The reproductive ecology of a northeastern Pacific nudibranch, Janolus fuscus, with an examination of its endoparasitic copepod, Ismaila belciki

Abstract

The arminacean nudibranch Janolus fuscus (family Zephyrinidae) is found in rocky intertidal habitats along the northeast Pacific coast. Adult J. fuscus are conspicuous from April to October but absent in the early winter at two sites, North Cove and Fossil Pt., in the Coos Bay region of Oregon. Over four years of intertidal surveys at these sites, the density of J. fuscus peaked with the abundance of their bryozoan prey, Bugula pacifica and Tricellaria circumternata , in spring and summer, while adult absence in winter was correlated with strong winter storms. To describe the timing of development and determine the life cycle of J. fuscus , embryos and larvae were reared in the laboratory and examined with light, scanning electron, and confocal microscopy. Larvae reared in the lab and juveniles collected from the field were monitored to quantify growth. Janolus fuscus exhibited typical spiral cleavage and hatched as planktotrophic veligers that grew for over a month before they reached competency, settled, and metamorphosed on their prey, B. pacifica . Juvenile growth was rapid, and adults reached maximum sizes of over 50 mm before dying. These demographic and developmental studies suggest that J. fuscus is a subannual species with a life span of approximately five months. Janolus fuscus is often infected with an endoparasitic copepod, Ismaila belciki . In the field, prevalence of I. belciki increased with host density and size. The distribution of I. belciki was weakly aggregated in the host population. The large female parasite was generally found in the anterior portion of the host hemocoel, and one or more dwarf males were typically associated with each female. Infected J. fuscus produced significantly smaller egg masses with fewer larvae than did uninfected individuals. Infection did not influence growth rate but did cause decreased survival in older nudibranchs. To examine the life cycle of I. belciki , naupliar larvae were reared in the lab and incubated with potential hosts. Additionally, copepodid stages were described from dissected J. fuscus collected from the field. Ismaila belciki has a least three planktotrophic naupliar stages and four copepodid stages in its life cycle.