Why There Are So Few Fish in the Sea

Most saltwater fish may have evolved from a freshwater ancestor, according to a new study that traces the family tree of both marine and freshwater fish.

The finding may seem topsy-turvy given that life originated in the oceans, but the results could help explain why the sea is relatively low in diversity compared with the land. The ocean takes up 70 percent of the Earth's surface, but contains only 15 percent to 25 percent of the Earth's total estimated species.

"It's a striking pattern that we haven't really explained yet," said study researcher John Wiens, a professor of ecology and evolution at Stony Brook University in New York.

The new results may provide a clue, Wiens told LiveScience. Freshwater fish could have diversified from saltwater ancestors, only to see those ancestors wiped out in ocean extinctions. Such extinctions would free up space for some freshwater fish to evolve, once again, to thrive in the ocean.

Diversity mystery

To investigate the relative dearth of fish in the sea, Wiens and his colleagues turned to the largest group of marine vertebrates on Earth, the ray-finned fishes. This group contains 96 percent of fish species, meaning they're "pretty much every fish you've ever eaten or kept in a fish tank or seen in a coral reef, with the exception of sharks and rays," Wiens told LiveScience. [Gallery: Glowing Aquatic Life]

Studying ray-finned fish was a "good place to start," Wiens said, not only because the group is so large but because it allowed for direct comparison between land and sea. Fish aren't land animals, of course, but they do live in a distinct non-marine environment: freshwater.

"Looking at a group in which all these species are aquatic … helps us to isolate what's special specifically about the ocean," Wiens said.

He and his co-authors pulled information on all living fish species from a comprehensive database called FishBase. Next, they combined that information with a family tree of ray-finned fish that shows relationships between groups and clades (groupings of organisms consisting of an individual species and all its descendents). The researchers also put together a tree for known fossil fishes.
Source:Yahoo