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'Ligia' 
Submitted by Allen Heide 
For completion of the 
M.S. in Biology 
August, 1971 
Dr. Bayard H. Mcconnaughey, 
Advisor 
l 
Introduction 
Adaptation to land from a water environment is a common 
topic in many textbooks. Most of the organisms discussed, 
though, have already adapted t hemselves to solve the most 
vital problems -0f water retention and temperature control, 
Animals such as mammals, reptiles, and birds have all developed 
outer coverings that are i□permeable to water and allow these 
animals a great deal of independence from t~eir water sources. 
Mammals and birds have physiological adaptations tha t allow 
the□ to regulate their body temperature, while reptiles 
have relied mostly on behavioral patterns to cope with this 
problem. 
In this paper I would like to discuss the Isopod Ligia 
that is considered terrestrial in habitat, but is yet to 
evolve a complete independence from the sea. 
Ligia belongs t o the family Oniscoidae, which is the 
Family of Crustacea containing species living a completely 
terrestrial life. Within the Isopods there are species such 
as CRrolana. harfordi and Idothea wasnesenskii tha t live a 
marine existence and others such as Porcellio and Amadillidium 
that are completely terrestrial. Ligia represents an inter-
esting point of study in that it appears to be an intermediate 
species in the transition from the sea to land. 
I would like to discuss some of the . physiological and 
• 
morphological characteristics of Li~ia that place it 1.nthe 
intermediate position and its general habitat and behavioral 
patterns that may help to give a fuller understanding of 
this uni que genus. 
J 
Habitat 
Ligia is a common inhab i tant of the upper spray zone 
and has been reported 1n Engl a nd, Medltt er a nean , Bermuda, and 
both coasts of North America . The "Sea Sister," as it is 
commonly called, spends most of its time . hiding under rocks 
and in crevices above the hig h water line, Li g i a is usually 
nocturnal and can be found on non-moonlite nigh ts feeding 
in large nu~bers on Fucus , Ul va , a nd the s cum and algea 
tha t cover the expos ed ar eas a t low tid e . 
Stevenson (29) reported Li ~ia oceanica (Dana} emerging 
in large numbers during th e ea rly morning hours at Cape Neddick, 
Maine, This daylight appea r a nce is probably more the ex-
ception than the rule. Stevenson did mention finding vary-
ing amounts of these anima ls on his later visits to the same 
place. It is possible that he saw mostly young specimens for 
they show less of an aversion for light than the older a nd 
larger s pecimens. Also a hig h humidity in the air due to 
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early morning fog or dew may have pr mpted t r.is unusual appear-
ance. 
Ligia pallasii (Brandt) the corn.~on species found along 
the Oregon coast has been studied in three different habitats 
by the authors Winchester Bay, Pacific Moura.ge in Florence, 
and Devils Elbow. 
The rocky beach areas at the base of the jetty's at 
4 
Winchester Bay have produced large groups of Ligia upon turn-
ing over large flat roc k just above the high water line. They 
appear to occupy a simllgr niche as Li'1'.ia baudiniana (Edwards) 
in Bermuda (2). Barnes notices that the younger speci~ens 
were found closest to the water which ls similar to my find-
ings at Winchester Bay, but unlike Ligia baudiniana larger 
specimens of Ligia pallasii seem to be voluntarily submerged 
by the incoming tide. 
At Winchester Bay Ligi a appears to feed mostly on pieces 
of Ulva and Fucus left behind by the outgoing tides, but 
many sme.ller specimens have been found under the rocks at the 
waters edge where they would probably feed on living fucus 
and the soum on the rocks. 
At Pacific Mourage in Florence Ligia pallasii occupies 
a similar niche as Ligia oceanica in the quays at Plymouth 
(20). Here they are found among the large crevices between 
the boards of the bulkade and behind those pilings tha t were 
not heavily covered with tar. At night they migrate 2 or 3 
feet down the pilings or bulkade to graze on the sparse pop-
ulation of ULva and the general scum left by the outgoing 
tide. Very definite clea r areas with little Ulva growth can 
be seen along the bulkade where populations of Ligia are 
hiding. Although no Fucus can be found in the immediate area 
it has likely been grazed off for substantial growths are 
found in the unpopul a ted areas at the same tidal level. 
