Phreatic mysteries: Diversity and distribution of fossorial and aquifer-dwelling synbranchid eels of southern peninsular India, and implications for conservation (Pisces: Synbranchiformes: Synbranchidae)

Abstract

Subterranean biodiversity continues to be poorly known as a result of uncertainties, challenges and hazards associated with sampling in microhabitats such as aquifers and caves. Focusing on the narrow, lateritic aquifers and associated groundwater habitats in the Western Ghats freshwater ecoregion (southern peninsular India), we investigate the genetic diversity of an enigmatic group of eel-like fishes (family Synbranchidae). A maximum-likelihood phylogenetic analysis based on mitochondrial cytochrome oxidase subunit 1 (cox1) gene sequences recovered these synbranchid eels into two distinct clades comprising genera Ophichthys (fossorial eels ‘with eyes’) and Rakthamichthys (aquifer-dwelling ‘blind’ eels). Additionally, three species-delimitation approaches (based on the mitochondrial cox1 gene), revealed the presence of 11 Evolutionarily Distinct Lineages (EDLs) within Rakthamichthys separated by an inter-lineage divergence between 5.8–20.3%, and an intra-lineage divergence between 0–4.5%. Rakthamichthys in southern peninsular India exhibited a distribution pattern comprising both restricted-range and wide-ranging lineages. Fossorial eels of the genus Ophichthys, on the other hand, are widely distributed in southern peninsular India, with clear geographical boundaries separating the two known species. The genetic network of Rakthamichthys and Ophichthys revealed multiple haplotypes within various EDLs, with a large number of mutations separating the haplotypes within, and between species and/or lineages. Though represented by high levels of genetic divergence revealing the potential existence of at least 11 EDLs, their remarkable morphological uniformity combined with a complex distribution pattern makes it difficult to assign known species names to various Rakthamichthys lineages. Most subterranean habitats in southern peninsular India are under severe anthropogenic threats. Therefore, resolving the taxonomy of, and developing conservation actions for groundwater-dependent species is a priority, for which we suggest future steps.