radular Teeth per Microscopic Field (1 mm) for Littorines of Equal Size. Species Number of Number of Mean Range Standard 1'leasure:nlents Animals Error L. sitkana 15 3 22.3 24.-24 L. scutulata 15 4 26.6 25-29 .70 I,. planaxis 26 5 31.6 28-37 1.16 The mean number of rows of radular teeth for L. scutulata and L. planaxis were significantly different. to: 3.8* 97 · * indicates significant difference of X2 comparisons. * 0( = .05 ** eX = .01 99 TABLE 3-11 Growth Response of L. p1anaxis caged at various Densiti.es in the Splah Zone above the High Kater Hark from January to :Hay 1972. Number of ~~ima1s per Cage Mean Growth Increment (mm) Comparing tbe Growth Responses 10 .29 20 .29 40 .23 20 versus 40 Animals per Cage F, d. f. == 1/OQ =21.22*** 100 TABLE 3-12 Survey of Hermit Crab Shells for Acanthina drill marks. L. planaxis L. scutulata shells shells N ,: drilled N %drilled Hopkins Marine Station June 2, 1973 10 40 19 15.8 June 3, 1973 21 24 63 4.7 June 6) 1973 1l~ 43 34 9.8 Total 45 33.3 116 7.7 *** Hopkins Marin~ Station Sept. 3, 1973 19 42 59 13.5 * Monterey Boat \oJorks Sept. /73 10 20 33 22 Bird Rock Sept.!73 12/16 of Acanthina were feeding on 1ittorines 19 32 13 31 Fan Shell Beach Sept./73 30 16.7 130 25 Lover's Point Sept./73 no Acanthina present 11 O· 35 0 Bodega Bay Marine Station no hermit crabs were found * Indicates that X2 values of comparison between species were significantly different *ato<.=.05 *** at c><::: .001 TABLE 3-13 Number of L. scutu1ata drilled more and less tban 8 rom in length. 101 L. 8 nun ,. 81l'.tn Drilled 16 43 x2 = 19.28*** Not Drilled 157 106 1'ABLE 3··14 The Effect of Adding Acanthina on the Distribution of Littorilles. in Experimental Cages. 102 June 1973 (total of 4 runs) with without Acanthina Acanthina X2 ------ L. p1anaxis 11 in levier 1/8 of cage 2 24 :fI in upper 7/8 of cage 238 166 135.55 **"1= L. scutulata f! in lower 1/8 af cage 41 63 /I in upper 7/8 of cage 199 127 112.69 *** September 1973 L. planaxis II in lower 1/8 of cage 6 36 II in upper 7/8 of cage 44 15 33.31 *** L. scutulata /I in lower 1/8 of cage 29 47 /I in upper 7/8 of cage 15 3 10.18 *** TABLE 4-1 Position in the enclosure of Li.ttorina planaxis conditioned to the splash zone ~~d to a lOWer level. at 4:1.5 pw. No Acanthina. 103 Position in Enclosure Upper 20 em Lower 20 em \'11 th Aeanth!~ Upper 20 em Lower 20 em L. pla...'1.axis condItioned to splash zone 36 42 79 21 L. planaxis conclitioned to a lower level 11 63 32 57 ",.2A Effect of Acanthina High Conditioned Low Conditioned 19. 26il'** 8.20** Effect of Conditioning 1<10 Acanthina With ~thina 16.44***34.24*** 104 Table 4--2 Number' of low and high con.ditioned Li ttq~~ina planaxis found in the enlosures 2.nd on rock faGe ab .ove the enlosures after two consecutive high .f.. • ~J~c.es • Above Enclosure i.n Enclosure I~m~ conditioned 17 84 Chi~Square'= 11.37 ** High conditioned. 50 82 TABLE 4-3 Number of 10\" and high conditioned Littorina p1anaxis remaining on substratum. 105 Remaining on substratum Dislodged lew Conditioned 101 99 High Conditioned 132 68 x2 = 26.97*** 106 ~\ABLE 5-1 Comparisons of the Three Species of Littorines~ scutulata planaxis Development Life span (Years) Mortality ;rate Recl"ui tment dir.ect; 1...2 rapid high high 7 planktotrophic '(+ slow loti low Variation in Recruitment Intraspecific Crowding Effects great great little little negligi.ble Diet Radular Teeth !'colera..."l1ce to Desiccation Ability to Remain attached Need for Shelter diatoms diatoms macroscopic algae black lichen COal"Se least least greatest blaok l1.chen fine most most least Relative Range Latitudinal Vertical Along Wave Exposure Gradient 2 2 2 ~ ~ 1 3 :3 :3 TABLE 5-2 Growth Responses to Varying Grazer Densities. 107 grazing area ') per animal{cm"') Ju1y·15-Aug.ll/69 ne\'1 length (mID) Apr:i.l l7-June 22/70 37 18.,,5 9 *** 0 .. 88 *** 2 04e j.,i t tori.na ~sc1J.tulata July 15-Aug.l1/69 April 17-June 22/70 l,i ttorina nlanaxis • ~ I 04 __ 0 .. 21 o.~ *** 0.00 1.31 ** 0.76 gra.zing area r. pel" animal (cmt;) Dec. 29/71- May 23/72 16 0.29 8 4 *** 0 23• * values are signigicantly different at 0\ =.05 '* 0( =.01 ** ~ ==.001 *** LITERATURE CITED 108 109 kyala, F.J. 1972. Competition between Species. American Scientist, Vol. 60, 348-357. Barnes, C.A. I A.C. Duxbury and B.A. I>lorse, 1972. Circulation and selected Properties of the C:Jlumbia River Effluent at Sea. In Pruter and Alverson (eds.). The Columbia River Estuary and Adjacent Ocean Waters. Univ. Wash. Press, Seattle, and London. 868 p. Behrens, S., .1971. The Distribution and Ablli'1.da'"1ce of the Inte::::d.dal Prosobranchs Littorina scut~lata (Gould 1849) and I-:. ~itkana (Philippi 1845). M.Sc. thesis University of British Columbia, ZOology Department. Behrens, S., 1972. The Role of vlave Impact a'1d Desiccation on the Distribution of Littorina sitkana. 'rhe Veliger, Vol. 15:2, 129- 132. Bigler, E., 1964. Attrition on L"1e Littorina planaxis population. Student repo~~, Hopkins ~arine Station, Pacific Grove, California. Bock, C.E. I R.E. Johnson, 19G8. The role of Behavior in DeteDnining the Inte1tidal zonation of I.i~torina planaxis and L. scutulata. Velige~, 10, No. 1:42-54. Bowlus, R.D., 1966. Comparison of the ability to resist and withstand desiccation of Littorina scutulata and L. planaxis. Student report, Bodega Marine Laboratory, Vol. 60 Zoology S-212. Colwell, R.K. and D.J. Futuyma, 1971. On the Measurement of Niche Breadth and Overlap. Ecology Vol. 52, No. 4:567-576. Connell, J .H., 1961. Effects of Competition, Predation by Thais lapillus and other Factors on Natural Populations of the Barnacle Balanus balanoided. Eco!. Mon. 31:61-104. Dall, W.H., 1921. Summary of the Marine Shellbearing Mollusks of the Northwest coast of America, from San Diego, California to the Polar Sea. u.S. National Museum Bull. 112, 212 pp. Gause, G.F., 1934. The Struggle for Existence. Wilkins Co., Baltimore. Hardin, G., 1960. The Competitive Exclusion Principle. Science, 131: 1292-1297. Harger, J.R.E., 1972, Competitive Coexistence among Intertidal Invertebrates, American Scientist Vol. 60; No. 5:600-607. 110 Haseman, J.D., 1911. The Rhytrunj_c Hovements of f'ittorina littorea Synchronous with Ocean Tides. BioI. Bull., Woods Hole, 21: 113-121. Hertling, H., W.E. p~kel, 1927. Bemerk~~gen Uber Laich lli~d Jugendformen von Littorina und ~acuna. Wissenschaftliche MeeresuntersuchLmg8n- Kommission zur Untersuchung der Deutschen £"ieere in Kiel und der Biologischen Anstalt