The largest specimens were observed in' the caves at 
Devils Elbow. They could be found scattered in small groups 
5 
over the ceiling and in .the moist crevices at the openings to 
the caves. Here males Jj_ by 22 mm were coomon , and females 
26 by 12 could be found. These finds agree with Nicholl's 
{20) who found his l argest specimens in the more rocky areas. 
The largest specimens were found in the darker reaches within 
the caves, with popula tions, intermediate in size, in the outer 
crevices. A large scattering of young could be found on the 
outer rocks in complete daylight. Most of the animals here 
were seen feeding at night on the green algae scum that would 
form on the face of the cliffs above the caves. This was 
caused by the constant seepage of ground water from the wooded 
areas above the caves. Only a very few animals were found to 
migrate toward the exposed tidal areas to feed on the Ulva and 
Fucus. 
Physiological Adaptations 
If Li~ia is truly an intermediate animal in the trans-
ition from the sea to land you would expect it to show un-
usual physiological abilities when co~pared to the marine and 
terrestrial forms of Isopods. 
Osmotic Regulation 
All crustacea regulate ionic concentration and ba' lance 
to some degree. There are two basic classes of crustacea 
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to consider when discussing control of body fluids. One group 
has their body fluid concentration isotonic with their en-
vironment and consists mostly of marine forms. For this group 
the main problem is regulation of specific ionic balances. 
The other group which consists of all terrestrial forms, 
those that have left the sea for brackish or fresh water, 
and some that have left fresh for salt water and a few salt 
water f~rms. For this group the control of the concentration 
of body fluids a nd ionic regulation is necessRry (28). 
Ligia spends most of its life on land and would appear 
to belong to the second group discussed. above, but on nu.111er-
ous occasions I have observed Ligia pallasii being submerged 
by the incoming tide. When chased they would retreat into 
the water with no apparent hesitation. Barnes (4) observed 
both young and adults submerged, but did indicate the young 
appeared much more at home in the we ter and were found there 
more often and for longer times. 
fait (29) tried a series of immersion experiments on 
Li~ia oceanica to test its ability to withstand different 
concentrations of sea water and to shed possible light on 
whether Li~ia inhabited land by a direct route from the sea 
or via a brackish estuary. 
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Table 1 
Emerslon Experiments 
Concentration of sea water Survival times 
100,% sea water 35,51,52,58,65,85 days 
50% sea water 16,18,24,34,37,42 days 
2.5.% sea water 3,5,7 to 9, 15 days 
Distilled water 8-9 hours, 36 hours 
It is evident th8t Ligia is quite well adapted to live 
in sea water over prolonged periods of tiMe . It can also be 
noted that a reduction tn the concentra tion of sea wate r 
reduces survival time. These results lead Tait to the con-
clusion that Ligia ls probably a direct descendent from the 
sea. Barnes (2) recorded similar results for Ligia baudlniana 
though the general survival time was much less tha n Tait's 
results with Ligia oceani ca. Tait noticed the.t oedema was 
common in those dying in distilled water and indicated loss 
of essential salts to be a likely cause of death. This was 
confirmed by Ba rnes (2). 
Brusca (6) compared Isopods of va rying degrees of adapt-
lbillty to land in emmersion and humidity tests. 
Carolina hartfordi - completely marine 
Idothea occ identalis - Partial exposure at low tide 
Ligia occidehtalis - Upper spray zone 
Porcello scaber - completely terrestrial 
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Table 2 
HumidiLy tolcrnnce tests for spring popu lations of thrPf' spPciPs o!' 
isopods from Dillon Bc,1ch. California 
Species No. indi11irlunls RPI. lw111. 511% rli·rrth tim,· 
Cirolnnn hnrfordi G 00 '½, 2 hrs. 
6 ~'> '½) 2 hrs. 
G 50 % (j .-i hrs. 
6 75% 20 hrs. 
6 100% -Vi It rs. 
Ligia occidentnlis 6 00 % ·1 f hrs . 
6 2'i% 4(i hrs. 
G 'i0 % :12 hrs. 
6 75 % $2 h rs. 
G JOO % I l8+) 
/>orcellio scnber 6 00% 20 ltrs . 
6 2'i % (·1·8+) 
6 50 % ( ~8+ ) 
6 75% < f8 + ) 
48 100% (·1·8+ ) 
Table J 
Salin ity to lcrn ncc lC'Sls for w i11 IC' r po pul;i 1io 11 s or fo1 1r S]>l'C i!'s or 
isopocl s from Dil 1011 B1•;i cli . Cd i rorn i,1. 
