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.


Introduction
The branchial fish parasitic isopod genus Elthusa Schioedte & Meinert, 1884, was recently revised 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 andElthusa nemo Aneesh, Helna, Raj, &Kumar, 2023 from the southwest coast of India.
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 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. (2019Aneesh et al. ( , 2020bAneesh et al. ( , 2021aAneesh et al. ( , 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).
Additional characters: Mandible palp articles all slender.Maxilla mesial lobe distinct (not fused).Maxillula with 4 acuminate terminal RS.Maxilliped with oostegital lobes; mouthparts not covered by oostegites of pereopod 1. Pereopodal bases each without a prominent carina, without setae; articles not dilated or expanded.
Etymology: The genus is named in honour of the late Alexander James Bruce, known to all as 'Sandy' (the Scottish diminutive of his first name), in tribute to his memory and in recognition of his significant contribution to the taxonomy of decapod crustaceans.The gender is feminine.

Remarks
Sandythoa gen.nov.can be distinguished from all other branchial cymothoid genera by the following combinations of female characters: cephalon anterior margin with acute rostrum, rostrum narrowly rounded, not folded; pleonite 1 is laterally strongly reduced; the presence of a moderate gap between pleonites; pleon short 15% BL, less than 0.70 times as wide as maximum pereon width; antenna with more than 10 articles; maxilliped with oostegital lobe; pleopod peduncle lateral lobes absent; uropods short, extending approximately halfway along pleotelson lateral margin.
Pereopod 1 basis, 1.70 times as long as greatest width; ischium 0.75 times as long as basis; propodus as long as wide; dactylus slender, as long as propodus, 3 times as long as basal width.Pereopod 2 basis, 2.33 times as long as greatest width; ischium 0.60 times as long as basis; merus lateral margin with bulbous protrusion; propodus 1.50 times as long as wide; dactylus slender, 0.65 times as long as propodus, 1.65 times as long as basal width.Pereopod 3 basis, 2.1 times as long as greatest width; ischium 0.70 times as long as basis; propodus 1.4 times as long as wide; dactylus slender, 1.1 times as long as propodus.Pereopod 4 basis, 2.85 times as long as greatest width; ischium 0.70 times as long as basis; propodus 1.8 times as long as wide; dactylus short, 0.7 times as long as propodus.Pereopod 5 basis, 2.63 times as long as greatest width; ischium 0.70 times as long as basis; propodus 1.8 times as long as wide; dactylus slender, 0.8 times as long as propodus.Pereopod 6 basis 2.5 times as long as width; ischium 0.85 times as long as basis; propodus 0.77 times as long as wide, 0.40 times as long as ischium; dactylus as long as propodus, 2.10 times as long as basal width.Pereopod 7 basis, 2.50 times as long as greatest width; ischium 0.80 times as long as basis; merus 0.65 times as long as wide, 0.33 times as long as ischium; propodus 1.75 times as long as wide, 0.58 times as long as ischium; dactylus 0.85 times as long as propodus, 2.40 times as long as basal width.
Uropod 0.60 times as long as pleotelson; peduncle 0.5 times as long as exopod, 1.1 times as wide as long, lateral margin without setae; rami without marginal setae, apices narrowly rounded.Exopod 2.6 times as long as greatest width, 1.4 times as long as endopod, lateral margin convex.Endopod 0.7 times as long as exopod, apically narrowly rounded, exopod curved to mesial, 1.75 times as long as greatest width, mesial margin concave, lateral margin convex.
Antennula, antenna maxillule, and mandible similar to those of ovigerous female.Maxilla basally widest; inner median lobe with 1 and outer lateral lobe with 3 small, slightly recurved RS.Maxilliped slightly narrower than in non-ovigerous female, article three with 2 terminal recurved RS.
Pereopods similar to those of ovigerous female.Penes conical, apices blunt and medially united.Appendix masculina straight, small, shorter than endopod.
Uropod slightly larger than in female, about half the length of pleotelson; rami unequal in length, curved and apically rounded, exopod longer than endopod.
