http://www.cosmosmagazine.com/node/1702Male fish fake it for oral sexThursday, 15 November 2007
by John Pickrell
Cosmos Online
SYDNEY: A bizarre mating behaviour in some 'maternal mouthbrooding' cichlid fish involves the female collecting eggs in her mouth and then allowing the male to fertilise them by depositing sperm there.
Now researchers at Konstanz University in Germany have found a gene linked to the behaviour, which allows males to produce oval-shaped spots of bright yellow or orange pigment cells on their anal fins.
These spots – known as egg dummies – are a crucial part of the mating process of more than 1,000 species of haplochromine cichlids, found in East Africa's great lakes.
Maternal mouthbrooding
Firstly, females with ripe eggs approach the territory of courting males and lay their eggs on the lake bed, they then turn around and pick up the small batch in their mouths.
Then, attracted by the egg-spots on the males' anal fin, "the female attempts to ingest these dummies and brings her mouth into close proximity to the males' genital opening – this is when the male discharges sperm," write the experts today in the journal BMC Biology. "In this way, the fertilisation of the eggs takes place within the females' mouth."
After fertilisation, the female broods the fish larvae in her mouth, where gill rakers prevent her from swallowing them accidentally. Watch a video here of the mating process in action in the cichlid species Astatotilapia burtoni (Konstanz University, 10MB file).
To find out more about the evolutionary basis for the behaviour, zoologist Axel Meyer and his team studied the genes of cichlid fish and found that a gene called colony-stimulating factor 1 receptor a (or csf1ra) was responsible for the development of the eggs spots in a number of different species.
Evolutionary explosion
Now they hope to use egg-spot genes to help pinpoint exactly why cichlid fish have been so successful.
"We are looking for the underlying genetic basis of morphological and behavioural traits that possibly were, at least partly, responsible for the evolutionary success of species," said Meyer. "There are about 1,800 species of cichlids that belong to this one subgroup of cichlids – that's about seven to eight per cent of all know species of fish. So they are doing something right."
Maternal mouthbrooding and the egg spots on the male's anal fin are two features that characterise the group, so the trait could be one of the explanations for their evolutionary success.
"Finding a gene that's at least partly responsible for development of the egg spots is the beginning of this research program," he said.
Though several other species of fish show mouthbrooding behaviour, it's usually the male who is left holding the babies, as they are the last to fertilise the eggs.
