Zoologger: The amphibious fish that mates with itself

From New Scientist: Zoologger: The amphibious fish that mates with itself

Species: Kryptolebias marmoratus

Habitat: Mangrove swamps on the east coast of North and South America

Faced with the inevitability of death, some people draw up a "bucket list": a checklist of things they plan to do, like learning a musical instrument or visiting the Grand Canyon. The Bucketlist website collates these ideas, including such gems as "play chicken with a train and lose". Yet nowhere on the site has anyone expressed a desire to have sex with themselves while living in a tree.

This shows a deplorable lack of initiative, because it wouldn't even be a world first. The mangrove killifish is way ahead of us. It lives in pools that are prone to drying up, so it can also live on land for months at a time – often inside hollowed-out logs – where it survives by breathing through its skin. It's also one of only two vertebrates – the other being the closely-related ocellated rivulus – that can self-fertilise.

These abilities give rise to a peculiar society made up of groups of clones that compete with each other for survival.

Fishy business
Plenty of animals are hermaphrodites, with both male and female sexual organs. But they still tend to mate with others to mix their genes up a bit.

Mangrove killifish don't tend to bother. Adults have both ovaries and testes, so when they want to reproduce they release sperm and eggs simultaneously. After the eggs are fertilised, they lay them in gravel.

Constant self-fertilising means the fish lose any genetic variation. Mangrove killifish that have self-fertilised for just a few generations become homozygous: that is, they have two identical copies of every gene. So within each population, there are a few groups of genetically identical killifish, each cloned from a different ancestor.

Having said that, some populations do have a few males. And though self-fertilisation is the norm for the hermaphrodites, they do occasionally mate with a male. That allows different clonal groups to trade genes, using males as middlemen. The killifish in these populations are not homozygous – they have two different versions of some of their genes.

Personable clones
Nevertheless, most of the time there aren't any males, just groups of hermaphrodite clones. These groups can be quite different, for instance having differing sex ratios, growth rates and numbers of offspring.

Mathew Edenbrow and Darren Croft of the University of Exeter in the UK wondered if the groups also differed in their personalities: for instance, how willing they were to explore new places.

One idea is that the "life history" decisions an animal makes, like when to reproduce, help determine its personality. But when Edenbrow and Croft monitored 120 young fish as they hatched and grew to adulthood, they couldn't find any correlations between life history and personality.

Instead they found that the young killifish were remarkably malleable, with each individual changing its behaviour substantially as it grew up. "They're not constrained in their personality expression," Edenbrow says. He speculates that the young fish are learning from their experiences and shaping their personalities accordingly.

But not all of the fish were so flexible. Edenbrow and Croft looked at six clone groups and found that some were more malleable than others. That could give them an advantage: faced with a changeable environment, they may cope better than their stuck-in-the-mud cousins.

Given that the clone groups must compete with each other, do animals within each group work together? We don't know yet: Croft says it's not even clear if clones can recognise each other. But closely related animals are more likely to cooperate – think of worker ants in a nest, all descended from the same queen – and clones are as closely-related as you can get.

Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2011.07.003