Nearly all marine fish came from freshwater ancestors

[muriel_volestrangler]

I’m sure that most of us think that marine fish evolved in the sea, but a new paper by Greta Vega and John Wiens in the Proceedings of the Royal Society (B) says that that just ain’t so. The vast majority of them evolved from ancestors who lived in fresh water (themselves derived from marine ancestors) and then re-invaded the sea—just as marine mammals evolved from terrestrial mammals whose distant ancestors were aquatic.

As Vega and Wiens point out, compared to the land, the sea is biologically depauperate: marine habitat covers 70% of the Earth’s surface but contains only 15-25% of Earth’s species. It gets worse if you count “habitable space”: since the ocean is three-dimensional, Vega and Wiens claim that it contains “90-99% of the volume of the habitable biosphere.”
...
My own guess, which is also that of Vega and Wiens, is that geographic barriers, which are the first step in most speciation events, are simply less common, or arise less frequently, in open water than in terrestrial habitats (riverine fish, of course, are geographically isolated in river systems). It’s telling that the greatest diversity in the ocean is found in the Indo-Pacific, which Vega and Wiens describe as “geographically complex,” limiting dispersal and facilitating the formation of new species.

Vega and Wiens try to resolve this issue by doing a DNA-based phylogeny of the actinopterygians (the ray-finned fish, so called because of the bony struts in their fins). Ray-finned fish are by far the most numerous of all “fish”—about 96% of them. What they found was surprising: here’s the phylogeny of different actinopterygian groups and the number of species in each group , with the colors indicating where they live (red is freshwater, blue marine, and mixed colors indicating that members of the group occupy both habitats. Notice that the three “basal groups”, Polypteriformes, Chondrostei, and Amiiformes, are all exclusively freshwater
...

http://whyevolutionistrue.wordpress.com/2012/02/16/nearly-all-marine-fish-came-from-freshwater-ancestors/

[xchrom]

1. interesting. nt

[hunter]

2. Sharks and cephalopods, maybe?

And untold parasites...

The oceans were a pretty tough neighborhood.

[Odin2005]

3. Lobe-finned fish, too.

Coelecanths were common in the oceans during the Mesozoic.