Distribution of amphipods Ischyrocerus on the red king crab, Paralithodes camtschaticus: Possible interactions with the host in the Barents Sea

The determination of costs and benefits experienced by crustaceans as a result of occupation by their symbionts has received increased attention from marine ecologists. However, the interactions between some important species and their associates remain unclear. We examined the distribution of amphipods in the genus Ischyrocerus on the red king crab Paralithodes camtschaticus, a commercially important species, in two areas of the Barents Sea. Ischyrocerus commensalis was found on 30.5% of crabs in Dalnezelenetskaya Bay (DZB) with the mean number per crab being 55.1, in Dolgaya Bay (DLB) these rates were 28.6% with 19.3 specimens per crab. Sympatric species Ischyrocerus anguipes was found on 13.5% of crabs in DZB with a mean of 7.3 individuals per host, in DLB it had much lower occurrence (1.3% and 1.5 specimens per host). There were no significant differences between proportions of male and female crabs infested by amphipods in both areas examined. Prevalence of amphipods was similar among years examined except for I. commensalis on small crabs (carapace length CL <90 mm) and I. anguipes on large crabs (CL >90 mm) in DZB. We found that I. commensalis and I. anguipes are not egg predators of P. camtschaticus in the Barents Sea, at least in summer. High numbers of I. commensalis occur in crab gills, and both mean intensity of the amphipods and their empty tubes increased with crab size. In the gills, I. commensalis predominated in the section nearest the mouth parts. Possible negative impacts for the hosts due to gills infestation are discussed. In contrast, I. anguipes were predominately found on the carapace and limbs of crabs and appears to be a less specific symbiont of P. camtschaticus. Both amphipod species seem to be commensals, however possible negative impacts for the host could not be excluded.