Life - Most fish in the sea evolved on land

 

Family histories don't come much more bizarre. Three-quarters of the fish in the sea can trace their origins back to a freshwater ancestor. The finding highlights how important rivers and lakes are as a source of new species, just as that supply is under threat from disappearing freshwater habitats.

Fish first evolved in the sea. The oceans have been teeming with them for almost half a billion years, so there is no reason to doubt that the fish living there today did all their evolving in salt water - until you take a closer look at their family tree.

Greta Vega and John Wiens at Stony Brook University in New York noticed something peculiar while studying the evolutionary tree of ray-finned fish, a mega-group comprising 96 per cent of all freshwater and marine fish species on the planet.

They realised that all the fossils belonging to the ancestral group that gave rise to ray-fins some 300 million years ago – known as the polypteriformes – came from freshwater deposits. In fact, according to Vega and Wiens's tree, the ray-fins may not have taken to the sea in large numbers until about 170 million years ago. Their descendants now make up three-quarters of all marine fish (see diagram).

We've seen this kind of topsy-turvy evolution before. Most whales, dolphins and porpoises, live in the sea, but like the ray-finned fish, they all evolved in rivers.

Michael Benton of the University of Bristol, UK, says that combined with what we know about whales and dolphins, the new study may point to a more general pattern: that most major groups of vertebrates came from land-based ecosystems. But we'll need many more studies to confirm that, he says.

What could be driving such a pattern? Wiens says it is possible that seas may be more prone to extinctions than land, rivers or lakes; while rivers and lakes form an "arc of survival" that can reseed the oceans when marine species are lost.

"I don't think our results show that seas are strongly inhospitable, but they may become so at certain points in time," he says. Unfortunately, the strong ocean acidification that is predicted for the near future means we may be heading for one of those times now, he adds.

Today, however, rivers and lakes may not be healthy enough to help re-supply the oceans. "Freshwater ecosystems suffer from a higher rate of species loss than any other major ecosystem," says Peter Bosshard, policy director at International Rivers, a non-profit NGO based in Berkeley, California. "This study shows that by damming, diverting and polluting the world's rivers, we may deplete the seed bank of future generations."

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2012.0075

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