Most fish in the sea evolved in rivers

Most fish in the sea evolved on land
New Scientist, 8 February 2012, by Colin Barras
www.newscientist.com/article/dn21441-most-fish-in-the-sea-evolved-on-land.html

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

(For the full research paper, see
www.stonybrook.edu/sb/docs/diversity.pdf. For an International Rivers
blog post on the topic, see www.internationalrivers.org/en/node/7160.)
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