A new species of branchial fish parasitic deep-sea isopod, Brucethoa Aneesh, Hadfield, Smit & Kumar, 2020 (Isopoda: Cymothoidae) from the Indian Ocean, with the transfer of two Elthusa Schioedte & Meinert, 1884 species

Syst Parasitol (2024) 101:26 https://doi.org/10.1007/s11230-024-10149-0 A new species of branchial fish parasitic deep‑sea isopod, Brucethoa Aneesh, Hadfield, Smit & Kumar, 2020 (Isopoda: Cymothoidae) from the Indian Ocean, with the transfer of two Elthusa Schioedte & Meinert, 1884 species Panakkool Thamban Aneesh Ameri Kottarathil Helna · Appukuttannair Biju Kumar · Received: 4 September 2023 / Accepted: 6 January 2024 / Published online: 13 March 2024 © The Author(s) 2024 Abstract Brucethoa isro n. sp., a new species of deep-sea cymothoid is described and illustrated from the host fish Spinyjaw greeneye, Chlorophthalmus corniger Alcock, 1894, at depths of 265 to 458 metres from the southwest coast of India. Brucethoa isro n. sp. is recovered from the base of the gill cavity, facing the head towards the anterior, and the dorsal body closely adpressed against the gill, while the ventral brood presses against the inner wall of the operculum. Brucethoa isro n. sp., the second species of the genus, is characterized by: head weakly immersed in pereonite 1, very elongated body (3.15 times as long as wide); body dorsum not vaulted, almost flat; all coxae 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, Trivandrum, Hiroshima 725–0024, Japan e-mail: ; P. T. Aneesh Travancore Nature History Society (TNHS), MBRRA, Mathrubhumi Road, Vanchiyoor, Trivandrum, Kerala 695035, India A. K. Helna (*) Regional Forensic Science Laboratory, Kannur, Kerala 670002, India e-mail: A. B. Kumar Department of Aquatic Biology and Fisheries, University of Kerala, Karyavattom, Thiruvananthapuram, Kerala 695 581, India short, 0.5 times as the length of corresponding pereonites; sternite 7 with prominent posterior lobes. All adult life stages of the new species are described [including females (ovigerous and non-ovigerous), males, transitional, and juvenile. The species is currently known from the southwest coast of India and is the type locality. Additionally, this research provides valuable ecological insights into Brucethoa isro n. sp. and its habitat. As part of the taxonomic contributions, two species, Brucethoa alvaradoensis (RochaRamírez, Chávez-López & Bruce, 2005) comb. n. and Brucethoa epinepheli (Trilles & Justine, 2010) comb. n., are transferred from the Elthusa genus to the Brucethoa genus. Introduction Fish parasitic isopod family Cymothoidae Leach, 1814 globally includes 365 species under 41 valid genera (Aneesh et al., 2023b). Among them, eleven genera are monotypic. More than 70 % of the species (282 out of 365) are from the following eight genera: Anilocra Leach, 1818 (61 species); Ceratothoa Dana, 1852(26 species); Cymothoa Fabricius, 1793 (42 species); Elthusa Schioedte & Meinert, 1884 (41 species); Ichthyoxenos Herklots, 1870 (19 species); Mothocya A. Costa in Hope, 1851 (30 species); Nerocila Leach, 1818 (44 species); Renocila Miers, 1880 (20 species) (Aneesh et al., 2016; Kawanishi et al., 2023). Nineteen genera with less than five Vol.: (0123456789) 13 26 Page 2 of 27 Syst Parasitol (2024) 101:26 species, at least 15 genera are known to infest the branchial cavity, including the recently described Brucethoa Aneesh, Hadfield, Smit, & Kumar, 2020 and Glyptothoa Helna, Aneesh, Kumar & Ohtsuka, 2023 (Aneesh et al., 2020b, 2023a; Helna et al., 2023). Indian cymothoid fauna is well-documented, with 62 valid species under 22 genera (Ravichandran et al., 2019; Aneesh et al., 2020a, 2021a, 2022a, 2023a; Helna et al. 2023). Among them, at least 18 species are originally described from Indian waters, which indicates the scope of exploration and description of more new taxa, especially from non-commercial and deep-water hosts. Parasitic cymothoids infesting commercial fish from Indian waters are well documented (Ravichandran et al., 2019; Aneesh et al., 2020a, b, 2021a, b, 2022a, b, 2023b; Nashad et al., 2022). On the other hand, cymothoid infestation of non-commercial deep-sea- trash fishes is not well documented. The marine fish Spinyjaw greeneye, Chlorophthalmus corniger Alcock, collected at depths of 265 to 458 metres from the southwest coast of India, and the occurrence of a branchial parasitic isopod, specifically attached to the base of the gill cavity, facing the head towards anterior, with some unique morphology, always creates curiosity among taxonomists. While identifying the present cymothoid specimen, it was clear that it belonged to the recently described genus Brucethoa, by the following characteristics: cephalon with a rostral point; bilobed brood plate 1; sternite 7 with prominent posterior lobes; larger pleopods. Accordingly, we described a new species of Brucethoa in the present study based on the following life stages, female, male, transitional, and juvenile. The ecological remarks of the new species are also provided. The two Elthusa species transferred to Brucethoa are: B. alvaradoensis (Rocha-Ramírez, Chávez-López & Bruce, 2005) comb. n., and B. epinepheli (Trilles & Justine, 2010) comb. n. The collected cymothoids were processed following the techniques described by Aneesh et al., (2019, 2021c). One ovigerous female was designated as the holotype, and one paratype and a few non-type specimens were minimally dissected to conserve the specimens (the dissected appendages were kept in separate vials along with the said specimen). Methods for dissection, mounting, and drawings of appendages were according to the techniques described in Aneesh et al., (2019). The specimens were microphotographed using a multi-focusing dissection microscope Leica-M205A and image capturing software (Leica Application Suit). Drawings were digitally inked using Adobe Illustrator and a WACOM CintiQ DTK-1300 drawing pad. Distribution of Brucethoa bharata Aneesh, Hadfield, Smit & Kumar, 2020 and Brucethoa isro n. sp. is provided in Fig. 22. Sources for the fish taxonomy and host nomenclature were Fish Base (Froese & Pauly, 2023) and Catalogue of Fishes (Fricke et al., 2023). The type specimens are deposited in the Western Ghat Field Research Centre of the Zoological Survey of India, Kozhikode (ZSI/ WGRC) and other collections are deposited in PTA and AKH’s personal collection, extended in India (CAH). Abbreviations: RS robust seta/e; BL body length; W width. Materials and methods Remarks: Aneesh et al., (2020b) recently described the deep sea cymothoid genera Brucethoa as a monotypic genus for B. bharata. The following combinations of characters characterise the genus: cephalon weakly immersed in pereonite 1, cephalon anterior margin with acute ventrally directed rostral point, pereonites 6 and 7 with posterolateral margin expanded; pleon with gaps between the pleonites; Fresh specimens of unidentified cymothoid samples were collected from the branchial cavity of the deepsea fish Spinyjaw greeneye, Chlorophthalmus corniger Alcock (Aulopiformes: Ch (...truncated)