Most people have seen the empty shell of a clam or mussel, but significantly fewer have seen the actual animals that live in there. Get a good look before they’re gone – these animals are some of the most threatened by habitat alteration and pollution today.
Danger Zone
As stated above, freshwater mussels are some of the most threatened animals in existence. Just in Iowa, there are currently nine endangered and six threatened species, and these account for about a third of the species living in the whole state.
A statewide survey is currently underway to determine the status of mussels in Iowa, as well as efforts to reintroduce mussels in areas where they have been wiped out.
Lazy Lovers
Mussels don’t move much, even for love. An adult mussel holds on to the ground with a muscular foot, and moves as little from that place as possible. This is possible because mussels are filter feeders, eating tiny bits of algae and plankton they pull into their body with a siphon.
When mussels mate, the male still stays put, releasing his gametes into the water to be carried to females downstream. The female comes into contact with the gametes as she pulls water through her siphon.
Cling to Life
Mussel larvae, called glochidia, are initially sheltered within a specialized chamber of their mother’s gills, but they can’t stay there forever. Instead, all larval mussels must go through a parasitic phase in which they inhabit a cyst on a fish or salamander.
But to get from stationary mom to swimming host can be difficult, so some female mussels have a trick to get their fishy friends in close. The glochidia-bearing mother extends a bit of her mantle out past the safety of her shell and wriggles it until a predatory fish attacks. Often this mantle flap closely resembles a bait fish, bug or even a crayfish.
The attack ruptures the structure holding her glochidia, which quickly attach themselves to the fish’s gills and fins. There, the immature mussels become so entrenched that the fish’s tissues grow over them, sealing them into safety. The process is not harmful to the fish, and after they have transformed into juvenile mussels, they fall off the gills, hopefully into suitable habitat.
Cute as a Button
As stated above, there are remarkably fewer mussels in Iowa today than there were even a hundred years ago. Populations were heavily reduced in the early 1900s because mussel shells could be used to make pretty mother-of-pearl buttons, and at one time there were 41 button factories in Iowa alone. It’s estimated the 23,840 tons of shells were harvested from the Mississippi River in Iowa in 1899 alone.
These factories shut down with the introduction of cheaper plastic buttons in the 1940s, but a new market for mussel shells emerged in the 1950s with the growth of the cultured pearl industry in Japan. In order to get oysters (a sea bivalve) there to produce pearls, shells of mussels in Iowa were cut and polished into beads that could be inserted into the oyster’s shell. The presence of the bead would irritate the oyster and cause it to cover the invader with layers of a solution called nacre, which eventually formed a pearl.