Imagine the shock when you check your aquarium with a lone female fish only to discover it's somehow managed to produce four babies.

This recently happened to researchers at Hull University in the UK - the fish in question was a hybrid of two common fish varieties from the cichlid family, a popular type of tropical freshwater fish you'd often find in home aquariums.

An experimental cross between the genetically close Pundamilia pundamilia and Neochromis omnicaeruleus, the unassuming yellow-coloured female was kept in isolation in a small aquarium to be individually photographed, just like 80 of its female siblings.

To researchers' surprise, one day they found that despite isolation, this fish had managed to spawn, producing actual fish babies.

While such feats in fish are not entirely unheard of, this wasn't really supposed to happen, and was documented as a super-rare case of 'selfing' in a sexually reproducing vertebrate species. The results were published this week in the journal Royal Society Open Science [open, DOI: 10.1098/rsos.150684].

Now, selfing stands for self-fertilisation in biology: it's when an organism has all the necessary tools to reproduce and doesn't need the help of a member of the opposite sex to have babies. You'll recognise that as a pretty common occurrence in flowers (think self-pollination), and also hermaphroditic animals such as freshwater snails.

However, vertebrates normally stick to actually having sex of some sort, exchanging reproductive material between the male and the female.