Description of Sandythoa gen. nov., a fish parasitic branchial cymothoid (Crustacea: Isopoda: Cymothoidae) from the Indian Ocean, with five species including one new species.

The comparative analysis of records of Elthusa samariscii (Shiino, 1951) from Japan and India, alongside corresponding illustrations, indicates that the records of E. samariscii from Samaris cristatus Gray in India represent a distinct and previously undescribed species. This study introduces Sandythoa tiranga gen. and sp. nov., providing comprehensive descriptions of various lifecycle stages, including the female, male, transitional, premanca, and manca larvae. The following combinations of characters identify the genus: cephalon anterior margin with acute rostrum; pleonite 1 is distinctly narrow, not extending laterally; presence of a narrow gap between pleonites; antenna with more than 10 articles; maxilliped with oostegital lobe. Sandythoa tiranga sp. nov. is specifically identified along the southwest coast of India. Furthermore, we propose transferring the following species from Elthusa: Sandythoa arnoglossi (Trilles and Justine 2006) comb. nov.; Sandythoa parabothi (Trilles and Justine, 2004) comb. nov.; Sandythoa samariscii (Shiino, 1951) comb. nov.; Sandythoa moritakii (Saito and Yamauchi, 2016) comb. nov. A revised key to the global marine branchial cymothoid genera is provided.