October 23, 2015 — Last week I wrote a little ditty about North America’s lampsilis mussels, which do unbelievable impressions of fish. And why would they? So that real fish attack them, busting loose clouds of mussel larvae that infiltrate the attackers’ gills. Here the larvae clamp onto the filaments and suck out nutrition, developing in safety until they drop out of their host. This, as you can imagine, is bad for the fish, to the point where it can kill them.
Lampsilis has the most highfalutin way of infesting fish with their larvae, but freshwater mussels the world over do it too—simply by releasing the young into the water column. There’s one group of fishes in Europe and Asia, though, that won’t suffer such parasitization without retaliation: the bitterlings. These guys flip mussels’ reproductive strategy back on them. Using a tube-like structure, the female fish inserts her eggs into a mussel’s gills, then the male fires his sperm in as well. The fertilized eggs get a nice little home in the host. The host mostly just gets embarrassed.
So let us explore the strange game that is different kinds of parasitic river critters trying to impregnate each other with their offspring.
Typically fish reproduction is about as basic as it gets in the animal kingdom. Males and females get near each other, then dump their sperm and eggs into the water. That’s it. The problem with that, though, is it opens the young up to all kinds of problems, particularly predation. Only a fraction will make it. The rest will end up in stomachs.
For the bitterling, this won’t do. Its reproduction begins with good mussels, and it’s up to the male to find them, preferably large hosts with more room to hold the young. When he chooses a victim, or even several in a given area, he posts up. Should rival males take an interest in his property, his coloration intensifies as he head-butts his foes for control of the territory.