Description of Elthusa aquabio sp. n. (Crustacea: Isopoda: Cymothoidae), a branchial fish parasitic isopod from Indian waters Author Aneesh, Panakkool Thamban Author Helna, Ameri Kottarathil Author Raj, Smrithy Author Kumar, Appukuttannair Biju text Journal of Natural History 2023 2023-08-17 57 21 - 24 1193 1205 http://dx.doi.org/10.1080/00222933.2023.2242099 journal article 55339 10.1080/00222933.2023.2242099 d9d90eef-7b56-4196-a161-4fbdc607dddd 1464-5262 8270960 Elthusa aquabio sp. n. ( Figures 1–5 ) The new species name is registered in ZooBank under: urn:lsid:zoobank.org: act: 5F5C029D-A8E0-49ED-BE1C-F2D5A2AE3821 Type material: Holotype . 1 female [16.0 mm L, 8.8 mm W (maximum width) ovigerous], from an unknown host, coll. Raj S , in March 2022 (Reg. No. ZSI / WGRC /I.R./INV. 23,886). Description Holotype female ( Figures 1–5 ): Body weakly curved to the right, elongated, dorsal surfaces smooth, 1.80 times as long as greatest width, widest at pereonite 6, most narrow at pereonite 1. Cephalon symmetrical, sub-triangular, 1.20 times wider than long, anterior margin rounded, turned down from a rostrum. Eyes oval with distinct margin, dorsolateral to the cephalon, 0.40 times as long as cephalon, right eye 0.30 times as wide as cephalon. Coxae 2–4 very narrow, slightly visible in dorsal view, much shorter (0.6–0.7 times) than pereonites; coxae 5–7, flat, broad, laterally expanded, 5 nearly as long as pereonite, 6–7 longer than the pereonites. Coxae 7 overlapping lateral margin of pleonite 1–3. Pereonite 5 longest, 2 shortest; pereonites 1–6 slightly increase in width. Pleon short, 0.16 times as long as total body length, pleonites all visible, pleon 0.75 times as wide as pereion maximum width. All pleonites visible dorsally. Pleonites 1–3 entirely overlapped by pereonite 7 posterolateral expansion and coxa 7. Pleonite 1 narrower than others, pleonites 1–3 progressively wider towards posterior, pleonites 2 and 4 sub-equal, 5 narrower than 4. Pleonites 1–5 sub-equal in length; Pleonite 3 widest; posterior margin broadly rounded, lateral margin of pleonites curving posteriorly. Pleotelson 2.00 times wider than long, slightly narrower than pleonite 5; 0.90 width of pleonite 5, posterior margin broadly rounded. Antennula shorter than antenna, with eight articles, widely separated at base; article 1 widest, 1.10 times as wide as long; article 2 longest, 0.70 times as wide as long; articles progressively decreasing in length and width from 2–8; articles 6–7 as long as wide; terminal article shortest; articles 5–7 with 1–3 setae, terminal article with few terminal aesthetes. Antenna , narrower than antennula, with 10 articles; article 1 widest, 4 longest; progressively decreasing in length and width from 4 to 10; terminal article shortest, with few short simple setae; article 2 widest, 1.50 times as wide as long; article 5 longest; articles 5–12 decreasing in width. Mandibular molar process present, with curved incisor processes. Mandible palp slender, article 1 longer than 2, article 2 is 2.50 times as long as wide, article 3 shortest, 2 times as long as wide, with 4 short terminal recurved setae. Maxillula with 1 large and 3 small sub-equal acuminate terminal robust seta/e (RS). Maxilla mesial lobe with one acute apical RS, lateral lobe with 2 acute RS. Maxilliped without oostegital lobe, article 3 with four recurved RS. Figure 1. Elthusa aquabio sp. n. ovigerous female holotype (minimally dissected) (Reg. No. ZSI/WGRC/I. R./INV. 23886). A , dorsal view; B , ventral view; C , frontal view; D , lateral view. Figure 2. Elthusa aquabio sp. n. ovigerous female holotype (minimally dissected) (Reg. No. ZSI/WGRC/I. R./INV. 23886). A , dorsal view; B , cephalon dorsal view; C , cephalon ventral view. Pereopods 1–3 propodus inner-lateral margin with few robust spines. Pereopod 1 basis 1.70 times as long as greatest width; ischium 0.55 times as long as basis; merus 0.35 times as long as wide; carpus 0.50 times as long as wide; propodus 0.40 times as long as basis, 1.40 times as long as wide, inner-lateral margin with three robust spines; dactylus slender, 1.10 times as long as propodus, 3.20 times as long as basal width. Pereopod 2 basis 1.6 times as long as greatest width; ischium 0.65 times as long as basis; merus 0.40 times as long as wide; carpus 