Eremitione Williams & Boyko, 2019

Williams, Jason D., Boyko, Christopher B. & Madad, Asma Z., 2019, Branchial parasitic isopods (Crustacea: Isopoda: Bopyridae: Pseudioninae) of hermit crabs (Crustacea: Decapoda: Diogenidae) from the western Pacific, with descriptions of a new genus and three new species, Raffles Bulletin of Zoology 67, pp. 83-118 : 108-109

Eremitione Williams & Boyko , new genus

Diagnosis. Female body elongate, curved dextrally or sinistrally (straight in E. tuberculata , new combination); all segments distinct. Frontal lamina weakly developed, smooth. Barbula with either two lateral projections (outer more than twice as long as inner) or single elongate projection on each side meeting medially; medial region smooth or with numerous low lobes. Maxilliped with single palp, setae on projecting tip (if present). First oostegites with tapered to sharply recurved posterolateral points; internal ridge smooth or with few low projections. Coxal plates weakly developed on pereomeres 1–4 on at least one side; tergal projections not present; pereomeres 2 and 3 subequal in breadth, broader than 4; lateral margins of pereomeres 1–4 rounded, 5 and 6 tapered or rounded and resembling pleomeres; gradual transition in shape between pereomeres 1–6, 1–4 not abruptly different than 5 and 6. Pereopods with short carpi, scales on outer surface of meri, and inflated, scale-covered, dorsal surfaces of bases. Six pleomeres, first five produced into weakly developed lateral plates (greatly developed lateral plates in E. tuberculata , new combination), directed posterolaterally; sixth pleomere visible in dorsal view; overall pleon much narrower than pereon (except in E. tuberculata , new combination where they are not markedly narrower); five pairs of biramous, lanceolate pleopods, edges and surfaces relatively smooth (tuberculate in E. tuberculata , new combination); uropods lanceolate, uniramous, edges and surfaces smooth (tuberculate in E. tuberculata , new combination).

Male body gradually tapered anteriorly and posteriorly from widest pereomere; all body regions distinct. Anterior pereopods not markedly larger than others. Pleon of six distinct pleomeres; pleopods uniramous, tuberculiform; posterolateral margins of pleotelson slightly to strongly produced into posterolateral lobes (no posterolateral lobes in E. tuberculata , new combination); no uropods. Worldwide distribution in temperate and tropical waters, infesting hosts in Lithodidae and Paguroidea.

Remarks. Eighty-eight species and subspecies have been described or placed in Pseudione Kossmann, 1881 , at one time or another; some were transferred to other genera and some were synonymised so that 53 species remained in the genus until recently ( Boyko et al., 2017). These 53 taxa are a morphologically heterogeneous assemblage, with the number of pairs of pleopods being essentially the only unifying feature (females with five pairs of biramous pleopods; males with five pairs of ovate tuberculiform uniramous pleopods). Based on a study of those species of Pseudione found parasitising mud shrimp, ghost shrimp and lobsters, Boyko et al. (2017) defined Pseudione on the basis of the characters of its type species, P. callianassae Kossmann, 1881 , and restricted the genus to contain only the seven other species which shared numerous characters with the type species and whose characters were not present in the other 45 species of Pseudione sensu lato. Two species were placed in a new genus ( Robinione Boyko, Williams & Shields, 2017 ), P. compressa Shiino, 1964 , was transferred to Ionella Bonnier, 1900 , and P. panopei Pearse, 1947 , was synonymised with Progebiophilus upogebiae ( Hay, 1917) (Boyko et al., 2017) . This left 41 species in Pseudione sensu lato not congeneric with the type species of Pseudione . In the present study, we made a detailed examination of the descriptions and illustrations of all those species of Pseudione sensu lato found parasitising hermit and king crabs. Nine of these previously described species appear to form a monophyletic grouping that is distinct from Pseudione sensu stricto and to which none of the other Pseudione sensu lato species found on other hosts (e.g., galatheoids, carideans) belong. Therefore, we propose a new genus to contain the species parasitising paguroid and lithodid hosts: Eremitione Williams & Boyko. We also provisionally place Pseudione brandoi Brian & Darteville, 1941 , in Eremitione , new genus, based on the type host, even though the species was described solely from what appears to be a juvenile female and is in need of rediscovery and redescription.

