Identification and observations of parasitic isopod larvae (Isopoda: Epicaridea) from the northeastern Pacific: pelagic distribution and association with copepod intermediate hosts

Zooplankton samples from the northeastern Pacific, USA were analyzed to identify and determine the abundance and distribution of parasitic isopod larvae and copepod hosts. Vertically stratified samples were collected at stations spanning the continental shelf off Oregon, USA. Two species of epicarideans were identified: Hemioniscus balani (endoparasite of barnacles) and Argeia pugettensis (ectoparasite of shrimps). Cryptoniscus larvae of these species can be distinguished based on the antennae (four basal and five flagellar articles in H. balani, four basal and four flagellar articles in A. pugettensis), tooth-like structures on the antennules and coxal teeth (present in H. balani, lacking in A. pugettensis), and pereopods (1, 2 gnathopodal and 5, 6 ambulatory in H. balani, all gnathopodal in A. pugettensis). Epicaridean larvae were rare in zooplankton samples with concentrations up to 10 m(-3). Larvae were often found within 10 km of shore, with highest concentrations below the Ekman layer at 20-50 m depth near the bottom and between the 8 and 10 degrees C isotherms. Likely as a result of dislodgment, only 26 copepods with epicaridium or microniscus larvae attached were confirmed. Larvae were mostly (78%) found attached to two species of Acartia, suggesting they were the preferred host, but five other genera of copepods were documented as hosts. The concentration of copepods and larvae exhibited a significant positive correlation when the analysis was limited to stations where both larvae and copepods were caught. The distribution of host copepods was significantly shallower than epicaridean larvae and generally closer to shore. The planktonic phase of the life cycle of epicarideans remains a black box for most of the +860 known species. Future work should include molecular data to link their larval and adult life cycle stages and connect the parasites with their intermediate hosts.