Odiomarinae nov. subfam., a new subfamily for two primitive genera of Hymenosomatidae MacLeay, 1838, with preliminary remarks on the family (Crustacea, Decapoda, Brachyura)

Abstract

A new subfamily Odiomarinae nov. subfam. is erected to receive two primitive genera of the eubrachyuran family Hymenosomatidae MacLeay, 1838: Odiomaris Ng & Richer de Forges, 1996, and Amarinus Lucas, 1980, mostly from fresh and estuarine waters of the Indo-West Pacific region. The new subfamily is characterised by the presence of "intercalated platelets" on the male abdomen, either articulated and moveable or relatively less well demarcated. The hymenosomatid platelets are actually vestigial uropods that are similar to those, also showing as dorsal plates, of the podotreme Dynomenidae Ortmann, 1892, and Dromiidae De Haan, 1833. The hymenosomatid uropod differs from the podotreme ones by the deep socket that is excavated at its ventral side and thus corresponds to the typical eubrachyuran press-button system. The odiomarine socket is particularly interesting because it provides morphological and phylogenetic criteria for identifying podotreme uropods and eubrachyuran sockets as homologues. In addition to several other plesiomorphic characters, the retention of dorsal uropods in the Hymenosomatidae, a unique known case in the Eubrachyura Saint Laurent, 1980, and evidence of an ancient lineage, allows re-defining and preliminarily interpreting the exclusive combination of characters of the family and to reconsider its status within the Eubrachyura.

The position of the Hymenosomatidae MacLeay, 1838, within the Brachyura (Crustacea, Decapoda)

“…Several distinctive characters may be enumerated (Lucas 1980;Ng & Chuang 1996;: peculiar disposition of the eyes (absence of orbits and proepistome in the plesiomorphic state; orbits remaining incomplete in the derived hymenosomatid taxa) and cephalic appendages (antennules vertically folded and dorsally exposed; no antennal fossa and antennae extended forwards); thoracic sternum with sternites 4-8 considerably enlarged (Fig. 1B); sutures 4/5-7/8 laterally restricted; sterno-abdominal cavity of males generally reduced in length; male and female abdomens never having more than five somites, always with the formation of a pleotelson (somite 6 fused to the telson) and, often, fusion of additional somites to the pleotelson so that the abdomen may consist of only three elements; press-button for abdominal locking situated on the undivided part of the thoracic sternum; abdominal sockets located at the base of the pleotelson, thus belonging as usual to the last abdominal somite (somite 6); presence in some genera (Odiomaris and Amarinus) of defined intercalary platelets, completely articulated, moveable, homologous to the uropods of the Dromiidae De Haan, 1833, but with the sockets situated ventrally, the platelets being slightly discernible (Holthuis 1968: 115;Lucas 1980 (Barnard 1950: 67;Richer de Forges 1976;Guinot 1979a: 186); axial skeleton regularly compartmented, with a parallel arrangement of the phragmae in the anteroposterior plane (Secretan 1998(Secretan : 1763Guinot 2011…”

“…2, 4) supported the heterotreme status of the Hymenosomatidae, although with indecisive results "regarding a hymenosomatid-majid alliance, either in terms of topological robustness or resolution, although majoids and hymenosomatids are always in proximity to or near the 'base' of the eubrachyurans". The odiomarine Amarinus lacustris (Chilton, 1882), basal in the family (Guinot 2011), was recovered as the sister taxon of the dorippid Dorippoides facchino (Herbst, 1785), indicating a dorippid + hymenosomatid clade within the Heterotremata. Significantly, the majoidhymenosomatid assemblage was always associated with the dorippids, the majoids having a low position among the eubrachyurans.…”

“…The presence in the Hymenosomatidae of seminal receptacles connected to sternal openings of sternite 5, the vulvae (Guinot 1979a: 186), the salient apomorphy of the Eubrachyura Saint Laurent, 1980, supports a eubrachyuran assignment. Nevertheless, the retention in the basal Hymenosomatidae (Odiomarinae Guinot, 2011) of dorsal uropods as in the basal Podotremata (Guinot & Bouchard 1998;Guinot 2011), so far unique among eubrachyurans, merits discussion. In addition, a wide range of other plesiomorphic features is present in hymenosomatids, notably a weak cephalic condensation, with the absence of orbits and proepistome in basal representatives and incomplete orbits even in the more derived taxa.…”

Coevolution of post‐Palaeozoic arthropod basibiont diversity and encrusting bryozoan epibiont diversity?

“…Podotreme crab classification follows Karasawa et al (2011Karasawa et al ( , 2014, Davie et al (2015), and Schweitzer et al Thoma et al (2014), and Davie et al (2015). For the classification and arrangement of superfamilies, families, subfamilies and genera within the Majoidea, we follow Guinot (2011Guinot ( , 2012 and Windsor & Felder (2014). Classification of Pinnotheroidea follows Davie et al (2015) and Theil et al (2016).…”

A new genus and species of anchialine Hymenosomatidae (Crustacea, Decapoda, Brachyura) from Samar, Philippines

“…Conservation. Many members of the family Hymenosomatidae are small in size, cryptic in habits and known to inhabit littoral to sublittoral zones that are sensitive to human activities (Chuang & Ng 1994;Guinot 2011). In the case of Samarplax principe new species, the conservation challenges are associated with the cave environment.…”