Species of Eremitione , new genus can be distinguished from those of Pseudione sensu stricto (see Boyko et al., 2017) by the following characters of females: barbula of female with two lateral projections or single elongate projection on each side meeting medially (single pair of short, smooth lateral projections in species of Pseudione s. s.), maxilliped with single palp (usually with dual palp in species of Pseudione s. s.), setae on projecting tip of palp (both lobes of palp setose on distal margins in species of Pseudione s. s.), first oostegites with tapered to sharply recurved posterolateral point (with tapered posterolateral point in species of Pseudione s. s.), internal ridge of oostegites smooth or with few low projections (internal ridge with few low projections in species of Pseudione s. s.), coxal plates weakly developed on pereomeres 1–4 on at least one side (coxal plates moderately developed on pereomeres 1–4 on at least one side in species of Pseudione s. s.), six pleomeres, first five produced into weakly developed lateral plates, except in E. tuberculata , new combination where the lateral plates are large but rounded (moderately to greatly developed tapered lateral plates in species of Pseudione s. s.), overall pleon much narrower than pereon, except in E. tuberculata , new combination (overall pleon as wide as or wider than pereon in species of Pseudione s. s.), and uropods lanceolate, uniramous (uniramous or biramous in species of Pseudione s. s.). Males are difficult to distinguish morphologically but in Eremitione , new genus males invariably have a pleon with six distinct pleomeres, whereas those of Pseudione s. s. usually have all pleomeres distinct but the last two or three fused in some species. Eremitione , new genus infests hosts in Lithodidae and Paguroidea whereas species of Pseudione s. s. infest hosts in Axiidea and Astacidea (Nephropidea).

Species of Eremitione , new genus can be distinguished from those of Bopyrissa by the following characters: body straight ( E. tuberculata , new combination) or curved dextrally or sinistrally (S-shaped in species of Bopyrissa ), sixth pleomere visible in dorsal view (sixth pleomere ventrally displaced and concealed by medially adpressed lateral plates of fifth pleomere in species of Bopyrissa ).

Eremitone tuberculata , new combination has several differences from those of “typical” Eremitione , new genus: the body of the female is linear (curved dextrally or sinistrally in other species of Eremitione , new genus), the dorsolateral bosses on pereomeres 5–7 are greatly inflated and anterior ones overhanging posterior ones (not inflated, not overlapping in other species of Eremitione , new genus), the surfaces of the oostegites and pleopods are tuberculate (smooth in other species of Eremitione , new genus), the length of the pleon is ca. 40% of total body length (length of pleon <33% of total body length in other species of Eremitione , new genus), the lateral plates are much wider than the medial region of the pleomeres (lateral plates at most slightly expanded at distal margins compared to medial region of pleomeres in other species of Eremitione , new genus), and the male pleotelson is globular with no posterolateral extentions (posterolateral extentions in most other species of Eremitione , new genus, although some specimens of E. hyndmanni , new combination have been reported with essentially no posterolateral pleotelson extensions). However, all these differences, with the exception of the presence of tubercles, are differences of gradation in character. For example, the strong expansion of the dorsolateral bosses and lateral plates in the females of E. tuberculata , new combination is an expansion of structures that are present in all other species of Eremitione , new genus. We prefer to adopt the conservative position that E. tuberculata , new combination is a derived species of Eremitione , new genus, with morphological adaptations to its lithodid hosts (see Miranda-Vargas & Roccatagliata, 2004), especially in terms of what we interpret as secondarily-derived symmetry. It should be noted that some controversy on the evolutionary relationships of lithodoids and paguroids remain, although a “hermit to king” transition has been supported in multiple molecular analyses (see Morrison et al., 2002; Bracken-Grissom et al., 2013; Noever & Glenner, 2018; Hall & Thatje, 2018). Importantly, parasites such as bopyrids and rhizocephalans could be used as independent lines of evidence to test hypotheses on the evolutionary relationships of these anomurans (e.g., Boyko & Williams, 2009; Noever, 2017). Thus, E. tuberculata , new combination provides an opportunity to test (via molecular data) the hypothesis that lithodoids evolved from a hermit crab ancestor using coevolutionary studies. Alternatively, E. tuberculata , new combination may be found as a distinct lineage and require erection of a new genus.

Type species. Phryxus hyndmanni Bate & Westwood, 1867 .

Type locality. Groomsport, Ireland; type host: Pagurus sp .