How the Eastern and Western Ghats are different and similar
India is a triangle of land that tapers southward into the Indian Ocean. Two mountain ranges run along its edges, the Western Ghats along the west coast facing the Arabian Sea, and the Eastern Ghats along the east coast facing the Bay of Bengal. Both ranges sit on the the same basement of the ancient supercontinent Gondwana, compressed and reshaped by tectonic events over billions of years.
How Old They Are and How They Were Shaped
The biggest difference between the two ranges is age, and that helps explain many of the differences between them.
Image : Eastern Ghats shaped by billions of years of erosion
The Eastern Ghats began forming around 3 billion years ago. To put that in perspective, the most complex life on Earth at that time was still single-celled organisms. The Eastern Ghats have had billions of years of rain, wind, and river erosion working on them continuously, carving the range into disconnected clusters of hills.
The Western Ghats formed between 150 and 200 million years ago, a time when dinosaurs still walked the Earth. The Deccan Traps volcanic eruptions, one of the largest volcanic events in Earth's history, caused the western edge of the Indian plate to bulge upward and the entire plate to tilt eastward to form the Western Ghats. The only break in the Western Ghats is the Palakkad Gap in Kerala, a crack left behind when India and Madagascar split 90 million years ago that the Deccan Traps eruptions could not lift up.
How Water Divides and Connects Their Worlds
Understanding the fish of both ranges starts with understanding how differently each range experiences monsoon. From June onward, the southwest monsoon hits the Western Ghats and drenches them, with some areas receiving more than 6,000 mm of rain in a single season. By the time the monsoon crosses the Western Ghats and reaches the Eastern Ghats, most of its moisture is spent, but then the Northeast monsoon arrives from the Bay of Bengal in October and the Eastern Ghats receive between 1,000 and 2,000 mm annually.
Image : Monsoon clouds over Western Ghats
That rainfall difference shapes everything downstream. Western Ghats streams run year-round, fast, cold, and highly oxygenated, rushing over large granite and basalt boulders and dropping steeply toward the coast. Eastern Ghats streams run strong during monsoon months and slow to a trickle or dry out entirely by summer. They are mostly warmer, shallower, and gentler, with sandy and gravelly beds rather than boulder fields.
Image : Underwater shots of an Eastern Ghats hillstream (left) and a Western Ghats hillstream (right)
What connects both ranges is the rivers. The Kaveri, the Godavari, and the Krishna originate in the Western Ghats, travel eastward across the peninsula, and pass through the Eastern Ghats before emptying into the Bay of Bengal. These rivers are the physical link between both ranges.
Looking at the Fishes
Freshwater fish cannot cross land. Their entire world is the river or stream they were born into, which makes geography the single most important force shaping what a freshwater fish looks like, where it lives, and whether it exists anywhere else on the planet.
Image : Bhavania australis in a Western Ghats hillstream. It is endemic to the Western Ghats.
The Western Ghats, with their steep terrain, created dozens of separate river basins, each one sealed from the next by a ridge. Fish in west flowing rivers that drain toward the Arabian Sea are separated from fish in east flowing rivers that drain toward the Bay of Bengal. Any fish population on one side of that ridge has no path to the basin on the other side without crossing a mountain, and over millions of years those isolated populations diverged into entirely new species. The result is that roughly 70% of all freshwater fish species in the Western Ghats exist nowhere else on Earth.
Image : Glossogobius giuris (Tank Goby) in an Eastern Ghats hillstream.
The fish community of the Eastern Ghats is more connected and generalist. The streams here are seasonal, the terrain is fragmented, and the rivers that cut through these hills also connect them to other parts of the peninsula. As a result, many fish species are more widely distributed, and fewer are exclusive to the Eastern Ghats when compared to the Western Ghats.
Image : Rasbora dandia in a Western Ghats hillstream. It is also found in Eastern Ghats.
Then there are some fish species that are found in both the Western Ghats and the Eastern Ghats. One possibility is that these fishes belong to ancient lineages that had already spread across the peninsula before the Western Ghats fully formed, leaving their descendants in both ranges over time. Another possibility is that river systems flowing from the Western Ghats toward the Eastern Ghats acted as natural freshwater pathways, allowing more habitat generalist species to disperse between the two. The exact story of how they got there is still being understood.
How this Is Featured in our Tshirts
We have tried to represent this natural history in our Eastern Ghats Ichthyology and Western Ghats Ichthyology tshirts. Each fish on them is illustrated in field guide style to bring out the naturalist side in the wearer. Apart from being arranged in the order of their size, they are also arranged in the order of where they actually live in the water.
The Eastern Ghats tshirt has Aplocheilus lineatus, the Striped panchax, a surface dwelling killifish that hunts at the surface. Devario malabaricus, Rasbora dandia, and Dawkinsia filamentosa, mid water fishes that move through open water in loose groups. And at the bottom, Channa kelaartii, a snakehead that stays low, tucked into sheltered spots near the bed, waiting in ambush. The arrangement goes from smallest at the top to largest at the bottom.
The Western Ghats tshirt has Opsarius ardens, a surface hunting fish that feeds on insects at the waterline, found only in the Sita and Swarna river systems of Karnataka. Dawkinsia apsara, a mid water barb restricted to the Sowparnika and Sita rivers of Karnataka. Etroplus canarensis, the Canara pearlspot, a cichlid found only in the Kumaradhara and Nethravati rivers of coastal Karnataka. Waikhomia sahyadriensis, the Maharaja barb, that feeds along the substrate in shaded hill streams of the Krishna, Sharavati, and Tunga river systems in the northern Western Ghats. And at the very bottom, Mesonoemacheilus tambraparniensis, a zodiac loach that lives on the streambed, endemic to the Tambraparni river basin in Tamil Nadu. The arrangement here goes from largest at the top to smallest at the bottom.
Image : Looking for fishes in an Eastern Ghats hillstream
What is interesting is that you could visit a single stream anywhere in the Eastern Ghats and have a reasonable chance of finding all five fishes on that tshirt in the same water. Each of the five fishes on the Western Ghats tshirt, however, is endemic to its own specific part of the range. To find all of them, you would need to travel across the length of the Western Ghats, visiting different rivers in different states. And that is the difference between the fishes of these two ranges!
All images captured by Beta Mahatvaraj (Meenkaran)