The common skate (Dipturus batis), a type of ray, isn't common at all. The rare fish species is already critically endangered, but now new research indicates that the common skate is actually two species, so both are more at risk than previously thought.

According to a paper to be published this week in the journal Aquatic Conservation: Marine and Freshwater Ecosystems, the common skate is really two similar-looking species, the flapper skate (D. intermedia) and the blue skate (D. flossada). This long-standing misclassification has led to miscounting of skates caught by commercial fishermen, all of which have been counted as common skates, says lead researcher Samuel Iglésias of France's National Museum of Natural History. This has allowed the flapper skate to be overfished, and placed it in danger of extinction.

Classifying the common skate as two species isn't a new idea. Flapper and blue skates were previously considered two species until 1926, when researcher R. S. Clark recognized only D. batis as a valid species.

Iglésias spent two years examining skates to reassess Clark's classification, including looking at systematic molecular data, along with the species' life history and fishery statistics. "Our research clearly shows that D. flossada and D. intermedia are distinct and should be resurrected as two valid species," he said in a prepared statement.

Despite its listing as a critically endangered species by the IUCN Red List of Threatened Species, the common skate (or whatever it should be called) is still heavily fished in European waters, and stocks have been depleted in the Celtic Sea, North Sea, English Channel and other bodies.

Iglésias hopes his research will lead to reclassifying the skates, and to added protections for both species. "Without revision and recognition of its distinct status," he said, "the world's largest skate, D. intermedia, could soon be rendered extinct."

Image: Barndoor skate (Dipturus laevis), a close relative of the common skate. Via Wikipedia.