Miocene diversification of mountain loaches

How ancient river systems, geological barriers and climatic changes in the Western Ghats influenced speciation of balitorid loaches

Of the many endemic and evolutionarily distinct freshwater fish lineages of the Western Ghats Biodiversity Hotspot, the mountain loach genus Bhavania comprises a biogeographically fascinating group. Its morphological similarities to the sucker-loaches of Indo-China and Sunda Islands had fuelled speculations that this group originated from South East Asia, and colonized the Western Ghats during the Pleistocene. Despite being known since the 1840s, there were no efforts until this date, to understand the how this group of fishes evolved and diversified in the region.

(Above) The mountain loach, Bhavania australis is a ‘cryptic species complex’ endemic to the Western Ghats Biodiversity Hotspot in India. Photo credit: Beta Mahatvaraj. Mexico.

Most importantly, the apparent wide distribution of the genus in the Western Ghats from 9° to 13°N latitudes (an approximate south-north distance of 450 km), and the fact that only two species were known, encouraged us to investigate the true diversity within this genus. After our team carried out extensive sampling in torrential mountain tributaries of 16 river systems of the Western Ghats, collecting specimens which were then subjected to molecular phylogenetic and biogeographical analysis, the results did not surprise us.

Sidharthan A, Raghavan R, Anoop VK, Philip S & Dahanukar N (2020) Riddle on the riffle: Miocene diversification and biogeography of endemic mountain loaches in the Western Ghats Biodiversity Hotspot. Journal of Biogeography 47, 2741-2754
. https://doi.org/10.1111/jbi.13972

The genus Bhavania was shown to harbour many more species than previously known. Molecular genetic analysis revealed that the currently known Bhavania australis represented a ‘species complex’ comprising six or seven putative species, with restricted distribution ranges. This has now helped us to better understand their extinction risk, and inform future conservation action.  Unfortunately, naming them has become a challenge, as they are cryptic and hard to tease apart morphologically!

Collecting mountain loach.

We also found that these mountain loaches originated in the early Neogene, and subsequently diversified in the Miocene, and later in the Pliocene and Pleistocene. This laid rest to the speculations of early Indian ichthyologists, and biogeographers that these fishes originated in South East Asia and arrived into India in the Pleistocene. The current distribution pattern of these mountain loach species on the different hill-ranges and its associated drainages in the Western Ghats reflect the climatic (intensification of monsoonal rains) and physiographic (aridification, formation of land barriers, drying up of riverine connections, and fragmentation of streams) changes that could have occurred on this mountain during the Miocene. Together these would have resulted in dispersal of these loaches to new areas, and isolation in others, leading to the current pattern of diversity and distribution. A highlight of our study was what could be the first evidence for a large river system (the Cauvery) facilitating an east-west pathways for dispersal and diversification of endemic fish species of the region.

As a next step, we are embarking on an ambitious project to investigate the global phylogeny and biogeography of balitorid loaches so as to help improve our understanding of the evolution and diversification in this fascinating family of mountain loaches.

Written by:
Arya Sidharthan (1) and Rajeev Raghavan (2)
(1) PhD Candidate, School of Ocean Science and Technology, Kerala University of Fisheries and Ocean Studies, India; Assistant Professor, Department of Fisheries Resource Management, Kerala University of Fisheries and Ocean Studies, India.

Additional information:
@LabRajeev @AryaSidharthannswick

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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