Two branches of life grabbed the Devonian opportunity to colonize a new environment: the land. The fastest evolving were the tracheophytes, a group of plants which developed internal plumbing to transport water and nutrients, and rapidly spread across the continents, growing into new ecosystems of massive forests which captured atmospheric carbon and buried them deep underground. Once a land ecosystem developed, the second group, our remote ancestors crawled out into it to eventually find the buried carbon and release it back into the air. But this is not the story that fascinates me today. It is the story of the remainers, our cousins in nerve and bone, the fish, that I find gripping. Over the eons that passed, they evolved more exquisitely into new opportunities in the main element of our planet: the water.
I have seen mudskippers now and then, mostly in the muddy mangrove forests of the east coast, a littoral of the world’s center of biodiversity, namely the Indo-Australian seas. A year ago, when I visited Jamnagar I realized that I was in an unique place. The special shape of the Gulf of Cambay causes enormous tides, and creates an unique ecosystem: a huge expanse of tidal mud flats, and a second hot spot for mudskipper diversity. I watched one flopping in the exposed mud at low tide. Its fins were placed far ahead on its body, very close to the head. As I watched it walked on its fins from one shallow pool of water to another. Which species was it? I am not an expert on identification. But from its shape and the stripes on its body, my guess is that it was the Boleophthalmus dussumieri, common across east and west coasts of India all the way to the Arabian gulf.
The bulge behind its head came from a bubble of water it holds to keep its gills moist, as it breathes through its skin and the mucous membrane that lines its mouth. The one I concentrated on had its mouth closed, but the others kept opening their mouths to breathe. The males create burrows in the mud where females lays eggs. In high tide the burrows fill with bubbles of trapped air where they retreat to hide from aquatic predators. One mystery which has received a lot of scientific attention is how they survive in such low oxygen content. The other mystery dawned on me slowly as my companions left me to go watch the numerous birds which stalked the flats. Why do they not feed on these exposed mudskipper? They are eaten by the fishermen of Gujarat. Are they toxic to birds?
Why do I find the story of mudskippers so gripping? Its because I was wrong about them; they are not remnants of the ancient group of fish which flopped on to land and gave rise to tetrapods. They are part of most recently evolved group of fishes: the percomorpha (coloured in the evolutionary tree shown above), and are more recent arrivals on earth than the mammals. Mudskippers are close cousins to seahorses and mackerel and eels, and the many other percomorphs in modern seas. Mudskippers (Gobioidei) began to evolve along the western coasts of the giant ocean called the Tethys, just as the first mangroves were beginning to colonize tidal mud flats, and Gondwanaland was breaking up into Africa, Australia and India. That is a remarkably elegant and parsimonious explanation of the present global distribution of mudskippers. What a wonderful solution to that mystery.