Rhopalione Pérez, 1920

Boyko, Christopher B. & Williams, Jason D., 2024, New records of bopyrid parasites (Crustacea, Isopoda, Epicaridea) of pea crabs (Crustacea, Decapoda, Pinnotheridae) with descriptions of two new species of Rhopalione Pérez, 1921 and a review of the genus, Zoosystema 46 (7), pp. 149-175 : 156-158

Genus Rhopalione Pérez, 1920

TYPE SPECIES. — Rhopalione uromyzon Pérez, 1920 , by original designation.

REMARKS

The occurrence of female bopyrid isopods under the pleons of brachyuran crabs is apparently restricted to members of the genera Rhopalione and Spathione Boyko & van der Meij, 2018 . All other bopyrids that occur abdominally are in subfamilies other than Pseudioninae and occur on anomurans and caridean shrimps but never on brachyurans. This unique site of parasitization might correlate with the morphology of the host carapace. All species of Rhopalione parasitize genera of pinnotherid crabs (Pinnotherinae) in which, for females at least, the carapace is very soft and weakly calcified. This is in marked contrast to the carapace of other brachyurans infested by bopyrids, which is well calcified in both sexes and offers more protection to the parasite; this is true even in the gall crabs ( Cryptochiridae Paulson, 1875 ), which are parasitized by species of Spathione . The adaptation to an abdominal mode of infestation by species of Rhopalione might have allowed for maximal protection by the host, which is better obtained in an abdominal position; alternatively, Rhopalione is a relatively large bopyrid and the space afforded under the pleon is much larger than the branchial chamber (see further remarks below). In gall crabs, the parasitization of the pleon by species of Spathione may be a response to the host living in corals and making branchial parasitization more difficult. However, the soft carapace of pinnotherids and coral habitat of gall crabs do not preclude parasitization within the branchial chamber as four genera of keponine bopyrids ( Heterocepon Shiino, 1936 , Hypocepon Nierstrasz & Brender à Brandis, 1930 , Onychocepon Pérez, 1921 , and Rhizophoracepon Williams, Boyko &Tri, 2023 ) are composed of entirely branchial bopyrids of pinnotherids, while a fourth genus ( Dactylokepon Stebbing, 1910 ) contains one additional species known from pinnotherid branchial chambers. Likewise, the sole species of Carcinione Bourdon, 1983 is a branchial parasite of gall crabs. Clearly, the mode of infestation exhibited by species of Rhopalione and Spathione is unusual among bopyrids found on pinnotherids and gall crabs but further discussion here is restricted to species of Rhopalione (see Boyko & van der Meij 2018 for a fuller discussion on Spathione ).

In other brachyuran hosts, the female parasites obtain food by piercing the gill lamellae and rarely leave evidence of their feeding. In contrast, females of species of Rhopalione are oriented with the dorsal surface appressed to the sternites of the host ( Page 1985) and pierce the weakly calcified ventral surface of the pleon, leaving noticeable holes in the exoskeleton (see Remarks under Rhopalione rusa n. sp.). Females of species of Rhopalione are far more dependent on the secondary sexual modification of the host than other pseudionine species. Ordinarily, a female bopyrid occurring in a brachyuran branchial chamber is relatively unaffected by the sex of the host, as the host branchial chamber is not sexually dimorphic. However, in species of Rhopalione the parasite is found either on female crab hosts, which have a much broader pleon, or on male crab hosts that are usually feminized by the parasite, becoming modified with broader pleons (see Page 1985). How the parasite accomplishes this feminization of the male host, which also has female-type pleopods accompanied by male gonopods ( Page 1985), is unknown but is likely hormonal in origin. Changes in sexual characters of male crab hosts has been documented in some bopyrids (e.g. Hiraiwa & Sato 1939; Reverberi 1942; Reinhard 1956; Bourdon 1968; Romero-Rodríguez & RománContreras 2011) but is usually limited to primary reproductive structures (gonopores, gonopods, pleopods) and with a much less pronounced effect on external morphology and secondary sexual characters. It is also highly variable, as all hosts are not equally affected by their parasites in terms of modification of sexual characters. A side-effect of the abdominal mode of life is that no specimens of any species of Rhopalione exhibit marked torsion of the body (dextral or sinistral), as commonly seen in many other bopyrid taxa.

The relationships of Rhopalione and Spathione to other pseudionine bopyrids are difficult to discern. Rhopalione was once considered a part of Ioninae H. Milne Edwards, 1840 sensu lato (now Keponinae Boyko, Moss, Williams & Shields, 2013 ) but was moved to Pseudioninae Codreanu, 1967 by Boyko & van der Meij (2018), as the pleonal lateral plates of the female are very well developed and proximally constricted, the dorsomedial regions of the pereomeres are raised into low bosses, and the maxilliped and oostegites are comparable to other genera in the subfamily.Within Pseudioninae, however, these two genera are difficult to place, as they bear no general resemblance to any other taxon. The males of Rhopalione , with their distinctive pleonal morphology and strong dorsoventral curvature, are also unlike other pseudionine species, and indeed other bopyrid taxa excepting Spathione . Bourdon (1983) cited Rhopalione , Gigantione Kossmann, 1881 , and Carcinione Bourdon, 1983 , as the only three genera infesting brachyurans that did not appear to belong to Ioninae, but he did not formally assign any of them to a subfamily. However, Bourdon’s (1983) consideration of the status of Rhopalione was based solely on information in Pérez (1920a). Subsequent work by Page (1985) and Markham (1990) suggested that Rhopalione was a member of Ioninae (of uncertain placement) but this was refuted by Boyko & van der Meij (2018) who demonstrated that the genus belongs to Pseudioninae. Gigantione may be paraphyletic (Boyko, unpublished) but its position, as defined by the type species G. moebii Kossmann, 1881 (see also Bourdon 1969), suggests placement within Pseudioninae as well. Carcinione was also placed in Pseudioninae by Boyko & van der Meij (2018).