Sp('('ir.f No. irulioirl11als 1\frrl i11111 sor; ) tl, ·nth ti11u · 
( ·;,-,,/,,ni lwrfrmli (i JiW I lt r . 
(i 1 O'X, S'vV l Ir r,. 
(i 2•i % S \\' $ f I, rs. 
(i :io ~{, SW OK+ ) 
6 7'i % SW 1 l·K+ J 
fj I OO 'X, S\V ( fK+ l 
l rlothra wosncsr·nskii (i F\V I., It,·, . 
6 IO 'X, S\V 12 Irr, . 
6 2i;~:) S\\' 1•7 .'i Irr , . 
6 ·,O'X, S\\' ( f8 + I 
(i Vi '½, S \\' l ·fK +l 
(i 100% S\\' ( f8+ 1 
Ligia occidrntalis (j l •W 10.-; Irr·, . 
(i 10% S\'\' 'I Irr ·,. 
() 2'i '¼, SW I I .'i ltrs. 
6 'i0% SW 2 1: , Irr ·, . 
(i 7•i % SW l f Irr, . 
6 100 % SW I f8 + J 
Porcdlio scab,·r 6 FW 2 ltr, . 
(i 10% S \V -; I, rs . 
(i 2S% SW l hr,. 
6 'i0%SW $.-i Irr ·, . 
12 7'i% S\V ·) _r) lir -.. . 
12 100% S\'\' f h r, . 
Brusca•s results showed as was expected that Ligia 
was slig htly less ad apted to t h e high salt concentration 
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that the marine species but was more tolerant at the lower 
salinities. It rwas better adapted to immersion at all con-
centrations tha~ the completely terrestrial Porcellio. Ligia 
showed a greater tolerance at all humidity readings than 
Carolina, but showed a lesser adaptation to all but one humid-
ity reading than the terrestrial for~ . This one discrepancy 
is probably due to the larger size of Ligia and will be dis-
cussed later. 
When comparing the normal osmotic pressure of the b~y 
fluids, using freezing point depression, Ligia was found to 
be higher 2.15° C than the marine species Idothea (1.96° C), 
which is usually isotonic to its environment . (J2). A compar-
ison to the more terrestrial species also shows a much higher 
concentration for Ligia (Farry 195J). 
Oniscus 1.04 C 
Armadillidium 1.18 C 
Porcellio l.JO C 
The osmoregulatory ability of Ligia oceanica and Idothea 
granulosa were compared (J2). Ligia was found to remain 
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osmotically steady between 75% and 100% sea water, but to 
slowly rise or fall at greater or lesser concentrations. 
Idothea dropped significantly throughout, something to be 
expected in an animal not normally concenred with maintenance 
of osmotic concentration. Ligia also appeared to have better 
regulatory control at the les ser concentrations showing a 
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high degree of osmotic independence for short periods of 
time (5). Libia was able to maintain a free 7 ing point de-
pression of 1.65 C compared with 0.90 C for Idothea at 25% 
sea water (0.48 C). The lethal point appeared to be 0,97 C 
and 1.16 C respectively for Idothea and Ligia. Ligia would 
remain hypotonic to high concentrati on a nd hypertonic to 
low concentrations (26). 
Idothea and Ligia do not show any c orresponding increase 
in weight or volume with the lowering of internal osr!lotic 
concentrations {JO). This would be expected if a passive in-
flux of water was responsible for the lowering of osmotic 
pressure. Todd (32) proposed a possible controlled exchange 
of salts take place. Croghan {8) proposed tha t in Ar temia 
salts and water are absorbed through the gut and the excess 
salts excreted at the brachial plates. With the use of 
phenol red dye it was indicated tha t Artemia swallows large 
amounts of water and ~llows it to pass through the gut 
wall. Oral drinking in small Crustacea has been assigned a 
strictly enema and food movement function by Fox {15), but 
he did indicate that the passage of water throug h the gut and 
out the excretory organs is likely. 
