Eggs released into the water by mussels for fertilisation send out a chemical message that tells sperm they are a compatible mate, say researchers.
Evolutionary biologist Dr Jonathan Evans, from University of Western Australia, and colleagues, report their findings today in the Proceedings of the Royal Society B.
"The business of finding an egg is not as straight forward as you think it would be," says Evans.
Mussels, like other marine invertebrates that are not mobile, release eggs and sperm into the water and these gametes must then find each other.
"These things are fixed to a rock. Their sperm and eggs go into a turbulent marine environment and they need some kind of clever mechanism to find each other," says Evans.
It is known that many species release chemical clues to attract sperm to fertilise them - a process known as 'sperm chemotaxis'.
Scientists knew these signals were species-specific and helped sperm find an egg of the same species. But, says Evans, compatibility can be an issue even within species.
"For example, if you mate with a sibling it's not a good thing," he says.
Evans and colleagues were interested in whether chemical signals from eggs can also attract sperm from specific mates, within a species.
The researchers set up two different experiments to investigate the question using sperm and eggs from the species Mytilus galloprovincialis.
In one experiment, sperm from one particular male was forced to fertilise eggs from two different females. This was repeated for 60 different combinations of males and females.
From this the researchers found which sperm-egg combinations were compatible and which were not.
In another experiment the same sperm were given the choice of fertilising an egg they were compatible with or an egg they weren't.
"We found that they swim towards the chemo-attractants from the female from which they were most compatible," says Evans.
"In other words, sperm are differentially attracted towards the eggs from different females and that that actually matches the pattern of compatibility when you force them to mate."
Evans says the research demonstrates a previously unseen mechanism in which females send out a very specific message to males.
"It's offering a little bit more information than just 'Here I am and what species I am'. It's saying 'how compatible am I with you'."
What's really surprising, says Evans, is that similar sperm-attracting signals are sent out by eggs in species that fertilise internally, including humans.