Pseudione Kossmann, 1881

Boyko, Christopher B., Williams, Jason D. & Shields, Jeffrey D., 2017, Parasites (Isopoda: Epicaridea and Nematoda) from ghost and mud shrimp (Decapoda: Axiidea and Gebiidea) with descriptions of a new genus and a new species of bopyrid isopod and clarification of Pseudione Kossmann, 1881, Zootaxa 4365 (3), pp. 251-301 : 273-274

Pseudione Kossmann, 1881

Diagnosis. Female: Body elongate; all segments distinct. Frontal lamina weakly developed, smooth. Barbula with single pair of smooth lateral projections; median region with series of low lobes. Maxilliped usually with dual palp, both lobes setose on distal margins. First oostegites with tapered posterolateral point; internal ridge with few low projections. Coxal plates moderately developed on pereomeres 1–4 on at least one side; tergal projections not present; lateral margins of pereomeres 1–4 rounded, 5–6 tapered or rounded and resembling pleomeres. Pereopods with short carpi, smooth outer surface of meri, and relatively smooth dorsal surface of bases. Six pleomeres, first five produced into moderately to greatly developed lateral plates, directed posterolaterally; five pairs of biramous, lanceolate pleopods, edges and surfaces smooth; uropods lanceolate, uniramous or biramous, edges and surfaces smooth. Male: Body gradually tapered anteriorly and posteriorly from widest pereomere; all body regions distinct. Anterior pereopods not markedly larger than others. Pleon of six pleomeres, usually all distinct but last two or three fused in some species; pleopods uniramous, tuberculiform; posteriolateral margins of pleotelson slightly to strongly produced into posterolateral lobes; no uropods. Worldwide distribution, infesting hosts in Axiidea and Astacidea (Nephropidea).

Remarks. 82 species and subspecies have been described in Pseudione , but after varied synonymizing and transfer to other genera, 53 currently remain in the genus ( Boyko et al. 2011). These 53 taxa are, however, a morphologically heterogeneous assemblage, with essentially only one unifying feature: the number of pairs of pleopods. Females of all Pseudione species have 5 pairs of biramous pleopods and males have 5 pairs of ovate uniramous pleopods. Clearly, other morphological characters need to be examined to sort these taxa into monophyletic groupings. A significant impediment to clear delineation of this genus has always been the question of identifying the characters of the type species, Pseudione callianassae Kossmann, 1881 , due to the poor quality of the original description. In the present work, we provide evidence that P. dohrni Bonnier, 1900 , is identical to P. callianassae (see Remarks under P. callianassae , below), thereby providing a suite of characters that can be used to define the genus, restrict it to a group of similar taxa, and identify those species of “ Pseudione ” that belong in other existing genera or for which new genera should be established.

The characters given above in the diagnosis restrict the concept of Pseudione to 8 species: P. callianassae Kossmann, 1881 (type species, = P. dohrni Bonnier, 1900 ), P. atlantica Bourdon, 1971 , P. borealis Caspers, 1939 , P. hanseni Nierstrasz & Brender à Brandis, 1923, P. longicauda Shiino, 1937 , P. murawaiensis Page, 1985 , P. nephropsi Shiino, 1951 and P. tanimbarensis Markham, 1999 . Of these, five ( P. callianassae , P. borealis , P. hanseni , P. longicauda , and P. murawaiensis ) parasitize hosts in Axiidea whereas three ( P. atlantica , P. nephropsi , and P. tanimbarensis ) parasitize hosts in Astacidea. Among the other 45 species previously considered to belong to Pseudione , none are known from astacidean hosts, whereas only three are known from axiidean hosts. Because the focus of this paper is on parasites of axiidean and gebiidean hosts, we do not deal with the 41 “ Pseudione ” species that are found on anomuran or caridean hosts. They will be arranged in natural groups as relevant material becomes available for revisionary work; we hope this will include molecular studies to test the monophyly of the groups and that Pseudione sensu lato can act as a testing ground for host/parasite co-evolutionary hypotheses.

We have analyzed the sole species of Pseudione purportedly described from a brachyuran host, P. panopei Pearse, 1947 , because it is a synonym of a species in a genus whose other species are known from gebiidean hosts (see Remarks under Progebiophilus upogebiae ). The three species that are found on axiidean hosts but do not share characters with Pseudione sensu stricto are also analyzed and transferred to other genera herein. Two of these species ( P. brattstroemi Stuardo, Vega & Cespedes, 1986 , and P. overstreeti Adkison & Heard, 1995 ) are placed in a new genus (see Remarks for Robinione n. gen.), whereas the third ( P. compressa Shiino, 1964 ) appears to be a species of Ionella Bonnier, 1900 (see Remarks for Ionella compressa , n. comb.).