Ligia pallasii was placed in a phenol red environment 
following CroghRns method, and showed no signs of excessive 
uptake of water by oral drinking. To demonstrate the possible 
release of ions across the pleopod membranes in~Ligia 
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Croghans method was used. Half a dozen Ligia were rinsed in 
distilled water for several hours to remove all external 
salts. They were then placed i n a AgNOJ solution for two 
minutes. Any chlorine ions being released from the body should 
combine with the silver to form a white precipitate. The 
animals were then rinsed for several hours in distilled 
water againto remove excess AsNo3• The organisms were then 
placed in developing fluid for 1-2 minutes. This would cause 
any area of the body t hat showed a significant darkening was 
the median and posterior edges of the last four pleopods. 
The first pleopod usually showed li t tle oedema and no sign-
ificant darkening, which may indicate little if any ion or 
water exchange taki ng place at this point. Croghan did find 
that the eleventh branchae of Artemi a showed no darkening 
from the developing fluid, which he interpreted similarly. 
Resistance to Desiccation 
The dry condition of air provides a very hostile envir-
onment for thos~ organisms inhabiting land from the sea or 
fresh wa ter. Prevention of rapid desiccation by the develop-
ment of an impermeable integument is a problem already solved 
by most successful terrestrial forms. The Oniscoedea have 
not yet developed such a protective covering , although there 
are species tha t do show greater adaptation than others. 
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Table 4 
Rate of Evaporation of Water in mg/cm2/hr when exposed 
to dry air for an hour at various temperatures 
10 20 JO 40 50 60 
.Armadlllidium 1.5 1.5 2.7 4.5 6.5 9.5 
Porcelllo 1.0 1.2 3.0 6.0 10.0 17 . O 
Cylisticus 1.0 3.5 7.0 9 .0 13.0 18.5 
Ligia 2.0 4.2 7.0 12.5 15.0 19.7 
Results from Edney {9) 
.As would be expected Armadillidium t he mo st terrestrial 
in habitat shows the least a mount of water loss at all temp-
eratures, while the inhabitant of the littoral zone, Ligia, 
, shows the least resistance to desiccation. This difference 
in the permeability of Isopod cuticles by different amounts 
of lipoidi which impregnate the endocuticle. If the temper-
atures are raised above the melting-point a ma rked increase 
in permeability results . {?). 
Figure 1 shows the relationship between the vapour 
pressure and the rate of evaporation. 
20 Figure 1 
I 
C 
0 
~ 10 
.0a.  . 
.>,  
0.,  
~ 5 
Re. s u I ts EJ Ney(q) 
0 
C 
20 30 40 so 60 
lJ 
Note tha t there is a very close relationship between 
the increased vapour pressure and the rate of evaporation for 
the 0niscoidea, indicating little resista~ce to drying, 
Blatella (common coc kroach ) shows a comparative curve for 
insects, Note the great degree of resistance at the lower 
temperatures, The sudden rise for Blatella is ~o =the break-
down of the waxy protective layer in the cuticle caused by 
the excessive temperature, 
Porcellio, one of the better adapted 0nlscoldea, was 
compa red to other terrestrial forms (16). 
Percent Weight Loss/Hour 
Porcellio 4,0% at 20 C 
mealworm 0,05% at 20 C 
Cockroaches 1,14% at ~JC 
Earthworm 18,0 % at 20 C 
It appears that the terrestrial Isopods are not well 
enough adapted physiolog ically to survive a terrestrial 
existence without some type of behavioral pattern to compen-
sate for their inability to resist drying. 
Porcellio shows a.n increased activity when the humidity 
drops below 65%, This ls believed to keep the organisms 
moving until it gets back into a suitably moist habitat (16). 
Ligia has been observed to group in various parts of the same 
cave, which would afford the m greater protectio~ from des-
14 
iccation, but no formal study has been made to indicate 
that this may be a behavior similar to Porcellio's, 
Table 5 
The highe s t temperature which variou ~ species 
of woodlice and other ar thropods can toler a te at 
different hlli~idity for va rious periods of time 
I 
Period of exposure .. . 15 min. l hr. 24 hr. 
