Nipponasellus Matsumoto, 1962

Sidorov, Dmitry, 2023, Trans-Japan Sea land-bridge disjunction: A case of vicariance in the subterranean genus Nipponasellus (Crustacea, Isopoda, Asellidae) in a largescale biogeographical context, Zootaxa 5357 (3), pp. 342-374 : 350

Genus Nipponasellus Matsumoto, 1962

Nipponasellus Matsumoto, 1962, p. 163 , figs 1–10, (orig. descrip.). — Matsumoto, 1966, p. 77.

Type species: Asellus aioii Chappuis, 1955 (type loc.— wells in Aioi City , Hyogo Prefecture, Honshu, Japan) .

Generic diagnosis (emended): Small and entirely stygomorphic isopods inhabiting exclusively groundwater close to continental-margin areas of the Japanese Sea. Dorsal surface smooth, with sparse bristles. Pereonites 1–7 wider than long in dorsal view. Sternal processes absent. Antennula flagellum with each article successively longer, terminal article of flagellum vestigial. Antenna without exopodite. Inner plate of maxillula with 5 apical pappose setae. Mandibles normally with 3-segmented palp, sometimes reduced (1-segmented). Pereopod 1 propodus not sexually dimorphic, palm with stiff setae. Pereopod 2–7 propodi terminating in biungulate dactyls. Pereopod 4 not sexually dimorphic. Pleopod 2 endopodite (appendix masculina) sickle-shaped or a little curved, distally pointed with distinct ‘cannula’ completely immersed in ctenoid cuticular scales. Female pleopod 2 operculate, approximately triangular, with broadly rounded terminal corner. Body length from 5.0 to 11.1 mm, commonly males larger than females.

Composition and geographic distribution: Seven species. Five species, Nipponasellus aioii (Chappuis, 1955) , N. hubrichti (Matsumoto, 1956) , N. kagaensis (Matsumoto, 1958) , N. takefuensis (Matsumoto, 1961) , and N. tonensis (Matsumoto, 1961) , from wells of the Japanese archipelago; N. sudzukhensis spec. nov., N. matsumotoi spec. nov. from groundwaters of South Primorye .

Discussion of affinities: The heuristic search performed in this study resulted in the identification of a single most parsimonious tree with a length of 329 steps from the initial set of 51 most parsimonious trees. The resulting cladogram (see Fig. 3 ) provides an overview of the distribution of apomorphic states. Based on the phylogeny obtained, the current generic arrangement within the Caecidotea-Proasellus macrogroup remains unresolved. However, it is clear that both the ‘insular’ and ‘continental’ groups of Nipponasellus form a monophyletic clade within the tree. In addition, Nipponasellus appears to be a sister clade to the Calasellus-Columbasellus group. The ancestry of Calasellus-Columbasellus is supported by unique synapomorphy observed in males, the ‘labial spur’ on the endopodite of pleopod 2. Furthermore, advanced structural features associated with the male pleopod 2 endopodite confirm the localisation known as the ‘ Asellus -pattern’, which is supported by another pair of synapomorphies. It should be noted, however, that given the numerous homoplastic and polymorphic characters found across taxa, and the uncertainties surrounding the biological significance of several characters, there is little justification for maintaining the current arrangement within Asellidae . The cladistic analyses conducted in this study provided insight into the relationships between proposed groups, but showed limited confidence in resolving phylogenetic relationships within the Caecidotea-Proasellus group. These results suggest that the available morphological characters have limited phylogenetic utility for determining generic relationships within Asellidae . It is difficult to determine the phylogenetic placement of Nipponasellus with confidence based on these results. A comprehensive study with an expanded dataset, including additional species and characters not included in this analysis, would be required to gain a more accurate understanding of these relationships. When relying on diagnostic characters at the generic level, discrepancies with Magniez’s scheme ( Magniez 1996) were found. In particular, Nipponasellus was observed to occupy an independent position, characterised by an unusually submerged ‘ctenoid cannula’ and the absence of a ‘labial spur’. Nevertheless, it appears to be closer to the Caecidotea-Proasellus group rather than to the ‘ Asellus -pattern’. Based on interpretation of these results, both Nipponasellus and other southern Far Eastern subterranean asellids show an independent centre of origin and diversification within the Asia-Pacific region.

Figures

FIGURE 3. Consensus tree of genera and informal groups of Asellidae showing distribution of character states, with Nipponasellus highlighted in gray and Asellus-pattern highlighted in green. Additive characters nos. 2, 7, 8, 9, 10, 11. Black rectangles – unreversed apomorphies, empty rectangles – homoplasious apomorphies.