Study: Ocean fish may have freshwater ancestor

A mystery looming over the biology world is the question of why oceans cover 70 percent of Earth, but only contain 15 to 20 percent of all species.

A researcher at Stony Brook University attempted to answer that question by studying the genetic history of and links between ray-finned fish, which are fish that have fins made largely of skin supported by boney or horny spines (i.e. rays) and which make up 96 percent of all fish species.

The result of the study by John J. Wiens and a team of researchers is that there may be far less fish in the sea than originally thought, reports Newswise.

"There are more fish species in freshwater than in saltwater habitats, despite the much greater area and volume of the oceans," he said, according to Newswise, noting that freshwater environments occupy only about 2 percent of the Earth's surface. "More remarkably, our results suggest that most marine fish alive today are descended from freshwater ancestors (even though fish and animals in general first evolved in the oceans)."

The researchers came to their conclusion after creating evolutionary trees based on molecular data and fossils, and using a large database on the habitats of nearly all living fish species, Newswise reports.

Of all living ray-finned fish, the researchers found 15,149 species live in freshwater and 14,736 live in saltwater, reports Live Science. The research also revealed that fish diversity only exploded in the last 100 million years or so.

Their findings led the team to speculate that a mass extinction of ancient ocean ray-finned fish may have caused their freshwater cousins to eventually evolve back into adapting to a saltwater environment.

While the lake-to-ocean evolution may seem a controversial conclusion, similar scientific conclusions have been reached before. Most evolutionary biologists agree, for example, that whales evolved from a land-dwelling ancestor.