A new species of giant clam, Tridacna costata, has been discovered in the Red Sea.

These giant two feet long clams might have helped feed prehistoric humans as they first migrated out of Africa.

Tridacna costata once accounted for more than 80 percent of giant clams in the Red Sea. Today, these mollusks, the first new living species of giant clam found in two decades, represent less than 1 percent of giant clams living there.

This novel clam, whose shell has a distinctive scalloped edge, was discovered while scientists were attempting to develop a breeding program for another giant clam species, Tridacna maxima, which is prized in the aquarium trade. The new species appears to live only in the shallowest waters, which makes it particularly vulnerable to overfishing.

“These are all strong indications that T. costata may be the earliest example of marine overexploitation,” said researcher Claudio Richter, a marine ecologist at the Alfred-Wegener-Institute for Polar and Marine Research in Bremerhaven, Germany.

No one had expected to discover a new giant clam species, “particularly in the Red Sea, one of the best investigated coral reef provinces,” Richter said. The fact that it was overlooked for so long “is a testimony as to how little we really know about marine biodiversity.”

Only six out of 1,000 live specimens the scientists observed belonged to the new species.

The scientists detailed their findings online on Aug. 28 in the journal Current Biology.

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