Carcinoplaxmistio Ng & Mitra, 2019 (Crustacea, Decapoda, Goneplacidae): additional records and genetic differentiation of allied taxa.

The goneplacid crab, Carcinoplaxmistio Ng & Mitra, 2019, was originally described from West Bengal, India, in the northern Indian Ocean. Additional material of C.mistio from off Tamil Nadu in the southeast of India revealed a high degree of size-associated variation in the structures of the anterolateral tooth of the carapace, chelipeds, and male and female pleons. In addition to an in-depth morphological examination of C.mistio, this study also records the natural coloration of the species and conducts a genetic comparison (with mitochondrial COI and 16S rRNA genes) with its close relatives, C.haswelli (Miers, 1884) and C.purpurea Rathbun, 1914. Molecular comparison of C.mistio with its morphologically closest congener, C.haswelli from northern Australia and the western Pacific, corroborates their status as separate species. The genetic sequence of C.mistio, however, is similar to that of C.purpurea from the West Pacific, although these two species can easily be distinguished by distinct carapace and ambulatory leg characters. The present study provides some possible explanations for the genetic and morphological incongruence observed between C.mistio and C.purpurea and highlights the need for a detailed molecular study for Carcinoplax H. Milne Edwards, 1852, to appreciate the evolution of various morphological characters in the genus.