Evolution in the dark: Unexpected genetic diversity and morphological stasis in the blind, aquifer-dwelling catfish Horaglanis Author Raghavan, Rajeev Department of Fisheries Resource Management, Author Sundar, Remya L. Department of Fisheries Resource Management, Author Arjun, C. P. https://orcid.org/0000-0002-2652-9805 Kerala University of Fisheries and Ocean Studies (KUFOS), Kochi, India & Malabar Awareness and Rescue Center (MARC), Kannur, Kerala, India Author Britz, Ralf https://orcid.org/0000-0002-0126-4660 Malabar Awareness and Rescue Center (MARC), Kannur, Kerala, India Author Dahanukar, Neelesh https://orcid.org/0000-0001-7162-9023 Senckenberg Natural History Collections, Dresden, Germany neelesh.dahanukar@snu.edu.in text Vertebrate Zoology 2023 2023-01-25 73 57 74 http://dx.doi.org/10.3897/vz.73.e98367 journal article http://dx.doi.org/10.3897/vz.73.e98367 2625-8498-73-57 45578678ECC741FC81E06FB045D2CF78 9B191FC9F94150C2B828F960AED7B94B Horaglanis populi sp. nov. Fig. 5 Holotype. KUFOS.F.2022.101, 32.5mm SL. from a dug-out well at Malapally, Kerala, India (21 m asl), collected by Remya L. Sundar, Arya Sidharthan and C.P. Arjun on 6 Dec 2020. Paratypes (n = 5). KUFOS.F.2022.102, 23.9mm SL, from a dug-out well at Thiruvalla, Kerala, India (7 m asl), collected by V.K. Anoop on 11 Dec 2019; KUFOS.F.2022.103, 26.8mm SL, from a dug-out well at Edanadu, Kerala, India (18 m asl), collected by Remya L. Sundar and Arya Sidharthan on 03 Dec 2020; KUFOS.F.2022.104, 27.4mm SL, from a dug-out well at Thiruvanvandoor, Chengannur, Kerala, India (5 m asl), collected by Remya L. Sundar on 10 Mar 2022; KUFOS.F.2022.105, 29.0mm SL, from a dug-out well at Thiruvanvandoor, Chengannur, Kerala, India (5 m asl), collected by Arya Sidharthan on 14 Dec 2020; KUFOS.F.2022.106, 29.4mm SL, from a dug-out well in Chengannur, Kerala, India (5 m asl), collected by Remya L. Sundar and Arya Sidharthan on 01 Dec 2021. Figure 5. Horaglanis populi holotype (KUFOS.F.2022.101, 32.5 mm standard length) in A life and B - F immediately after preservation. A , B Lateral view; C ventral view; D dorsal view; E lateral view of head; F ventral view of head. Etymology. The species name populi , genitive of the Latin noun populus = people, honours the invaluable contributions made by interested members of the public in the southern Indian state of Kerala, helping to document the biodiversity of subterranean and groundwater systems, including the discovery of this new species. Diagnosis. A species of Horaglanis as evidenced by the absence of eyes and pigment, a blood-red body in life, a highly reduced pectoral fin in which only a shortened spine is present, an elongate body with long dorsal and anal fins extending to the base of the caudal peduncle, and four pairs of well-developed barbels. Genetically, Horaglanis populi forms a distinct clade, the sister group to the other three congeners (Fig. 2 ), from which it differs by a genetic uncorrected p distance of 13.8-17.4% in the COI gene, and between 12.3-14.0% in the cyt b gene. Specifically, H. populi differs from all three known species in the barcoding gene (Supplementary Table S4) in positions 106 (C vs. T), 115 (T vs. C), 142 (T vs. C), 171 (G vs. A), 183 (T vs. C), 216 (A vs. C or T), 234 (C vs. T), 237 (G vs. A), 265 (T vs. G), 270 (C vs. A), 312 (A vs. C or T), 324 (A vs. C), 325 (T vs. C) 330 (G. vs. A or T), 350 (G vs. T), 363 (T vs. G), 421 (C vs. G), 448 (C vs. T), 481 (G vs. T), 489 (C vs. T), 496 (A vs. G), 517 (c vs. T), 528 (G vs. T), 533 (G vs. A), 538 (A vs. C), 539 (A vs. G), 542 (T vs. C), 565 (T vs. A), 576 (G vs. T or C), 597 (A vs. C), 618 (C vs. T), 633 (G vs. A) and 636 (C vs. T). Description. Body elongated (Fig. 5 ), round in cross section anteriorly, laterally compressed posteriorly, dorsal profile slightly convex to start of dorsal fin, straight more posteriorly. Ventral profile convex in head region, then straight posteriorly. Head large, 15.7-20.4% standard length (Table 6 ), with dorsally and laterally bulging adductor muscles. Snout truncated. Mouth wide, terminal. Eye absent. Four pairs of barbels: two mandibular, one maxillary and one nasal barbel pair; nasal and inner mandibular barbels shorter than maxillary and outer mandibular barbels. Maxillary and outer mandibular barbels reaching posterior border of pectoral fins when folded back. Gill opening large, extending to slightly above pectoral-fin base; gill membranes united with isthmus. Scales absent. Caudal peduncle laterally compressed, 9.3-16.9% of standard length. Dorsal fin long, with 22-23 soft rays (xiii-xiv unbranched/8-9 branched), originating in advance of pelvic fin origin. Anal fin long, with xiii-xvii unbranched rays, starting opposite dorsal fin ray number 9, ending opposite base of last dorsal fin ray. Pectoral fin vestigial, consisting only of modified pectoral fin spine covered by thickened skin. Pelvic fin short, wide, with rounded margin, with ii-iv unbranched and 2-4 branched rays. Caudal fin with rounded posterior margin, with 8-9 branched and 2-4 dorsal unbranched and 2-4 ventral unbranched rays. Head skeleton well ossified (Fig. 6 ); neurocranium with a single large cranial fontanelle, no epiphyseal bridge connecting frontals in dorsal midline; lateral neurocranium wall with large trigeminofacial foramen; supraoccipital with long, narrow and pointed crest; opercle small and subtriangular. Jaws massive, dentary and premaxilla studded with numerous rows of closely set, recurved villiform teeth. Figure 6. 3-D reconstructed CT-images of head and anterior vertebrae of Horaglanis populi , KUFOS.F.2022.106, 29.3 mm. A Lateral view, note large trigeminofacial foramen (marked by asterisk) in lateral wall of neurocranium; B dorsal view illustrating lack of epiphyseal bridge and large cranial fontanelle (margin marked by line of dots); C anterior view of upper and lower jaws, showing rows of sharply pointed recurved, villiform teeth. Distribution. Horaglanis populi is restricted to the lateritic aquifer systems in the Alappuzha and Pathanamthitta Districts of Kerala, southern India, where it has been collected from dug-out wells in the towns of Malapally, Edanadu, and Chengannur, and the nearby village of Thiruvanvandoor (Fig. 1E ).