It's long been known that freshwater molluscs get spread around the place by all manner of mechanisms, up to and including hitchhiking on fish gills. Recent research in Alaska has demonstrated another way in which they might disperse — tucked away in a fish's tum.
Some species are able to survive the dark and smelly journey through a fish's interior, emerging from the far end apparently none the worse for the experience. In the time taken for the mouthful of mollusc to pass along the digestive tract, the fish might have moved a considerable distance — almost certainly further than an adult mollusc would have made it on its own. When the fish poos, out pops the invertebrate in a completely different location.
Randy Brown, of the U.S. Fish and Wildlife Service in Fairbanks, Alaska, examined poo from wild-caught fish and found that they were filled with live pea clams (Pisidium idahoensis, Sphaeriidae) and valve snails (Valvata sincera, Valvatidae). Although (understandably) quiet at first, once out in fresh water the molluscs soon started moving around.
The number of live molluscs collected from each fish ranged from 312 to 769 pea clams and 432 to 1212 valve snails. Only a single pond snail (Lymnaea atkaensis, Lymnaeidae) made it through alive.
The survivors have at least one thing in common. Both species can seal their shells to protect themselves from a hostile environment. (And, let's face it, the inside of a fish's intestine is probably pretty hostile.) Unlike the valvatids, pond snails lack an operculum, so can't close the door. The fish's digestive enzymes do their work unhindered.
Earlier studies have shown that the tiny hydrobiid snails (which also possess the operculum) survive the journey through flounders and trout. It appears that hydrobiids aren't the only ones to be taken on the tour. Now there's a little project for the holidays.
Brown, R.J. (2007) Freshwater molluscs survive fish gut passage. Arctic 60(2): 124 –128.