A U.S. researcher says he\'s found that contrary to popular belief, there aren\'t plenty of fish in the sea, and he\'s measured surprisingly low fish biodiversity. Stony Brook University researcher John J. Wiens said he\'s been researching why the oceans contain only 15-25 percent of all of Earth\'s species even though they cover about 70 percent of the planet\'s surface. Wiens and student Greta Carrete Vega examined the biodiversity of ray-finned fish, the most species-rich group of marine vertebrates with 96 percent of all fish species, a Stony Brook release said Tuesday. The study found a surprising difference in diversity between freshwater and saltwater habitats, they said. \"There are more fish species in freshwater than in saltwater habitats, despite the much greater area and volume of the oceans,\" Wiens said, 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].\" Extinctions in marine habitats hundreds of millions of years ago may help explain the low present-day diversity of marine fish, he said. \"Our results suggest that ancient extinctions in the marine environment may have wiped out the earliest ray-finned fishes living in the oceans, that the oceans were then recolonized from freshwater habitats, and that most marine fish species living today are descended from that recolonization [leaving less time for biodiversity to build up in the oceans],\" Wiens said.