Abstract The present study reports the first confirmed occurrence of the poecilostomatoid copepod Hemicyclops tanakai in the Vellar estuary, southeast coast of India. Previously known only from Japan and Korea, H. tanakai is a burrow-dwelling symbiotic copepod associated with decapod burrows in sediment-rich estuarine habitats. Zooplankton samples were collected using a 158 µm mesh net, and nauplii were cultured to adulthood under laboratory conditions. Adults were examined with standard taxonomic keys, and diagnostic features such as appendage segmentation and genital somite structure validated their identity with the original description. The Indian population displayed reversed sexual size dimorphism, with females larger than males, differing from earlier reports. Molecular confirmation was obtained by sequencing the 18S rDNA gene (GenBank Accession: PQ845087.1), which showed 99.53% similarity to reference H. tanakai sequences. Basic Local Alignment Search Tool (BLAST) analysis revealed close affinity with other Hemicyclops species and clausidiid relatives. Maximum likelihood phylogenetic analysis of partial 18S rDNA sequences placed the Vellar specimen within the H. tanakai clade with strong bootstrap support (99%), corroborating morphological identification. This record extends the known distribution of H. tanakai to the tropical Indian subcontinent and suggests possible range expansion via coastal currents or anthropogenic vectors such as ballast water. The finding adds to Indo-Pacific copepod biogeography and underscores the value of integrative taxonomy for reliable identification. It also highlights the need for continued biodiversity monitoring of Indian estuaries, which face mounting pressures from anthropogenic activity and climate change.
Beaven et al. (Thu,) studied this question.