0.50 times as long as wide; propodus 1.40 times as long as wide, 0.40 times as long as basis, inner-lateral margin with three robust spines; dactylus 1.25 times as long as propodus, 3.00 times as long as basal width. Pereopod 3 basis twice as long as greatest width; ischium 0.70 times as long as basis; propodus 1.70 times as long as wide, inner-lateral margin with two robust spines; dactylus slender, 1.20 times as long as propodus, 3.00 times as long as basal width. Pereopod 4 basis 2.50 times as long as greatest width; ischium 0.60 times as long as basis; merus 0.75 times as long as wide; carpus 0.60 times as long as wide; propodus 1.40 times as long as wide, 0.30 times as long as basis; dactylus 1.40 times as long as propodus, 3.00 times as long as basal width. Pereopod 5, 2.15 times as long as greatest width; ischium 0.80 times as long as basis; merus 0.50 times as long as wide; propodus 1.30 times as long as wide, 0.35 times as long as basis; dactylus 1.20 times as long as propodus, 2.40 times as long as basal width. Pereopod 6 basis 1.60 times as long as greatest width; ischium 0.90 times as long as basis; merus 0.50 times as long as wide, 0.55 times as long as ischium; carpus 0.3 times as long as ischium, 0.50 times as long as wide; propodus 1.6 times as long as wide, 0.45 times as long as ischium; dactylus 1.10 times as long as propodus, 2.20 times as long as basal width. Pereopod 7 basis 1.90 times as long as greatest width; ischium 0.85 times as long as basis; merus 0.50 times as long as wide, 0.25 times as long as ischium; propodus 1.20 times as long as wide, 0.35 times as long as ischium; dactylus 1.6 times as long as propodus, 2.50 times as long as basal width. Figure 3. Elthusa aquabio sp. n. ovigerous female holotype (minimally dissected) (Reg. No. ZSI/WGRC/I. R./INV. 23886). A , antennula; B , antenna; C , mandible; D , maxilla; E , maxillula; F , maxilliped. Brood pouch formed by 4 pairs of alternatively overlapping oostegites arising from pereopods 2–4 and 6; oostegites 4 and 6 larger than 2 and 3. Pleopods exopod as long as or slightly longer than endopod, all rami laminar. Pleopod 1 exopod 1.05 times as long as wide, lateral margin strongly convex, distally broadly rounded, mesial margin weakly convex; endopod 0.95 times as long as exopod, 1.35 times as long as wide, lateral margin weakly convex, distally broadly rounded, peduncle short. Pleopod 2 without appendix masculina, exopod 1.15 times as long as wide; endopod 0.90 times as long as exopod, 1.30 times as long as wide. Pleopod 3 exopod 1.10 times as long as wide; endopod 0.95 times as long as exopod, 1.30 times as long as wide. Pleopod 4 exopod 1.20 times as long as wide; endopod 0.90 times as long as exopod, 1.35 times as long as wide. Pleopod 5 exopod 1.40 times as long as wide, lateral margin weakly convex, distally broadly rounded, mesial margin weakly convex; endopod 0.80 as long as exopod, 1.20 times as long as wide, lateral margin weakly convex, distally broadly rounded; peduncle 3.40 times as wide as long. Figure 4. Elthusa aquabio sp. n. ovigerous female holotype (minimally dissected) (Reg. No. ZSI/WGRC/I. R./INV. 23886). A–G , pereopods 1–7. Figure 5. Elthusa aquabio sp. n. ovigerous female holotype (minimally dissected) (Reg. No. ZSI/WGRC/I. R./INV. 23886). A–E , pleopods 1–5; F , brood pouch; G , uropod; H , pleotelson and uropods. Uropods 0.70 times the length of pleotelson; peduncle 1.10 times longer than wide, 0.70 times long as exopod, lateral margin without setae, apices rounded. Endopod 1.05 times as long as exopod, 2.50 times as long as greatest width, apically rounded, lateral margin distally convex, without setae. Exopod apically rounded, 3.00 times as long as greatest width, without setae. Colour. Fresh female, colour is pale tan with dark chromatophores on dorsal surface of pereonites 3–7 and pleonites; after preservation, it became much more tan. Distribution. Known only from the type locality. Hosts. Unknown. Etymology The specific name ̛ aquabio ̍ is an abbreviation of the name ′Department of Aquatic Biology and Fisheries, University of Kerala ̍, a pioneer research institute in Kerala state , India , which has significantly contributed to aquatic animal research, especially of marine fish parasitic crustaceans in India . Furthermore, it