Porcellanid crab Pachycheles stevensii parasitized by rhizocephalans Lernaeodiscus spp.: population biology in the northwestern Sea of Japan

Abstract

We examined the population and reproductive characteristics of the anomuran crab Pachycheles stevensii Stimpson, 1858 (Decapoda: Porcellanidae) heavily infested by 2 rhizocephalans of the genus Lernaeodiscus in the northern part of the species range (Peter the Great Bay, Sea of Japan) in 2020-2021. The prevalence of infestation reached 32.3%, and L. rybakovi proved to be much more numerous than L. kasyanovi. The sex structure of the P. stevensii population was represented by males (38.8%), females (50.6%), and modified specimens (10.6%), which were extremely feminized males. The latter had a broadened abdomen, smaller chelae, shortened gonopod, and 3 additional pairs of female-type pleopods. Thus, the initial sex ratio in the P. stevensii population approximated the expected 1:1, as in most porcellanid crabs. No significant modifications of the secondary sex characters of the female hosts were observed. The size structures of males and females did not significantly differ. P. stevensii produced 1 clutch yr-1. Oviposition and incubation lasted from summer to early fall; recruitment was noted in the fall. There was a correlation between the reproductive cycles of the host and parasites: rhizocephalans with mature externae were found from June-October. A large number of ovigerous female P. stevensii simultaneously carried parasitic externae of L. rybakovi; fecundity was considerably higher in non-infested females.