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

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 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 (Rocha-Ramírez, Chávez-López & Bruce, 2005) comb. n. and Brucethoa epinepheli (Trilles & Justine, 2010) comb. n., are transferred from the Elthusa genus to the Brucethoa genus.

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., (2019Aneesh et al., ( , 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) 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; both antennae slender, with antennula shorter than antenna, and large pleopod rami that extend laterally beyond the pleotelson margins; ovigerous females have proximally thickened bilobed oostegite 1, and the marsupium is posteriorly partly enclosed by two sub-rectangular fleshy lobes.A detailed generic diagnosis and comparison of Brucethoa with other branchial cymothoid genera were provided by Aneesh et al., (2020b).(Figs. 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20) Material examined:
Antennula shorter than antenna, with eight articles, narrowly separated by rostrum; terminal article with few short simple setae.Antenna slender, with ten articles; article 4 longest; terminal article shortest, extending to anterior margin of pereonite 1. Buccal cone positioned strongly to anterior, overriding and obscuring antennal bases, visible in frontal view.Mandible with molar process, palp articles all slender, article 1, twice the length of article 3; palp article 3 with 9, simple setae.Maxillula with four acuminate terminal RS.Maxilla mesial lobe distinct, with two acute apical RS, lateral lobe with two acute RS.Maxilliped article 3 with two recurved RS.Maxilliped with oostegital lobes, lateral margins with many plumose setae; mouthparts not covered by oostegites of pereopod 1.
Pereopods articles not dilated or expanded; dactyli relatively short, strongly curved.Pereopod 1, basis 1.6 times as long as greatest width; ischium 0.7 times as long as basis; merus 0.55 times as long as wide; propodus 1.6 times as long as wide; dactylus 1.25 times as long as propodus, 3.3 times as long as proximal width.Pereopod 2 basis twice as long as greatest width; ischium 0.6 times as long as basis; propodus 1.8 times as long as wide; dactylus 3.4 times as long as proximal width.Pereopod 3 basis 2.1 times as long as greatest width; ischium 0.65 times as long as basis, 1.8 times as long as wide; propodus 1.6 times as long as wide; dactylus, 1.4 times as long as propodus, 3.2 times as long as proximal width.Pereopod 4 basis 1.6 times as long as greatest width; ischium 0.85 times as long as basis, 1.9 times as long as wide; propodus 1.6 times as long as wide; dactylus, 1.4 times as long as propodus, 3.1 times as long as proximal width.Pereopod 5 basis twice as long as greatest width; ischium 0.85 times as long as basis, 1.8 times as long as wide; propodus twice as long as wide; dactylus, 1.2 times as long as propodus, 3.0 times as long as proximal width.Pereopod 6 basis 1.7 times as long as greatest width, ischium 0.9 times as long as basis, 1.7 times as long as greatest width; merus 0.5 times as long as wide; carpus 0.8 times as long as wide; propodus 1.8 times as long as wide; dactylus 1.35 times as long as propodus, 3.0 times as long as greatest width.Pereopod 7 slightly longer than pereopod 6; basis 1.9 tims as long as greatest width; ischium as long as basis, 2.5 times as long as wide; merus 0.85 as long as wide, 0.35 times as long as ischium; carpus 0.6 times as long as wide, 0.2 times as long as ischium; propodus 1.8 times as long as wide, 0.4 times as long as ischium; dactylus 1.2 times as long as propodus, three times as long as basal width.
Pleopods large, slightly visible in dorsal view, extending laterally and posteriorly slightly beyond pleotelson margins; rami all simple, without proximomedial lamellar lobe, folds or thickened ridges.Pleopod peduncle lateral lobes absent.Pleopod 1 exopod 1.6 times as long as wide, lateral margin convex, distally broadly rounded, mesial margin convex; endopod 0.85 as long as exopod, 1.4 times as long as wide, lateral margin weakly convex, distally broadly rounded; peduncle twice as wide as long.Pleopod 2 exopod 1.55 times as long as wide, lateral margin convex; endopod 0.8 as long as exopod, 2.1 times as long as wide.Pleopod 3 exopod 1.6 times as long as wide; endopod 0.9 as long as exopod, 1.7 times as long as wide.Pleopod 5 exopod 1.5 times as long as wide, lateral margin convex; endopod 0.9 as long as exopod, 1.7 times as long as wide.Pleopods decreasing size from 1-5.
Uropod 0.8 times as long as pleotelson; peduncle 0.6 times as long as endopod, marginal setae absent.Endopod 3.8 times as long as greatest width, 0.85 times as long as exopod, lateral margin convex.Exopod curved to mesial, 4.0 times as long as greatest width, mesial margin concave, lateral margin convex.