Relative humidity( % ) .. . 0 I 50 100 0 50 100 0 50 100 
- -- -- - - - - - - -- - - ----
Armadillidium v u/gare 43 46·5 42 41 42 40 18 22 37·5 
A. uasntum 42·5 45 ·5 41 41 41·5 40 l l 17·5 37·5 
Porce//io 42·5 43 38 39 39·5 37·5 13 17·5 36 
Oniscus 40 41·5 37 34 36·5 33·5 9 15 3 I ·5 
C1"listicus - - - 35 ·5 36 37·5 12·5 13·5 35 
Pi,iloscia - - - 32·5 36 ·5 34 9 13 3o·5 
Lit ia 41·5 40 34·5 39 35 32·5 9 - 29 
Glomeris - - - 42 . 42·5 42 27 I 33·5 38 
Edney (9 . ) 
In Table 5 it can be seen that Ll~ia has the lowest 
temperature tolerance for all the arthropods tested. It ls 
interesting to note that at lower humidities Ligia's temper-
ature tolerance is much closer to the rest of the animals 
than at 100% relative humidity. It is probable that the 
true temperature tolera nce is more accurately i ndicated 
at 100% relative humidity when the least amount of evapor-
ation can take place, 
15 
38 Figure 2 
60 70 
Time in minutes 
Temperature curves for various woodlice and 
the cockroach, Blatta, exposed to slowly moving air 
at 37° c. Edney bsj. 
Figure 2 demonstrates the cooling ability of evapor.-
ation on the body sutface of woodlice. Note the amount of 
cooling obtained by Ligia, which has been shown earli er to 
have the greatest rate of evapora tion. Ligia 1s able to main-
tain body temperatures up to 7o C below ambient temperature 
for up to JO minutes. 
Edney (12) wa s able to measure the body temperature of 
animals in the field to see if the temperature reducing effects 
of evaporation could be used by Ligia in its natural habitat. 
., 
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Figure 3 
34° C. )8° C. 20° C. -- 70 % R.H. 
Microclimatic( condi tions on verticle section 
of base of red sandstone cliff and shingle inhabitated 
by Ligia. Edney (10 ). . 
In Figure 3 Ligia has ~oved to a seemingly more hostile 
location with beneficial results. The protected roc ky cover-
ing, which is t h e usual place of hiding during the day, has 
reached a lethal temperature level, and forced Ligia to make 
unusual da ylight appearance in order to reduce its body temp-
erature by rap id evapora tion . It is likely that this behav-
ioral pa tt ern allows Li~ ia to range within a l arger ecolog ica l 
habitat. 
Miller (18) made the following co~parisonsi 
On basis of moisture of habitat 
Ligidium Ligia = Actoniscus Porcellio Armadillidium 
Based on survival in suboptimum humidities 
Ligidium Porcellio Armadillidium Li~ia 
Numanoi found that in a dry a t r:iosphere Ligia could lose 
• I 
17 
about 13% of its body weight before dying and about 7% in 
saturated conditions. These results are contrary to exper-
iments performed on frogs, which would die quicker with a 
lower net loss when evaporation was faster. In Numanoi 's 
experiments it is possible that animals living in saturated 
conditions may have died of partial starvation or metabolic 
disorders rather than water loss. The animals were kept at 
25° C ± 2° with no way of cooling themselves due to the 
heavy humidity. Numanoi did show that there was definitely 
greater weight loss in living than dead animals which he 
attributed to metabolic activity. Ligia has been shown to 
gain weight in humidities of 98% at 15° C (9). The highest 
temperature Ligia can tolerate for even 24 hours is 29° c. 
Numanoi was working very close to the optimum t emperature for . 
Ligia and reported no deaths at 100% relative humidity until 
about JO hours. 
Respiration 
In the sea respiration requires thin flat membranes of 
considerable surface area to facilitate the proper circul-
ation of water and an adequate supply of oxygen. On land the 
great increase in partial pressure of oxygen and the less 
dense quality of the air allow for a reduced respiratory 
surface and a more closed system. The latter is especially 
18 
important to eliminate excess loss of water from the gill 
areas. 
Insects have developed small respiratory tubes, known 
as spiracles, that have the ability to close during severe 
drying conditions. In the Isopods species has developed 
respiratory organs as a~ those of the insects, but those, 
which are more terrestrial in habitat, do show definite adapt-
ive changes in their respiratory structure. The aquatic 
Isopods such as Idothea have plate like abdominal appendages 
that serve as gills. The more terrestrial forms like Arma-
dillidium have a tree-like branching system of tubules, bathed 
irt blood, throug h each exopodite of the first two pleopods, 
or, sometime of all five pa irs (JJ) (18). Table 6 compares 
some of the comm on Isopods by habitat and respiratory develop-
ment. 
