On a freshwater species of the genus Sanyuania Zhao and Han, 1980 (Crustacea, Ostracoda, Loxoconchidae) from Thailand, with a discussion on morphological evolution of the freshwater Loxoconchidae

摘要

The genus Sanyuania Zhao and Han, 1980 is here transferred from the Cytherideidae to the Loxoconchidae, based on the morphology of valves and soft parts. Sanyuania segersi sp. nov., is described from a freshwater habitat, a river, in Thailand. This is the first record of this genus in Southeast Asia. The new species has a number of remarkable characters distinguishing it from other members in the genus and family: invicidont hinge, spearhead-like protruding posterior extremity of carapace in dorsal view, subtriangular shape of carapace in lateral view, sieve plate without a central seta (sensillum) and an aberrant chaetotaxy of first and second antennae. The new taxon is compared with other recent freshwater loxoconchids [Cytheromorpha fuscata (Brady, 1869), Elofsonia baltica (Hirschmann, 1909), Loxoconcha elliptica Brady, 1868, Loxoconcha galilea Lerner-Seggev, 1968, Pseudolimnocythere hartmanni Danielopol, 1979, Pseudolimnocythere hypogaea Klie, 1938] using (1) phylogenetic analyses and (2) analyses of the chaetotaxy of the first antenna and second and third thoracopods, by comparison with an ontogenetic series of Loxoconcha japonica Ishizaki, 1968. The results from both types of analyses are congruent and reveal that Sanyuania segersi sp. nov., Pseudolimnocythere and Loxoconcha are in a more derived clade in the Loxoconchidae and are closely related to each other, whereas Cytheromorpha and Elofsonia are more ancestral.