Habitat: Between 100 and 300 metres deep off the coast of south-east Africa, around the Comoros Islands
We thought they died out 400 million years ago till one popped up off the coast of the Comoros Islands in 1938. Their two unusual front fins show they are closely related to the first fish to clamber onto land around 400 million years ago. Now, coelacanths, dubbed "living fossils" since their unexpected reappearance, have sprung another surprise: they are serial monogamists.
Coelacanths are enormous, bottom-dwelling fish that lurk between 100 and 200 metres beneath the surface, on the rocky sides of volcanic islands in the Indian Ocean. The 1.5-metre-long giants are best known from sightings in the Comoros Islands, but have also been spotted off the eastern and south-eastern coasts of Africa. They spend their days in cavities inside submarine volcanic rocks, and only venture out at night to feed – mostly on squid, octopus, cuttlefish and other fish.
"We know hardly anything about their reproduction," says Kathrin Lampert of Ruhr University Bochum in Germany. What is known is that their gestation period is an impressive three years and they bear live young.
Now, for the first time, Lampert has DNA fingerprinted two dead pregnant females and their offspring. The fish were accidentally caught by fishing boats, one off Mozambique in 1991 and the other off Zanzibar in 2009, and weighed in the range of 90 kilograms.
"For both, it was very clear there was only one male involved," says Lampert. She and her colleagues were stunned by the discovery: they expected the females would mate with multiple partners to maximise the genetic diversity and therefore the survival prospects of their offspring. "Given the length of gestation, that's a big investment in offspring by the female," says Lampert.
Not so many fish in the sea?
Only a few hundred coelacanths are thought to exist, so it could simply be that there are too few males available for multiple mating. But previous research has shown that they live in groups in caves, providing ample opportunities for females to play the field. "So it's unlikely they are restricted by mate choice," Lampert says.
The study shows that a single male sires an entire clutch, says Lampert – but it's unlikely that mates stay faithful to each other throughout their lives. Lampert says it's most likely that they are serial monogamists.
Females don't appear to care for their offspring once they are born. They may even cannibalise them given half a chance. Only one or two juveniles have ever been observed in the wild alongside adults. "Maybe they go to deeper depths to avoid being eaten by their parents," says Lampert.
The pregnant fish give one clue to the puzzle. The offspring – 23 found in one female and 26 in the other, each weighing about 500 grams – are large at birth, about a third the length of adults. This may make them more difficult to catch and eat, and help them to quickly escape to deeper waters. It would also explain why the female undergoes such a long gestation: growing the young to a large size inside her boosts their chances of escaping a hungry neighbour.
Read more at New Scientist
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