Antarctic bivalves have surprised scientists who have discovered that the animals switch sex.
The reproduction of Lissarca miliaris was studied in the 1970s and the species was first described in 1845.
But their hermaphrodite nature had remained unknown until they were studied by scientists from the National Oceanography Centre, Southampton.
Researchers suggest the molluscs could switch between the sexes to efficiently reproduce in the extremely cold ocean.
The results are published in the journal Polar Biology.
"The previous reproductive study only looked at the large eggs and broods," said PhD student and lead author Adam Reed.
This earlier work showed how females brood their young for up to 18 months, from "large yolky eggs" to "fully shelled young", and found that females can support as many as 70 young inside their hinged shell.
But concentrating on reproduction at a cellular level, Mr Reed and colleagues discovered that the eggs were actually present in males.
"Curiously, we found huge numbers of very small eggs in functional males, which appear to be far higher in number than an individual could brood throughout the life of the animal," he told BBC Nature.
The team suggested that the bivalves reproduce as males while they are still in the "small" stages of development, switching to female organs once they are large enough to brood a significant number of eggs.
"At present the traits we describe are unusual for Antarctic bivalves, but in 10 years perhaps this will be common too," said Mr Reed.
"Hermaphroditism is not necessarily uncommon in Antarctic bivalves, and with many species still to study there may be many more to describe."