is the institution of the late renowned copepodologist Prof. N. Krishna Pillai, who laid the foundation for studies on the taxonomy of parasitic copepods in India . Remarks Elthusa aquabio sp. nov. conforms with the generic diagnosis of Elthusa recently revised by Aneesh et al . (2020a) , in having a weakly vaulted pereion, antennula shorter than antenna, bases not in contact, cephalon posterior margin that is not trilobed, coxae of equal size on both sides, coxae 5–7 flat and broad, coxa 7 equal to its respective pereonite, mandible palp slender, article 3 with distolateral setae, pereopods with relatively short dactylus, uropods short and not reaching the posterior margin of the pleotelson. Elthusa aquabio sp. nov. can be separated from all congeners by the following combination of characters: body slightly twisted, elongated; dorsal surfaces smooth; 1.8 times as long as greatest width; has the general appearance of a partially moulted cymothoid, due to the presence of larger posterior coxae; pleon short, ~16% body length (BL), pleon 0.75 times as wide as pereion maximum width; widest at pleonite 3 and most narrow at pleonite 1; coxae 5–7 flat, broad, laterally expanded; uropods 0.7 times the length of pleotelson; pereopods 1–3 propodus inner lateral margin with small robust spines; antenna with 10 articles. All the described species of Elthusa species have pereopods without spines. Elthusa aquabio sp. nov. is the only species with robust spines on the propodus of the first three pereopods in ovigerous females. The presence of spines on the anterior pereopod and the partially moulted-like appearance of the general body are the key identifying characteristics of this species. Furthermore, the new species closely resembles E. pseudorhombus in general body shape, number of articles in the antennula (10) and ratio of maximum pleon width to maximum pereion width (0.75). However, it can be distinguished from E. pseudorhombus by the following characteristics: pleonite 4 widest in E. pseudorhombus (vs pleonite 3 widest in E. aquabio sp. nov. ); pleotelson evenly rounded, 1.5 times as wide as long in E. pseudorhombus (vs pleotelson broadly rounded, 2.0 times wider than long in E. aquabio sp. nov. ); cephalon 1.7 times wider than long in E. pseudorhombus (vs cephalon sub-triangular and 1.20 times wider than long in E. aquabio sp. nov. ). Table 1. Inter-specific characters of Elthusa aquabio sp. n. , Elthusa fistularia , E. pseudorhombus , and E. uranoscopus , described from Indian waters.

E. uranoscopus
E. pseudorhombus	E. fistularia Aneesh ,	Aneesh, Helna,
Aneesh, Helna, Kumar	Helna, Kumar and	Kumar and Trilles,
Character	E. aquabio sp. n.	and Trilles, 2020	Trilles, 2020	2020
Cephalon	1.20 times wider than	1.7 times wider than	1.2 times wider than	1.6 times wider than
long	long	long	long
Coxae	Coxae 2–4 very narrow,	Coxae 6 and 7 longer	Coxae 6 equal to	Coxae 6 and 7, flat,
slightly visible in	than the pereonites,	pereonite, 7,	broad, longer than
dorsal view, much	coxae 7 overlapping	extending	the corresponding
shorter than	lateral margin of	beyond, coxae 6–	pereonite;
pereonites; coxae 5–	pleonite 1–2	7 posteroventral	posteroventral
7, flat, broad,laterally	posteroventral angles	angles broadly	margin rounded
expanded, 5 nearly	acute	rounded
as long as pereonite,
6–7 longer than the
pereonites
Antenna	With 10 articles	With 10 articles	With 9 articles	With 11 articles,
slightly longer
than antennula
Pleon length	~16% BL	~14% BL	~15% BL	~16% BL
Ratio of maximum	0.75	0.75	0.80	0.84
pleon width to
maximum
pereion width
Pleonites	Pleonite 3 widest	Pleonite 4 widest	Pleonite 2 widest	Pleonite 3 widest
Pleotelson	Broadly round, 2.0	Evenly rounded, 1.5	Broadly round, 1.7	Pleotelson 1.8 times
times as wide as long	times as wide as long	times as wide as	as wide as long,
long, lateral	lateral margins
margins convex	convex
Rami	Endopod 1.05 times as	Endopod 0.8 times as	Endopod as long as	Endopod 1.3 times
long as exopod	long as exopod	exopod	longer than
exopod
Pereopod spines	Pereopods 1–3	Absent	Absent	Absent
propodus
innerlateral margin with
few robust spines

Inter-specific character differences between Elthusa aquabio sp. n. , Elthusa fistularia , Elthusa pseudorhombus and Elthusa uranoscopus , described from Indian waters, are listed in Table 1 .