Resolving taxonomic ambiguity and distribution of Tremoctopus gracilis (Cephalopoda: Tremoctopodidae) based on molecular evidence

The octopus of the genus Tremoctopus from the Arabian Sea has not been investigated in the past. This study reports on the rare pelagic octopus Tremoctopus gracilis from the south-eastern Arabian Sea. Twenty females with dorsal mantle lengths (DML) ranging between 62 and 160 mm and weighing between 31 and 719 g were examined. These specimens were collected from commercial fishing grounds, and amongst them was a single mature female with 12,618 oocytes in the ovary (size varied from 0.61 to 1.67 mm (mean = 1.13 mm). The detailed morphometric and morphological characteristics were recorded, and the specimens were identified up to the genus level. Further molecular species identification was carried out through mitochondrial markers (COI & 16S rRNA). Phylogenetic analysis, based on partial sequences of the above markers, generated a single clade that included sequences from the Indian and Pacific Oceans, including those misidentified as Tremoctopus violaceus from the Pacific. Evolutionary divergence estimation revealed a high inter-specific genetic divergence from T. violaceus sequences in the Mediterranean Sea and Atlantic Ocean, thus confirming the Tremoctopus species in our study as T. gracilis and authenticated that GenBank sequences of T. violaceus from the Pacific Ocean are indeed T. gracilis .