Endangered mussels squirt out offspring

You’ve heard of throwing the baby out with the bathwater, but researchers at Cambridge University have really pushed the boat out with the discovery that a rare mussel species squirts its young across a river so that fish mistake them for food and temporarily snap them up, and but then end up nurturing them for the next few months. David Aldridge tells the story of unio crassus, the thick shelled river mussel…

David - We've stumbled on something absolutely incredible and really very surprising. A freshwater mussel known to be endangered, which during the springtime, the females of this species move to the margins of the rivers in which they live and start squirting jets of water, about a meter long, back into the river. And we've found that these jets of water carry the tiny little larva of this freshwater muscle back into the river.

Chris - When you say they moved to the margins, these animals are almost beaching themselves. They're actually coming out of the water to send these jets through the air and then back into the water, aren't they? Because I've looked at the footage that you've published of them doing this. It's quite extraordinary.

David - Yes. These mussels are sort of exposed. They're just on the edge of the water and they suck up the water inside their shells and then they squeeze the shell together that squirts these jets and these beautiful loops back into the river. And what's happening is that the larvae of freshwater mussels have this amazing life history where the larvae are like miniature castanets. They're about a third of a millimeter long and they have little hooks and spines on them and they have to attach to the gills or the fins of a host fish in order to complete their life cycle. And so what we found is that each of these jets carries about 50 or so larvae. And when those jets of water land in the river, they are perceived as food by the fish, which are the best hosts for these mussels. And instead of getting food, they actually get parasitized by the larvae. And this is a really neat system for the mussel because by using fish as a host, it allows them to travel around a river system. And obviously mussels are very immobile. They sort of sit in the mud at the bottom of the river. But fish can particularly help to transport these larvae upstream.

Chris - How long are they in the fish for then?

David - Well, they can stay on the fish for about two or three weeks in some species. And over a year in other species in this particular mussel, it's probably, they're probably on fish for a month or so before they drop off.

Chris - And then it's down to the riverbed to spend the rest of their life maturing, becoming adults. And then they're doing this behavior later.

David - That's right. What we found with this muscle is that unlike most European muscles, this particular species, the thick shell river mussel, has a very narrow range of fish that it uses. That's probably one of the reasons why we've got this peculiar behavior that the mussel wants to make sure it gets its larvae on the right fish and not on the wrong fish because otherwise it would waste a lot of its larvae. And by having this close association with a particular range of fish species, those fish share the same habitat as this particular mussel. So there's a greater chance of the juvenile mussels leaving the fish in suitable habitat. So we've got this very close sort of coevolution going on.

Chris - There's a million questions going through my mind because it is such an extraordinary story. First is how does a shellfish that doesn't have a brain know it's at the riverbank, get itself partly emerged from the water and do that at the right time of year?

David - <laugh>, I think your guess is as good as mine. It's totally mind blowing that yes, as you say, this is an organism without a head or a brain that knows to do this really by specialized behavior. My hunch is that they are responding to the sound of the water on the margins of the riverbed. So that's sort of acting as a cue for them to move to the margins. But we really don't know. I mean, there's some opportunities for great experiments to try and try and sort of really work out what's going on.

Chris - Tell us then, how did you spot it? Were you just sitting on a riverbank counting mussels and you spotted one squirting and then that made you wonder?

David - <laugh> Like many of the greatest pieces of scientific research, this emerged from a conversation over some beers at a conference. These mussels were living in a river in Poland, and my colleague mentioned that he'd observed this peculiar behavior. And so a group of us said, let's go and spend a week in the Carpathian mountains in Poland and see what's going on. But I don't think anything really prepared us for what we were going to see. It was remarkable. Just all these mussels all the way along the riverbed squirting away.