Contrary to the popular saying, there are not plenty of fish in the sea. But why? Perhaps because a huge number of species became extinct in ancient times, say Greta Carrete Vega and John Wiens at Stony Brook University in New York.

Marine environments cover about 70% of Earth's surface but contain only 15–25% of all estimated species. To find out why, Vega and Wiens studied actinopterygian (ray-finned) fish — which encompass 96% of Earth's fish species — in marine and freshwater environments.

They found that both environments were similarly rich in actinopterygian species, even through the marine environment is much larger and has greater primary productivity. They also discovered that all extant marine actinopterygians descend from a freshwater ancestor, suggesting that ancient extinctions have robbed the seas of their species.

Proc. R. Soc. B http://dx.doi.org/10.1098/rspb.2012.0075 (2012)