Description of transitional stage (Figs. 13-14).
Body 2.9-3.1 times longer than wide; dorsal surfaces not vaulted, widest at pereonite 4-5, most narrow at pereonite 1. Cephalon similar to that of the female.Pereonites and pleonites of the late stage similar to those of the female.Antennula, antenna, mandible palp, maxilla, maxilliped similar to those of the male.Coxae similar to that of the female.Penial processes rudimentary.Pleotelson 1.3 times wider than long.Uropods similar to those of male.Pereopods and pleopods more similar to those of male in early stage, and those of late stage more similar to those of female.(Figs 15c,d,(16)(17)(18).Body symmetrical, 2.5 times as long as greatest width, dorsal surfaces smooth, lateral margins sub-parallel, widest at pereonite 3, pereonite 2-6 subequal in width, most narrow at pereonite 7. Cephalon 2.6 times wider than long, anterior border slightly triangular.Frontal margin similar to that of female.Eyes conspicuous, one eye 0.3 times width of cephalon.Coxae all dorsally visible.Pereonites 1-5 posterolateral angles not produced, pereonites 6-7 slightly produced.Pereonite 7 posterolateral margin slightly extending beyond the lateral margin of pleonite 1. Pereonites 3-6 more or less equal in width, 1 narrower than others.Pereonite 1 longest, 7 shortest.Pleon wide, 0.8 times as wide as pereon; pleonites 2-4 more or less equal in width, 1 narrower than others, 5 widest.Pleotelson 0.9 times   Pereopods, without carina, without spines or setae.Pereopod 1 basis 1.8 times as long as greatest width; ischium 0.6 times as long as basis; merus as long as wide; propodus 1.75 times as long as wide; dactylus, 1.2 times as long as propodus, 2.8 times as long as proximal width.Pereopod 2 basis 2.1 times as long as greatest width; propodus 1.4 times as long as wide, 0.55 as long as basis; dactylus, 1.4 times as long as propodus.Pereopod 3 basis twice as long as greatest width; propodus 1.7 times as long as wide, 0.7 as long as basis; dactylus, 1.1 times as long as propodus.Pereopod 4 basis 1.85 times as long as greatest width; propodus 1.3 times as long as wide, 0.4 as long as basis; dactylus, 1.65 times as long as propodus.Pereopod 5 basis 1.6 times as long as greatest width; propodus 1.4 times as long as wide, 0.55 as long as basis; dactylus, 1.7 times as long as propodus.Pereopod 6 basis twice as long as greatest width, ischium 0.8 times as long as basis; propodus 1.4 times as long as wide; dactylus 1.7 times as long as propodus.Pereopod 7 basis 2.1 times as long as greatest width; ischium 0.8 times as long as basis; merus as long as wide; propodus twice as long as wide; dactylus 1.1 times as long as propodus.

Description of male
Penial process acute, twice as long as basal width, separated by 28 % width of sternite 7, visible on sternite 7, basally mutually adjacent.

Description of juvenile
Antennula with eight articles, extending up to the anterior margin of pereonite 1; articles 3-8 with 2-3 spinules.Antenna longer than antennula, with ten articles, extends beyond the anterior margin of pereonite 1; articles 3-10 with 2-4 spinules.Mandible palp article 3 with one long and five short marginal setae; article 2 with four unequal setae.Maxillula, maxilla, and maxilliped similar to those of the male stage.

