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

Syst Parasitol (2024) 101:50 https://doi.org/10.1007/s11230-024-10163-2 Description of Sandythoa gen. nov., a fish parasitic branchial cymothoid (Crustacea: Isopoda: Cymothoidae) from the Indian Ocean, with five species including one new species Panakkool Thamban Aneesh · Niel Bruce · Ameri Kottarathil Helna Appukuttannair Biju Kumar · Received: 20 January 2024 / Accepted: 4 April 2024 © The Author(s) 2024 Abstract 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. This article is registered in ZooBank under urn:https:// www.zoobank.org:pub:474C86E9-081C-4D8F-B7FB- 1B07E0345413. The branchial fish parasitic isopod genus Elthusa Schioedte & Meinert, 1884, was recently revised P. T. Aneesh (*) Blue Innovation Division, Seto Inland Sea Carbon Neutral Research Center, Graduate School of Integrated Sciences for Life, Hiroshima University, 5‑8‑1 Minato‑machi, Takehara, Hiroshima 725‑0024, Japan e-mail: ; N. Bruce Water Research Group, Unit for Environmental Sciences and Management, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa P. T. Aneesh Travancore Nature History Society (TNHS), MBRRA, Mathrubhumi Road, Vanchiyoor, Thiruvananthapuram, Kerala 695035, India N. Bruce Biodiversity and Geosciences Program, Queensland Museum, South Brisbane BC, PO Box: 3300, Brisbane, QLD 4101, Australia Introduction A. K. Helna (*) Regional Forensic Science Laboratory, Kannur, Kerala 670002, India e-mail: A. Biju Kumar Department of Aquatic Biology and Fisheries, University of Kerala, Karyavattom, Thiruvananthapuram, Kerala 695 581, India Vol.: (0123456789) 13 50 Page 2 of 28 by Aneesh et al. (2020a), and a restricted generic diagnosis was provided based on the type species, Elthusa emarginata (Bleeker, 1857). Thirteen species of Elthusa that did not fully conform to the new diagnosis were placed into Elthusa incertae sedis, while the remaining 26 species were retained in combination with Elthusa (See Aneesh et al., 2020a). That recent revision of Elthusa has allowed for a better understanding of the characters within the genus and for the allocation of species, firstly to the recently described Glyptothoa Helna, Aneesh, Kumar, & Ohtsuka, 2023 (three species), and here four species placed into a new genus (see Helna et al., 2023). Aneesh et al. (2023a, b) recently described Elthusa aquabio Aneesh, Helna, Raj, & Kumar, 2023 and Elthusa nemo Aneesh, Helna, Raj, & Kumar, 2023 from the southwest coast of India. Livoneca samariscii Shiino, 1951 was originally described from the samarid fish Samariscus japonicus Kamohara from Kochi, Japan. Bruce (1990) transferred it into the genus Elthusa. Elthusa samariscii (Shiino, 1951) was subsequently reported and redescribed from Kerala, southwest coast of India, by Kumar and Bruce (1997) and Aneesh et al. (2020a) from another species of samarid, Samaris cristatus Gray. Aneesh et al. (2020a), retained the species within Elthusa, but as incertae sedis. The present study initially set out to place Elthusa samariscii incertae sedis from India into the correct genus by examining the type specimen of E. samariscii deposited by Shiino (1951). As the type material appears to be missing, we compared the Indian specimen to the description and illustrations of Shiino (1951) (see Fig. 18). Based on that description (Shiino, 1951), it became clear that the specimens from India identified as Elthusa samariscii belongs to a different species than Elthusa samariscii from Japan. Furthermore, in the process of describing the new species from Samariscus cristatus from India, it became apparent that a new genus was needed for the new species as well as four other species currently placed incertae sedis in Elthusa. Together with recently described Glyptothoa, the new genus described here brings the total number of accepted cymothoid genera to 42 (Helna et al. 2023; Aneesh et al. 2024). A revised key to Vol:. (1234567890) 13 Syst Parasitol (2024) 101:50 the global marine branchial cymothoid genera is also provided. Materials and methods Fresh isopod specimens were collected from the branchial cavity of cockatoo righteye flounder, Samaris cristatus Gray (Samaridae), caught by commercial trawlers operating from Neendakara (08° 30.0′ N 76° 53.30′ E), Kollam district, Kerala state, and Muttom, Tamil Nadu state southwest coast of India. Methods for collection, preservation, dissection, mounting, and drawings of appendages follow Aneesh et al. (2019, 2020b, 2021a, b, 2022; 2024). One ovigerous female was designated as the holotype and one paratype was minimally dissected to conserve the specimens (the dissected appendages were kept in separate vials along with the specimen). The specimens were microphotographed using a multifocusing dissection microscope Leica-M205A and image capturing software (Leica Application Suit). Drawings were digital-inked using Adobe Illustrator and a WACOM CTL-472/K0-c drawing pad. Sources for the fish taxonomy and host nomenclature were FishBase (Froese & Pauly, 2024) and Catalogue of Fishes (Fricke et al., 2024). Classification of the cymothoid follows Brandt & Poore (2003). The type specimens are deposited in the Western Ghat Field Research Centre of the Zoological Survey of India, Kozhikode (ZSI/WGRC) and and PTA’s & AKH’s personal collection in India (CAH). Abbreviations: RS, robust seta/e; BL, body length; W, width; ZSI/WGRC, Western Ghat Field Research Centre of Zoological Survey of India, Kozhikode. Results Taxonomy Suborder Cymothoida Wägele, 1989 Superfamily Cymothooidea Leach, 1814 Family Cymothoidae Leach, 1814 Genus Sandythoa gen. nov. urn:lsid:zoobank.org:act:98058CA37B41-4C64-BA8E-6EDC5BDF0721 Syst Parasitol (2024) 101:50 Type species: Sandythoa tiranga sp. nov.; original designation. Diagnosis of female (bold = diagnostic characters). Body vaulted dorsa (...truncated)