Table 6 
Name Habitat Respiratory Organ 
Idothea aquatic thin gills 
Asellus aguaticus aquatic thin gills 
Ligia oceanica high water line gills, with stouter 
exopodites 
Omiscus assellus damp places gills, with special 
air chambers at edge 
of exopodites 
Porcellia scaber drier places trachea in first two 
pairs of exopodites 
Armadillidium dry places same as above 
19 
It is evident that compared to other terrestrial forms 
Ligia has a very poorly adapted respiratory system. If you 
assume that in Isopods, the rate of evaporation at the 
respiratory sights is indicative of t he permeability of the 
respiratory membranes, the following compar ison ca n be made. 
If you compare t he rate of weight los s per unit area of the 
total body weight lost from drying animals you receive values 
of 83 for Armad illidium, 97 for Porcellio, and 58 for Ligia. 
The lower rate of evaporat ion per unit area at the respira tory 
sights in ~igia ates ts to its thickened quality and only 
partial adaptation to land. 
The sensitivity of marine Crustacea to decreased levels 
of o2 was · tested (14). The amphipods Gammarus :pulex, e.nd g_ . 
locusta showed increa s ed gill movement when 02 levels were 
dropped below the saturation level between air and water. 
Movements of the scaphogmathites of crayfish Astacus showed a 
similar increase . Though Ligia shows an increa se pleopod 
beat with rising temperetur e s (20) (21), it did not show the 
sensitivity to change of o2 level as in the other aquatic 
crustacea tested. Pe rhaps Ligia's relatively receht emergence 
from the sea has been long enough to allow it to lose some 
of its sensitivities to an aquatic life . 
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Behavioral Patterns 
In this last section I would like to discuss some of my 
personal observations and shed s ooe light on possible reasons 
why Li .z: ia has not been able to advance much beyond the littoral 
area. 
Eating Habits 
Ligia is mostly herbivorous in its nor~al habitat, but 
has been observed in the laboratory to be quite capable of 
eating animal remains (Jl) (21). Those animals that were 
unable to adjust to laboratory conditions were usually 
devoured in two nights of feeding. Ligia showed a preference 
for the internal orga ns, but would eat the exoskeleton 
once the more choice areas were finished. When both plant 
and animal material was available they showed an equal liking 
for both. 
A variety of foods were fed to the laboratory animals 
and yielded the following preference list. Mossy filamentous 
algae (in fresh water)= scum consisting of unicellular 
algaes, small filamentous algaes, diatoms, and small micro-
scopic protozoa and crustacea = Fucus Ulva Alaria eel 
grass (more consumed). Moss capsules and parts of the 
syncytium of Vancheria were found in the gut contents of 
Ligia oceanica (20). The favorite food of some Ligia kept 
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21 
in a simulated seashore habitat was the synthetic .aellulose 
sponge I had used as their hiding place. Although fresh 
Fucus was placed in the aquarium it was never consumed . 
Only the s ponge and on several occasions the green scum 
covering the gravel made up the diet of t his populati on . 
It is apparent that Ligia has a very diversified appetite, 
which has no doubt helped it to take advantage of most of the 
suitable niches along the coast. In the three areas studied 
by the author Li~ia appeared to be subsisting on different 
diets, with the largest animals feeding on fresh water algae 
forms. 
Range and Grouping Pa tterns 
Barnes (2) discussed how Li~ia baudiniana would move with 
the incoming and outgoing tides, while feeding on the exposed 
plant life. I was never able to see such a clear example of 
orderly movement, although I did see some indication that 
this may be the case with the younger specimens at Winchester 
Bay. 
For the most part I found that Ligia ranges very little 
from its normal hiding place. At Pacific Nourage in Florence 
this amounted to only about 2-3 feet in most cases. The 
most adventuresome animals may have travelled as far as five 
feet. During the day ani mals were mar~ed with luminous paint. 
Up to 3 weeks l ater over half those marked could be found in 
22 
the same _approximate area behind the pilings. It~ probable 
tha t a higher percentag e was1. continually returning but 
due to the difficulty in being able to see all individuals 
because of the closely pa cked populations and the occasional 
shedding of the paint observati ~ns were g ross at best. At 
Winchester Bay the large adults would be found on or within 
a foot of the l arge rocks they would hide under. It seems 
almost essenti a l that they would have to migrate closer to 
, the water for enough food. but this was never observed. 
