There’s a rare catfish in your well

A rare species of catfish, has been found in aquifers in Kerala, opening a window to the rich biodiversity that exists even in deep and dark underground subterranean environments where unique organisms survive by adapting to the harsh surroundings informs this paper titled 'Evolution in the dark: Unexpected genetic diversity and morphological stasis in the blind, aquifer-dwelling catfish Horaglanis' published in Vertebrate Zoology.

This was a unique effort made by scientists to involve citizens in identifying these rare species as a part of the ‘citizen science’ campaign that helped raise awareness on the subterranean fauna of southern India, and led to increased reports on the occurrence of these rare and interesting fish species. This also led to more specimens becoming available for research.

What lies beneath

While a lot is known about the biodiversity and the range of interesting organisms –vertebrates as well as invertebrates found in freshwater ecosystems such as streams, lakes, rivers, ponds, what kind of life exists in the water beneath the surface of the earth still remains a mystery.

As much as 97 percent of the earth’s freshwater resources other than the polar ice caps are stored as groundwater. Groundwater is stored in aquifers - bodies of rocks that are capable of storing large quantities of water in pores and open spaces that are large enough and interconnected to transmit water. And aquifers can go very deep, even as deep as 30,000 feet.

Groundwater from the aquifers can be accessed by pumping or drawing out water through dugwells, borewells, tubewells and used for drinking and irrigation. Occasionally, this water can bring along with it wonders of the natural world that include rare varieties of animals living inside the deep hidden waters in caves or subterranean environments, unknown to the world outside. 

Subterranean aquatic habitats often harbour unique type of fish such as bony fish, with 289 species being currently known to dwell in subterranean aquatic habitats in every continent except Antarctica. These include cave-adapted species or lineages of ‘living fossils’ . These unusual fish species have been aptly called the ‘wrecks of ancient life’ and ‘ghosts in the water’.  While subterranean habitats are known to harbour extraordinary varieties of such animals, many of these remain under studied and unknown because of their inaccessibility.

Chance encounters with these subterranean animals often happen through dug wells and local communities are often the first or sometimes the only people to encounter these species and can thus  play a significant role in improving scientific knowledge on these rare and mysterious deep dwelling animals in environments that are dark, have reduced concentrations of nutrients, carbon and dissolved oxygen, and free space.

The lateritic landscapes of Kerala

The lateritic landscape in the state of Kerala in India is a global hotspot for subterranean fish and harbours  a rare and rich variety of subterranean fish with it presently including 10 endemic species in five genera (Aenigmachanna, Horaglanis, Kryptoglanis, Pangio and Rakthamichthys) and two monotypic families (Aenigmachannidae and Kryptoglanidae). Some of these fish exhibit unusual morphological characters such as absence of eyes and body pigments, absence of dorsal or pelvic-fins or even both fins.

Horaglanis, is a type of catfish that is blind, pigmentless and of blood-red coloration, is of a tiny size (< 35mm) and occurs in unique habitats (lateritic aquifers), but is rarely found appearing only occasionally in dug-out wells. Though three species of this catfish are currently known,  two continue to be poorly described and there is no information on the differences between the three species. 

The study

The paper discusses the findings of a six-year exploratory and citizen science-backed survey across the lateritic landscape of Kerala that resulted in extensive information on Horaglanis. Samples were obtained from dug-out wells, borewells, natural wetlands adjacent to lateritic zones, home-gardens and plantations, as well as lateritic caves.

Community involvement in the study was encouraged through a series of workshops, focus-group discussions and informal interactions. Local villagers were informed of the importance of the species and their conservation needs, and they were asked to share information, photographs or videos if the species were encountered and/or collected.

This approach was complemented by targeted collection efforts by researchers through draining of wells and overhead storage tanks, the use of scoop nets in shallow wetlands and in water channels in home gardens and plantations, as well as the use of baited traps in dug-out wells in homesteads, ponds and caves.

The study findings

• Horaglanis (a rare type of catfish)  is endemic to the south of the Palghat Gap in Kerala. While there is limited information on the number, size and extent of the aquifers populated by Horaglanis, the study found that the northern extent of its range was limited by the Bharathapuzha (the second largest river basin in the region), as well as a wide zone of rock formations in which laterite rock was absent - coinciding with the Palghat Gap.
• The major barrier separating species from each other was the Periyar-Chalakudy River basin (the largest river basin in the region), while some populations of fish that differed genetically from those close to the type locality of this species, occurred on the south of Periyar.
• All four species of Horaglanis were very similar in external appearance and morphology and demonstrated a  case of extreme morphological stasis (lack of evolutionary change over a long period of time) due to a number of factors such as lack of competition, the high energy costs of life in harsh environments that prevent any evolutionary change, low metabolic rates leading to low fertility to survive in stressful and low-energy environments that restrict further morphological changes, and to life in stable underground environments over a long period of time. 
• The small pore size of the lateritic rocks in the aquifers restricted entry of predators in the surroundings resulting in a predator-free environment for Horaglanis, thus severely limiting interspecific competition. The fish was also found to have a low number of (just 25 to 30) comparatively large eggs in response to  nutrient poor habitat. The lateritic aquifers seemed to have provided a stable and ecologically homogenous environment for Horaglanis over a long period of time making the external morphology of all species similar while having significant differences at the genetic level.
• While Horaglanis has received less attention due to its rare appearance , the ongoing ‘citizen science’ campaign has helped raise awareness of the subterranean fauna of southern India, and this has in turn increased the number of occurrence reports of these interesting fish.

Horaglanis have no legislative protection till now because of lack of adequate information although their habitats continue to be invaded more and more with human populations and human activities such as high extraction of groundwater for both household and agricultural purposes and mining  of laterite soil. . Also the fish are found in many areas near the coast which increases the risk of seawater intrusion into the aquifer systems thus killing the fish.

The paper argues that it is important to conserve these rare and beautiful catfish and ensuring the security of these fish in the lateritic aquifers of Kerala will require landscape-level planning and implementation involving a variety of stakeholders including local communities who can play an important part in building knowledge and protecting these enigmatic catfish.

This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0),

Image Source: By Rajeev Raghavan, Remya L. Sundar, C.P. Arjun, Ralf Britz, Neelesh Dahanukar - Evolution in the dark: Unexpected genetic diversity and morphological stasis in the blind, aquifer-dwelling catfish Horaglanis, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=128832101.