Etymology:
The new specific name 'isro', is an abbreviated name of the "Indian Space Research Organisation", the national space agency of India,  Brucethoa isro n. sp. is the second species under the genus.All life stages of the new species are described [including females (ovigerous and non-ovigerous), males, transitional, and juvenile for precise identification irrespective of the life stages.

Ecological remarks
From November 2017 to November 2021, along the southwest coast of India, we closely examined 52 species of deep-sea fishes for the presence of parasitic cymothoids.The cymothoid isopod, Brucethoa isro n. sp. was recovered only from spinyjaw greeneye, Chlorophthalmus corniger signifying its oligoxenous host specificity.A total of 76 individuals of C. corniger were examined from the available localities along the southwest coast of India.Of these, 37 individuals were infested with B. isro n. sp., the prevalence being 48.6%.Twenty females (22 ovigerous and 8 non-ovigerous), 12 transitional stages (4 early and 3 late), 18 males, and two juveniles were recovered from these 37-infested host fish.Parasites were usually found in pairs in the host fish, one in each branchial cavity; mostly male-female pairs were found; the relatively large ovigerous female was found in the branchial cavity, specifically attached to the base of the gill cavity, facing the head towards anterior, and the dorsal body closely adpressed against the gill, while the ventral brood presses against the inner wall of the operculum (see fig. 1).Males were found to occupy the opposite gill chamber in the same position.Host: Glossanodon macrocephalus Bineesh & Endo(Argentinidae) (Aneesh et al., 2020b).
Remarks: Recently, Aneesh et al., (2020b) described Brucethoa bharata, from specimens collected from Glossanodon macrocephalus from the Muttom, southwest coast, India.In the present study, we have collected 17 females (eleven ovigerous and six non-ovigerous) and eight males of Brucethoa bharata from the same host fish, G. macrocephalus, from the new locality Neendakara, Kollam district, Kerala state.Brucethoa bharata, can be identified by the following combinations of characters: the strongly vaulted dorsum, pereonites 6 and 7 with posterolateral margin expanded and coxae not visible in dorsal view, relatively wide pleon that has gaps between the pleonites, large pleopod rami that extend laterally well beyond the pleotelson margins, ovigerous females have proximally thickened oostegites, and the marsupium is posteriorly partly enclosed by two sub-rectangular fleshy lobes (see Aneesh et al., 2020b) (Fig. 22).Host: Epinephelus howlandi (Günther) (Argentinidae) (Trilles & Justine, 2010).
Remarks: Brucethoa epinepheli (Trilles & Justine, 2010) comb.n. was described initially based on the materials collected from the black-saddle grouper Epinephelus howlandi (Serranidae, Epinephelinae) from the coral reef of New Caledonia, the Southwestern Pacific by Trilles & Justine (2010), in the genus Elthusa.Based on the following characters: cephalon immersed in pereonite 1, coxae shorter than or as long as pereonites, relatively wide pleon, antennula narrowly separated by rostrum, buccal cone obscuring antennal bases, pleopods rami all simple, without folds or thickened ridges, sternite 7 with prominent posterior lobes, we place the species in combination with Brucethoa.

Taxonomy Suborder Cymothoida Wägele, 1989 Superfamily Cymothooidea Leach, 1814 Family Cymothoidae Leach, 1814 Genus Brucethoa Aneesh, Hadfield, Smit & Kumar, 2020
. 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.

(Rocha-Ramírez, Chávez-López & Bruce, 2005) comb. nov.
Synodus foetens (Linnaeus, 1766), was collected from the coast of central Veracruz, Mexico.Based on the following characters: antennula narrowly separated by rostrum, buccal cone obscuring antennal bases, pleopods rami all simple, without folds or thickened ridges, coxal plates not conspicuous in dorsal view, relatively wide pleon, with lateral gaps between pleonites makes it fit with the new genus Brucethoa.