RHIZOCEPHALA, F. Muller, 1862

McDermott, John J., 2011, Parasites of shore crabs in the genus Hemigrapsus (Decapoda: Brachyura: Varunidae) and their status in crabs geographically displaced: a review, Journal of Natural History 45 (39 - 40), pp. 2419-2441 : 2429-2430

Order RHIZOCEPHALA

Among the species of rhizocephalans known to parasitize H. sanguineus , Polyascus polygenea has been studied most extensively ( Lützen and Takahashi 1997; Takahashi and Lützen 1998). Originally described as Sacculina polygena, Glenner et al. (2003) established the new genus based on molecular and morphological data and the presence of multiple externae because of asexual reproduction. Prevalence of infected crabs (those bearing externae) in Japan reaches> 80% in some areas ( Yamaguchi et al. 1994; Takahashi and Lützen 1998), and is highest in mature crabs. Korn et al. (2005) recorded a prevalence of 41.5% in H. sanguineus (n = 1288) during a 2-year study in Vostok Bay, Sea of Japan; males and females were equally infected (272 and 262, respectively). Annual growth of parasitized males and females is less than normal (Takahashi and Matsura 1994). Feminization occurs in infected males as the result of pleopod changes (degeneration and some bifurcation), widening of the abdomen, and reduction in chela height; morphological changes are minimal in infected females ( Yamaguchi and Aratake 1997). Although usually only one externa appears per crab, as many as eight may be produced and there may be 50,000 embryos per externa; one externa may produce three generations of larvae ( Korn et al. 2004). In the Sea of Japan reproduction is continuous from spring to autumn, so high host prevalences of Polyascus polygenea prevail. Following the final release of nauplii both male and female crabs remove and consume externae ( Takahashi et al. 1997), after which additional externae are produced ( Korn et al. 2004).

Both Sacculina nigra and Sacculina senta were described from specimens infecting H. sanguineus collected in Japan ( Table 2). Lützen and Takahashi (1997) compared these two sacculinids with their description of Sacculina polygenea , also from Japanese waters, and concluded that they were similar but not identical. Later Glenner et al. (2003) suggested that Sacculina nigra and Sacculina senta might eventually be included within the new genus Polyascus . According to Lützen and Takahashi (1997) the unidentified species of Sacculina infecting H. penicillatus and studied by Shimoizumi (1970, 1973) may also have been Polyascus polygenea . Shimoizumi’s research showed that the barnacle caused damage to the host’s testes, ovarian castration, and degeneration of the androgenic glands related to feminization in infected male crabs.