Every so often I come across a news article that is relevant both to trans science and science fiction. Yesterday was one of those magical days, because I discovered the existence of self-fertilizing fish.

I should probably go back a few stages here by way of explanation.

To start with there is the whole question of what we mean by “biological sex”. Fish do not share the same XX/XY chromosome system familiar from mammals. In fact there are a wide range of different biological mechanisms that fish use to differentiate sex. When I talk about the biological sex of a fish here I mean whether the fish produces eggs, fertilizes eggs, or both.

Readers in the UK are probably familiar with the recent episode of Blue Planet II in which David Attenborough’s team filmed a Sheepshead Wrasse in the process of changing sex. There are, in fact, many fish species that practice what is called Sequential Hermaphroditism, in which the animal is male for part of its life cycle and female for another part. The much loved Clown Fish from Finding Nemo is another example. These fish only exhibit one sex at any one time in their lives.

There are also many fish species that exhibit Simultaneous Hermaphroditism. That is, they are capable of producing eggs and fertilizing eggs. Some species of Sea Bass have these abilities, which is something worth pondering next time you eat one. However, these fish have to have sex with other, similarly hermaphroditic, fish in order to make baby fish. How else would genetic diversity be achieved, right?

However, there is one species of fish (well, more properly two closely related species) that can make babies by individuals having sex with themselves. Enter the spectacularly named Mangrove Killifish.

The killifish is pretty amazing on several levels. It can live in both fresh and salt water, and it can survive for up to two months on land. But self-fertilization is seemingly the most miraculous ability because surely all of the fish would be clones, so how would they evolve? Mutation doesn’t seem an adequate explanation.

However, it turns out that killifish come in two sexes: both and male. The males are very rare, but very popular. If a clone family of both-sex spots one they’ll seek him out and make lots of baby fish. In that way a certain amount of genetic diversity is maintained.

For those who are interested, there is more scientific detail here.

And for those of you thinking of interesting ideas to use in alien biology, have at it!