At Winchester Bay I noticed tha t in the smaller rocks 
closer to the water there seemed to be a segregation of the 
sexes. If a rock wa a overturned it would yield almost ex-
clusively males or fe males . I d on't know if this ls a 
normal behavior pattern for the younger Ligia or is due to 
the inability for the larger males to get under many of the 
smaller rocks inhabit ed by fe males. At the other two places 
studied t h is separation was not so evident. 
At Devils Elbow it was easiest to follow the mov ement 
of various individuals because they were often found on the 
roof of the caves in easy view. In one cave approximately 
100 individuals found in a large gathering were marked. The 
next day only about 4 of the marked individuals could be found. 
The large gathering on the roof had scattered i n to the deeper 
crevices at the back of the cave. Continued markingsof in-
dividuals showed no consistant pa ttern of movement or tha t 
23 
animals would return to the same approximate area after 
leaving on an evening feeding trip. This consisted of 
traveling 10- 25 f eet to t he outside of the cave and up the 
face of the cliff. When animals mi gr a ted up the f a ce of the 
cliff they would usually follow established routes of water 
seepage. When they would return to the cave with no specific 
guides to follow they appeared to wander r a ndo :r.1. ly about toward 
the back of the cave until a suitable place could be fo und. 
The animals kept in my 2' x J' aquarium would always 
walk along t he sides to get to the wa t e r and back. Their 
hiding place was about four inches from one corner and re-
sulted in many missed attempts before t hey finally locate it. 
Ani mals released near the waters edge would almost 
always orient themselves toward the rocks and beg in moving. 
This was also observed by Barnes (4), who found inconsistent 
results when he tested his theory of possible geotaxic 
controls. 
Ligia apparently has very poor navigational abilities 
and must rely on some type of natural barri er to guide its 
movements or else revert to random wandering. 
Regulation of Moisture over- Pleopods 
Ligia has mostly been discussed as a terrestrial organ-
ism, but does on occasion submerge. It will exhibit a porpoise 
like swimming motion i n deep water, until it is able to reach 
PLATE 1 PLATE 5 
PLATE 2 PLATE 6 
PLATE 3 PLATE 7 
PLATE 4 PLATE 8 
24 
a solid substrate. In t he lab it ha s been seen to voluntarily 
submerge itself and graze on t he algae coating covering the 
gravel. When Li i:; ia does submerge the pleopods, that remain 
flat agai nst the abdomen while on land , begin to bea t. Warmer 
water or decreasing salinity will both cause an increa se in 
the rate of beating (21) (20) probably due to a reduced o2 
concentra tion or increased a smatic work. 
Some animals that have been observed grazing in -water 
will eleva te the ir abdomen to allow for a erial respiration 
(plate 8). Upon l eaving t he wa ter Lig ia will go through a 
series of movements to free the pleopods of excess water. 
This consists of flexing their pleopods with a raising and 
slight curli ng of tr. e abdo;'.len (pla tes 1-J). The result is 
the formation of a water droplet on the uropods. The animal 
will then lower the uropods to the substrate and deposit the 
excess water (plates 4-5). In plate 6 the uropods are seen 
to be held tightly together to facilitate the water release. 
Barnes (2) has observed t his arrangement of the uropods to 
be also used for drawing wate r up to the pleopods by capillary 
action when additional moisture is needed. Plate 7 shows 
the common stance taken by animals tha t have finished depos-
iting their excess water and apparently need some additional 
drying or increased respiration. This position 1s usually 
maintained for about 2-10 minutes probably depending on the 
dampness of the surrounding s a nd the length of time submerged. 
25 
Limiting Factors of Land Inhabitation 
While studying Lig ia I have become aware of its relative 
abilities co□pared to other for ms of more terrestrial wood-
lice. Then considering its great mobility, its conparative 
ability to withstand drying conditions, and to vary unselective 
feeding habits I began to wonder why it has not been able to 
range farther into estuaries or up stream beds where it could 
still have an adequate water supply. 
There appeir to be two main possibilities tha t I see 
as restraining Li~ia from further terrestrial adaptation. 
First is the observations of Barnes (2) and Hewitt (17) 
that Ligia must return to the sea to release its young from 
the brood pouch. Very little research has been done on the 
young because of its possible affect on experimental results, 
so that specific weaknesses or needs of the newly liberated 
Ligia are not well known. In immersion experiments young Ligia 
have been s hown to withstand rising temperatures in water 
as well or better than adults (20). Possibly- the small size 
of the young and a high rate of evaporation necessitates an 
aquatic beginning. 
