Editorial: 鈥�Why it鈥檚 worth collecting fish鈥�
Family histories don鈥檛 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.
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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 鈥� until you take a closer look at their family tree.
Greta Vega and at Stony Brook University in New York noticed something peculiar while studying the evolutionary tree of , 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 鈥� known as the polypteriformes 鈥� came from freshwater deposits. In fact, according to Vega and Wiens鈥檚 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鈥檝e 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.
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鈥檒l 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 鈥渁rc of survival鈥� that can reseed the oceans when marine species are lost.
鈥淚 don鈥檛 think our results show that seas are strongly inhospitable, but they may become so at certain points in time,鈥� 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. 鈥淔reshwater ecosystems suffer from a higher rate of species loss than any other major ecosystem,鈥� says Peter Bosshard, policy director at , a non-profit NGO based in Berkeley, California. 鈥淭his study shows that by damming, diverting and polluting the world鈥檚 rivers, we may deplete the seed bank of future generations.鈥�
Journal reference: , DOI: 10.1098/rspb.2012.0075
