Today’s factismal: The larvae of freshwater mussels were originally thought to be parasites.
In nature, one of the best ways to go extinct is to stay in one place. If conditions change, then you die. If your food source goes away, then you die. If a new predator shows up, then you die. So if you want to keep your species from going extinct, you make certain that it is spread out in many different places. But how do you manage that if you are a freshwater mussel?
Freshwater mussels are mostly sessile, sticking in one place with their single “foot” as they siphon in water and sediment and spew out water and used sediment. Though they can move short distances, they don’t travel far. So they have to leave the movement to new places to their offspring which they spew into the water come mating season. If they relied just on the water to transport the baby mussels, then they’d end up flowing downstream and into the sea which would seriously limit their options. So they’ve evolved a better way: they become hitchhikers.
The adult mussel waits for a fish to come by and sends out a cloud of larva that cling to the fish’s gills. Once the fish has moved into a new and more interesting neighborhood, the baby mussels drop off and settle into their new homes on the river bottom. Some species of freshwater mussel actually shape their larval bundles into a mass that resembles an insect, complete with eye spots, to attract fish. Interestingly, the larval mussels do very little damage to the fish while they cling to the gills. But that didn’t stop the biologists who studied them from classifying the baby mussels as “parasites” and calling them glochidia (“little pointy things”). Of course, part of the problem was that early microscopes couldn’t really make out that each glochidium was a baby mollusc. Now that we can see them, we’re stuck with the name just as the fish are stuck with the glochidia.
If you are the sort of person who finds this fascinating, then you are probably the type who would enjoy doing a citizen science experiment with mussels. In that case, float over to the Xerces Society for their Western Freshwater Mussel Study: