By Elizabeth Mitchell
Science reporter, BBC News
The new giant clam species has a deeply folded shell outline
A new species of giant clam has been discovered in the Red Sea.
Fossils suggest that, about 125,000 years ago, the species Tridacna costata accounted for more than 80% of the area's giant clams.
The species may now be critically endangered, researchers report in Current Biology journal.
The scientists believe their findings may represent one of the earliest examples of the over-exploitation of marine organisms by humans.
T. costata has "very peculiar characteristics" that set it apart from two other species of giant clam that are also found in the area.
The Latin word costatus means "ribbed" and T. costata has a disitinctive, zig-zag outline to its shell.
"The new species are mid-sized clams - up to 40cm long and a couple of kilograms heavy," explained co-author Dr Claudio Richter, from the Alfred-Wegener Institute for Polar and Marine Research, Germany.
The new species has a distant relative, T. gigas, which can grow up to 1.4m long.
Live specimens of T. costata appear to be restricted to very shallow waters. Other species were also found in deeper reef zones.
The clam has an earlier and shorter breeding season that coincides with the seasonal plankton bloom. Genetic analysis confirmed the status of the new species.
"One of the great features of the desert-enclosed Red Sea is that you can literally time-travel from the present, several hundred thousand years into the past," said Dr Richter.
The research team uncovered well-preserved fossil evidence that suggested stocks of these giant clams plummeted some 125,000 years ago - during an interval between Ice Ages.
They believe this period coincides with the appearance of modern humans in the Red Sea area.
Giant clams were abundant, large in size and easily accessible - making them an attractive food source for hunter-gatherers.
In "pre-human times", T. costata may have been up to 60cm long. Since then, shell size has also decreased dramatically.
"The overall decline in giant clam stocks - with the striking loss of large specimens - is a smoking gun indicating over-harvesting," said Dr Richter.
The scientists were not expecting to find a new species in an area as well studied as the Red Sea.
The research highlights how little is known about marine biodiversity in general, the scientists said.
"The coral reefs in particular... may still harbour very large surprises," said Dr Richter.