Bitterling

A Bitterling sweet symphony

Nature is horrible – that’s biology 101. One of the ways in which it is particularly horrible is what biologists refer to as ‘inter-specific brood parasitism’ – a situation where one species, the bully, murders the babies of a victim species that happens to be good parents. The bully then forces or tricks the victim species into raising its young, thus bringing up more bullies to haunt future generations of good parents. This description will likely make you go ‘cuckoo’ in your head, right? Well, you may be surprised to hear that there are plenty of examples of such outsourcing of parental responsibilities in the animal kingdom; many fish do it, many birds do it, and a lot lot LOT of insects do it. It’s sort of like the human parental tricks of Netflix or a Golden Retriever, but with murder! Anyways, one of the most fascinating forms of ‘inter-specific brood parasitism’ is present in a fish family called ‘the Bitterling’. Bitterlings are a big group of fish with silly, confusing species names: as far as I can tell, it goes 1 Bitterling, 2 Bitterling, 3 Bitterling… very confusing, I know! While there are some 40 species in this group, their name derives from a southern German habit of naming fish for some obvious trait and adding ‘-ling’ at the end. In the case of the Bitterling, it appears that the European Bitterling, Rhodeus amarus, does not taste great (‘amarus’ meaning ‘bitter’ in Latin). So it was first named ‘Bitterling’ in the areas where it occurs (southern central Europe) and subsequently gave this name to its whole taxonomic family, the Acheilognathidae – which in fact are mostly distributed throughout Asia. But 17^th/18^th Century European Naturalists couldn’t care less about such minute details. So here we are, stuck with a Bavarian name for a mostly east-Asian group of animals. Bitter.

Bitterling have a very special relationship with the mussels with which they share their habitat. But before we can dive into the details of this fascinating inter-specific tete-a-tete, we first need to know a tiny bit about bivalve biology: most of these shellfish are filter feeders, meaning that they suck in water, pump it over their gills, and expel it again. While biologically not quite correct, let us, for simplicity’s sake, refer to the opening through which a mussel sucks in water as its ‘mouth’ and the opening through which the water exits as its ‘butt’. While the water is running over and between the gills, oxygen is extracted (just like at the gills of fish), but also anything a mussel considers food is taken out; planktonic algae and other such things. So basically, dirty water comes in through the mouth and clean water exits through the butt. Not exactly what humans do. Anyways, mussels are pretty neat organisms from a fish egg development point of view: they provide great protection (shells!) and clean and well oxygenated water (pumping!) – most fish have to frantically fan their eggs to make sure the water around them doesn’t go stale and equally frantically drive away predators. By putting your eggs into a mussel, you are free to do whatever you want while still knowing your offspring are well cared for. A bit like a Montessori Kindergarten, huh? But while you pay copious amounts of hard earned cash to your sproglet’s nursing staff, the mussels that care for Bitterling eggs get very little in return – which is why many of them try to spit the eggs out. Only that ‘spitting out’ in this case goes via the butt. One might thus have used a different word, one that also rhymes with ‘spitting’. But we are far too civilised for such profanities here! But wait a minute, you will now say, how did the eggs get into the mussel in the first place? Well, Bitterling females have very long ovipositors which they insert into the mussel’s butt. In fact, they are so long as to ensure that the eggs are deposited deep inside the mussel, making it hard for the latter to get rid of them. If you plan on setting up a scam email scheme for Bitterling, make sure to target females’ hopes for ‘ovipositor enlargement’ – ‘penis enlargement’ will net you very few clicks; Bitterling males don’t have a penis. Bitter.

I hope the one thing you have been wondering thus far is: once the eggs are inside the mussel, how do they get fertilised? Especially without a penis in the mix? Excellent question. This is where the mussel’s mouth becomes important: Bitterling males simply squirt their semen into the rough direction of the chosen host’s mouth… who involuntarily inhales it and flushes the sperm straight to the eggs sitting in its gills. A few weeks later, happy little Bitterlings exit through the mussel’s butt. Now this is a case where oral sex leads to pregnancy. But not only oral sex, but inter-specific oral sex. Nature is weird, once you try to liken it to humans. But if you don’t, it is simply fascinating. I mean: here you have two fish sandwiching a bivalve so as to ensure their babies get the best possible start in life by coming out the mussel’s butthole. Fas-ci-nating and not one bit bitter! Unless you are the mussel.