Friday, January 27, 2023

[Ichthyology • 2023] Horaglanis populiEvolution in the Dark: Unexpected Genetic Diversity and Morphological Stasis in the Blind, Aquifer-dwelling Catfish Horaglanis (Siluriformes: Clariidae)

Horaglanis populi
Raghavan, Sundar, Arjun, Britz & Dahanukar, 2023

The lateritic aquifers of the southern Indian state of Kerala harbour a unique assemblage of enigmatic stygobitic fishes which are encountered very rarely, only when they surface during the digging and cleaning of homestead wells. Here, we focus on one of the most unusual members of this group, the catfish Horaglanis, a genus of rarely-collected, tiny, blind, pigment less, and strictly aquifer-residing species. A six-year exploratory and citizen-science backed survey supported by molecular phylogenetic analysis reveals novel insights into the diversity, distribution and population structure of Horaglanis. The genus is characterized by high levels of intraspecific and interspecific genetic divergence, with phylogenetically distinct species recovered above a 7.0% genetic-distance threshold in the mitochondrial cytochrome oxidase subunit 1 gene. Contrasting with this deep genetic divergence, however, is a remarkable stasis in external morphology. We identify and describe a new cryptic species, Horaglanis populi, a lineage that is the sister group of all currently known species. All four species are represented by multiple haplotypes. Mismatch distribution reveals that populations have not experienced recent expansions.

KeywordsCryptic species, groundwater, Kerala, molecular ecology, stygobitic, subterranean

Horaglanis populi in life. B Typical laterite rock showing tiny pores. C Homestead lateritic dug-out well in Kerala – habitat of Horaglanis.

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.

Horaglanis populi, sp. nov. 

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).

EtymologyThe 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.

Rajeev Raghavan, Remya L. Sundar, C.P. Arjun, Ralf Britz and Neelesh Dahanukar. 2023. Evolution in the Dark: Unexpected Genetic Diversity and Morphological Stasis in the Blind, Aquifer-dwelling Catfish Horaglanis. Vertebrate Zoology. 73: 57-74. DOI: 10.3897/vz.73.e98367