Elthusa aquabio Aneesh & Helna & Raj & Kumar 2023, sp. n.
Creators: name: Aneesh, Panakkool Thamban
name: Helna, Ameri Kottarathil
name: Raj, Smrithy
name: Kumar, Appukuttannair Biju
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Isopoda
Cymothoidae
Elthusa
Elthusa aquabio

description: 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.
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.
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.
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.).
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.
description: Published as part of Aneesh, Panakkool Thamban, Helna, Ameri Kottarathil, Raj, Smrithy & Kumar, Appukuttannair Biju, 2023, Description of Elthusa aquabio sp. n. (Crustacea: Isopoda: Cymothoidae), a branchial fish parasitic isopod from Indian waters, pp. 1193-1205 in Journal of Natural History 57 (21 - 24) on pages 1195-1202, DOI: 10.1080/00222933.2023.2242099, http://zenodo.org/record/8270960