The other area to consider in the unusually high osmotic 
concentration of Ligia's blood discussed earlier in this 
paper. The lethal level that Li~ia can drop to and still 
survive is still higher than the more terrestrial forms normal 
26 
osmotic conce ntration. Li gi a 's poor ability to resist loss 
of essenti a l salts in fresh water f or prolonged period s of 
time would provide a seri ous problem during times of heavy 
rain. 
This discussion brings up the problem of how Ligia 
maintains its high salt concentra tion in areas like Devils 
Elbow, where most of the anima ls don't appear to go down to 
the salt water. Instead they spend the daylight hours in 
dry-cool cave s and t he evening s foraging on rocks bathed by 
fresh water. A simple study wa s carried out to see if Ligia 
would be able to attain sufficient salt from its i mmediate 
I 
surroundings. Samples from the rock faces were taken by 
scraping a 9 cm2 area of all surface deposits and roc k . 
Other samples were taken to correspond to the normal size of 
the scrapings. A sample of moist beach sand was used as an 
example of a salty substrate. All samples were soaked in 
distilled wa ter for 4 hours and periodically mixed to allow 
time for the salts to dissolve. Samples were t hen titrated 
in Parts/Per/Million using 0.0141 Ag NOJ to determine the 
chloride ion content. 
27 
Table 7 
Location Conditions a-con tent 
(parts/per/million) 
Cave #1 at entra nce 20.0 
inside 24.0 
18 ft. above ground 24.0 
40 ft. to right (no 
Ligia here) o.a 
Cave #2 at entrance o.o 
at back wall o.o 
Cave #J inside on top 4.0 
25 ft. above sand 20.0 
forage (sever a l animals 
were grazing on) o.o 
·cave #4 Inside on top o.o 
Ground seepage dripping off forage 100.0 
Sand moist 210.0 
Measured sample 100/per/mil. 95.0 
Sea water 35 0/00 35,000.0 
Though the accuracy of the tests might be in ~question 
it appe ars tha t Ligia would have trouble getting a great deal 
of salt from its surroundings. This may indicate that 
periodic trips to the more salty tidal areas may be necessary. 
No indication of this was seen on four evening visits· to 
this area. It is possible that under normal conditions 
Ligia is able to obtain enough salt from its food to compen-
sate for any loss, but under severe conditions must return 
to the ocean for large amounts of salts. 
Barnes (J) observed tha t Ligia would show a preference 
• 
28 
for being submerged ln salt water over fresh water, but the 
opposite was the case if placed on damp filter paper. 
Both ideas just discussed are mostly conjecture with 
available evidence very lit1ited indeed. It is evident that 
more work has to be done on Ligia to gain a more lmowledgeable 
understanding of its place in nature. 
29 
summary 
Ligia was found to be a common inhabitant ~ of the upper-
spray zone with the ability to occupy a wide ranging habitat. 
When compared physiologically with other Isopods Ligia 
was found to be a superior osmoregulator to both the ter-
restrial and aquatic species. It exhibits a good ability 
to live submerged in salt water for prolonged periods of 
time, and was shown to be one of the most tolerant of the 
Isopods to severe drying conditions. Ligia's large size and 
rather permeable cuticle allow it to substantially cool 
itself when exposed to severe temperatures. 
Ligia's respiratory mechanism shows very little adaptive 
change from that of the aquatic Isopods. Elaborate behavioral 
patterns have been established to free the pleopods of excess 
water after being submerged. 
Ligia has a very omnivorous diet showing equal taste 
·for plant or animal foods. Ligia has a very limited range 
and will only move far enough to find a meal or a place of 
suitable moisture. The range may be limited by very poor 
navigational abilities. 
It was suggested that Ligia's continued advancement to 
a more terrestrial habitat may be hampered by the necessity 
of releasing the young in salt water or its unusually high 
salt requirements. 
• 
JO 
Ligia is definitely an internediate Genus of Isopod in 
the transition from the sea to a terrestrial environment. 
Whether Ligia's present position~1s the result of a relatively 
recent ascent to land; some poorly adaptable characteristics; 
or a combination· of the two, is difficult to say with the 
small amount of research that has thus far been done. 
• 
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