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 babied.
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.
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.
One of the only known exceptions so far is a species of fish called mangrove rivulus (Kryptolebias marmoratus). These little swimmers are what's called a 'simultaenous hermaphrodite', an animal that carries both male and female sexual organs and can self-fertilise.
However, for mangrove rivuluses selfing is a rare adaption, thought to occur because they have serious trouble finding mates.
Why the little cross-breed fish in the lab developed this odd skill is not entirely clear, although researchers speculate in the paper that this could be a genetic 'innovation' brought on because of the hybrid nature of the fish; they also pointed out that usually hybrids are not kept alone like this.
"Hermaphroditic selfing is likely to be underreported in vertebrates because of the unusual set of conditions that is required for it to be observed," the researchers note in the paper.
Passing down bad genes
This unexpected intersex fish lived for whole two years and went on to spawn a total of 46 babies, 17 of which survived to adulthood and were kept separately as well, to see whether they'd inherited this selfing ability (none of them did).
Once the fish was euthanised, the researchers did a careful dissection and discovered a blob of sperm-containing tissue nestled right next to its female reproductive organs - you could call that a testicle of sorts.
The reason vertebrates don't self-fertilise more often is most likely to do with the inbreeding risk that poses; without varied genetic material to make up the babies, harmful genetic mutations are more likely to be passed down the lineage, creating increasingly unhealthy individuals.
Best stick to good old reproductive sex, then.