Transitional stage (Fig. 12): Body 2.0 times longer than wide; slightly hunched towards one side, cephalon similar to that of male.Pereonites, pleonites, antennula, antenna, and mandible palp similar to those of ovigerous female and maxilla, maxilliped similar to those of male.Coxae similar to that Page 15 of 28 50 Vol.: (0123456789) of female.Penes not prominent.Pleotelson 1.40 times wider than long, shorter than pleonite 5. Uropods longer than that of female, reaching up to 0.50 length of pleotelson.Rami unequal, similar to those of male.Pereopods and pleopods similar to those of male and ovigerous female.Premanca (Fig. 13): Elongated and transparent body, 2.80-3.00times longer than wide.Eyes prominent and conspicuous in dorsal view.Cephalon 1.20 times wider than long.Yolk globules are visible in pereon, between pereonites 1 and 6.Pereonite 2 widest; gradually decreasing in width from 4-7.All pleonites visible and subequal in length and width.Pleotelson 1.40 times wider than long, without plumose setae.Antennula with 8 articles, extending slightly beyond anterior margin of pereonite 1. Antenna longer than antennula, with 12 articles; all articles without setae and spinules, extending beyond posterior margin of pereonite 1. Mouthparts not well developed; mandible palp articles without setae and spines; maxillule, maxilla and maxilliped with poorly developed apical spines.Apical spines not recurved.Six pereopods, all pereopods without spines.Propodus and dactylus of pereopods not toothed.Pleopods not visible in dorsal view.All pleopods without plumose setae.Uropod rami subequal, extending beyond distal margin of pleotelson, apically rounded without plumose setae.
Variation of adult female: Body sub-oval, slightly twisted to either left or right side, 1.50-1.80times longer than wide.Cephalon, 1.20-1.30times wider than long.Pleotelson 1.25-1.40times wider than long.Maxilliped without oostigite lobe in nonovigerous female.The number of eggs or larvae in brood pouch ranges from 70-220 according to size of female.
Colour: Female, male and transitional stage of light pink colour with scattered chromatophores throughout pereon; premanca and manca clear with scattered chromatophores.
Etymology: The specific name 'tiranga', is derived from two Hindi words, Tin and Ranga (originally from the Sanskrit word Tri and Ranga) which colloquially means the tri-colour of India's flag.Furthermore, 'Tiranga Point' is a location on the Moon near the lunar south pole where Chandrayaan-2's lander Vikram crashed.The site was named on 26 August 2023.It is located at the coordinates 70.8810° S 22.7840° E and it lies between 'Manzinus C' and 'Simpelius N' craters.
Distribution: Japan (North Pacific Ocean and East China Sea) (Saito & Yamauchi, 2016;present study) Host: Known only from the type host Ereunias grallator Jordan and Snyder (Saito & Yamauchi, 2016;present study).Remarks: Sandythoa parabothi (Trilles & Justine, 2004) comb.nov., was originally described from New Caledonia, based on specimens collected from the branchial cavity of lefteye flounders, Parabothus kiensis (Tanaka) (Bothidae) collected at depths of 385 to 401 meter (Trilles & Justine, 2004).Based on the following characters we place the species in combination with Sandythoa gen.nov.: body widest at pereonites 3 and 4; pleonite 1 is narrower than others; cephalon not deeply immersed in pereonite 1, anterior margin with acute rostral point; pereonites 2-7 coxae visible in dorsal view; pleon 0.40 times as wide as maximum pereon width; antennula separated by rostrum, shorter than antenna; pereopods dactyli relatively short, strongly curved.Interspecific character difference between the species of Sandythoa gen.nov.are listed in Table 1.
Sandythoa gen.nov.can be separated from Brucethoa by: pleopods not large, not visible in dorsal view (vs extensively large pleopods, visible in dorsal view in Brucethoa) and all coxae visible in dorsal view in (vs the coxae of pereonites 6 and 7 not visible in dorsal view in Brucethoa) (Aneesh et al., 2020b(Aneesh et al., , 2024)).
The genus Catoessa does have small gaps between the pleonites but differs from Sandythoa gen.nov. in having: a rotationally twisted pleon with a narrow pleonite 1, the anterior margin of the cephalon lacking a rostral point, and the uropods extend about halfway along to beyond the posterior margin of the pleotelson (Bowman & Tareen, 1983;Bruce, 1990;Trilles et al., 2012;Aneesh et al., 2020a;Helna et al., 2023).
Mothocya differs from Sandythoa gen.nov. in having the antennula being both distinctly longer and stouter than the antenna, lacking a distinct rostral point, the uropods extending to or beyond the posterior margin of the pleotelson, and the absence of a gap between pleonites (see Bruce, 1986;Hadfield et al., 2015;Aneesh et al., 2016;2020b;Kawanishi et al., 2023;Helna et al., 2023).
The genus Ichthyoxenos includes both flesh burrowers as well as gill-attaching species inhabiting both freshwater and marine water (Bruce, 1990).Sandythoa gen.nov.differs from Ichthyoxenos by following features: body slightly hunched (vs body strongly ovate and nearly circular in dorsal view in Ichthyoxenos); short gaps are present between all pleonites (vs no gap between pleonites in Ichthyoxenos); pleonite 1 laterally reduced (vs pleonite 1 laterally not reduced in Ichthyoxenos).

Conclusions
The branchial attaching species described here was found to differ consistently from all other known cymothoid genera; we describe Sandythoa gen.nov.with the type species S. tiranga sp.nov.The new genus Sandythoa is the 44th genus in the family and the fourth genus originally described from India.