By Rebecca Morelle
BBC News science reporter
A shrimp-like creature may have to forfeit its claim to be the longest abstainer from sex in the animal world.
The team found three males among hundreds of females
The discovery of three living male specimens casts doubt on the idea that the Darwinulidae family has been female and asexual for 200 million years.
Darwinulids produce eggs which do not need to be fertilised by sperm.
But a team of scientists, writing in a Royal Society journal, cannot say yet whether the newly found males actually perform a sexual function.
If they do not, if researchers can show the males are just some evolutionary hangover that is really no longer needed for reproduction - then the creatures will retain their famed celibacy status.
Not so dead end
Darwinulids are fresh water crustaceans with a hinged shell. They measure less than a millimetre in length.
Despite their diminutive size, however, they have an excellent fossil record.
And it is from studying this long history that scientists believe females have been producing young without the need for fertilisation from the Late Triassic onwards.
How these "ancient asexuals" persist is a mystery.
Evolutionary theory suggests they should accumulate so many damaging genetic mutations (errors) over the generations that they would die out within 0.5-1 million years.
On close inspection
The three male darwinulids just reported were found along with hundreds of female specimens on Yakushima, a small island in Japan.
The creatures represent a new species, Vestalenula cornelia.
"There are about 30 species known in the world belonging to this group," explained Dr David Horne, a palaeobiologist from Queen Mary, University of London, UK, and one of the authors of the paper that is published in the Proceedings of the Royal Society B.
The females store eggs in a brood pouch at the back of their bodies
"Thousands of specimens have been studied in the past, with no sign of a male; now, a bit like London buses, three have come along at the same time."
The male specimens were found to be shorter and smaller than the females; and did not have a brood pouch, which is where the females store their eggs and young.
That they are male can be seen in the differences in their copulatory organs; and the males also have hooks on their limbs, which would seem to be used to clasp the female during sex - as in other similar, but sexual, crustaceans.
Once in a while
Dr Horne - and colleagues Professor Takahiro Kamiya and Dr Robin Smith - say they cannot be certain yet whether the newly identified males play a role in sexual reproduction or not. It is possible the gender's presence has other explanations.
"These males could be functional males and this particular species is having sex; and they might be rare because they are only present for a very brief time in the year," Dr Horne told the BBC News website.
"Another possibility is that they are non-functional males - a throw back to times when darwinulids were sexual.
"Or it might be that they just appear very rarely - so it's almost an asexual species; but it is very, very occasionally having sex."
It is also possible that males are more prevalent but they are just not being recognised - juvenile females have a similar shell to the males.
"It gives us new information that we can use to go back to the fossil record and have a more careful look," said Dr Horne.
If it is shown darwinulids are having sex, then the record for the longest abstainers would pass to the rotifers, a group of microscopic aquatic creatures which are believed to have given up sexual reproduction about 40 million years ago.
The female (left) is larger than the male (right)