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.

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    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.

    Lampsilis reeveiana St. Francis River MOThe aforementioned lampsilis mussel, showing its mind-blowing fish lure and apparent identity crisis. Chris Barnhart

    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.

    Then there’s this thing called sexual selection. Females can be quite picky about the males they choose to mate with, whether they judge the fellas’ colors or calls or what have you. Interestingly with the bitterlings, females will more often follow large, colorful males back to their mussels, but if a male’s quarry is a crummy quality, she’ll split. So in a way, the male’s choice in mussels seems to be a function of his fitness for the ladies.

    But if she digs it, says Tommy Leung, a parasite ecologist at Australia’s University of New England, “she’ll extend this long ovipositor, this very long tube-like organ,” which, for the record, is weird for a fish. “And she’ll stick it into the mussel’s siphon and squirt a few eggs in there.” To do so, it may be that she’s first loading those eggs up in the ovipositor (literally, “egg placer”), then urinating to fire them out.

    When she’s done, the male steps up and fires his sperm in there as well. With any luck, the two parties will meet inside the gills of the mussel to make little bitterlings. The parents have, in effect, impregnated another species with their young. And like the deadbeat parents they are, they take off, leaving their kids inside the reluctant babysitter.

    Japanese Rosy bitterling (Rhodeus ocellatus). A Japanese rosy bitterling, so named for its sunny disposition. Nobuo Matsumura/Alamy

    But bitterling eggs aren’t your typical, spherical fish eggs you get at sushi. They’re oblong—lengthened likely to help them wedge in the mussel’s gills. The increased surface area will also help the eggs absorb more oxygen from the water, which the mussel certainly doesn’t appreciate. Host and parasite find themselves competing to breathe.

    When the eggs hatch into larvae, the bitterlings have still more adaptations that help them hold on as water rushes in and out of the mussel. Certain varieties have small spines that anchor them. “And there are some species that have these kinds of protrusions on the side of their head,” Leung says. “They look almost like Mickey Mouse ears, that come out and allow them to wedge themselves in there.”

    In this larval stage, the bitterlings face a new challenge: eating. While it’d be easy to assume they’re nibbling at the mussel’s tissues, it’s more likely that they’re picking off the food that’s pouring into the host, for mussels are filter feeders, hoovering up bits of organic matter floating around the river. All the larva has to do is pick the stuff off before the mussel can consume it.

    This, combined with the fact that the larva is stealing oxygen from its host, makes it a clear-cut parasite. The bitterling takes and takes and takes, and when it’s good and ready, it swims out of the mussel and into the world.

    But the mussel will have its revenge. Well, at least it’ll try. As I mentioned before, the mussel’s aim is to infest any fish in the neighborhood with its own young by dumping its larvae into the water. And it’ll work on a lot of fish—but not the bitterlings. They seem to be particularly resistant to the onslaught.

    bitterling (Rhodeus amarus, Rhodeus sericeus), juveniles leaving their shell. Young bitterlings bid their babysitter farewell. “Bye! Thanks for letting us stay up past midnight!” blickwinkel/Alamy

    Freshwater fish mount an immune defense by growing cells around the mussel larvae that clamp onto their gills. Sometimes this works, but other times the larvae can resist the attack and simply chill in the cocoon of cells. But not so with mussel larvae that attack the bitterlings. These fish are great at warding off the parasites. But why?

    Bitterlings clearly spend a lot of their time around mussels—that is, using them as sexual proxies. “And so in turn,” Leung says, “evolution would select for the [bitterlings] that are more resistant towards these larvae, because if they’re going to be around this thing a lot, being resistant to them would give bitterlings a fitness advantage.” In other words, over evolutionary time, individuals that were better able to resist infection were more likely to reproduce and pass along this resistance.

    It’s a remarkable relationship, and demonstrates just how intertwined species can be, at times in uncomfortably sexual ways. There’s just so much that science still doesn’t know about Earth’s rapidly changing ecosystems, regarding bitterlings and mussels or otherwise. Freshwater species like these in particular face tremendous threats from pollution, overfishing, and habitat degradation.

    “With conservation, people talk about how we should prioritize this species or we should prioritize this particular species,” Leung says. “But there’s still a lot of basic science and natural history work that needs to be done to work out, OK, just how interconnected are they? Are we missing something in terms of what kind of relationship that they have?”

    As for this particular relationship between bitterlings and mussels, science can probably classify it as … it’s complicated.

    Browse the full Absurd Creature of the Week archive here. Know of an animal you want me to write about? Are you a scientist studying a bizarre creature? Email [email protected] or ping me on Twitter at @mrMattSimon.

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    Absurd Creature of the Week: This Crafty Fish Turns Mussels Into Its Surrogate Parents