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The Clamor Over Clams

Life History


The freshwater mussel has an outer shell consisting of two halves held together by a dorsal elastic hinge. This hinge permits the valves to open and close, allowing the clam's foot and siphons to protrude. In defense, the clam will pull in its muscular foot and siphons and lock itself tightly shut.

Freshwater mussels have no head, eyes, or appendages. The insides consist of a mass of muscle, soft tissue, intestinal tract, and reproductive organs. A clam feeds by siphoning water through its internal digestive tract, where food such as zooplankton and algae are filtered out and digested.

Clams don't get around much and are usually found in colonies called beds. They often bury themselves in the mud or sand bottoms of streams, with only their back ends exposed. This provides protection while allowing the siphons to bring in food, water, and oxygen. The fact that clams occur in beds is also helpful to their reproductive efforts.

The sex life of a freshwater mussel is less than romantic and can best be described as a hit-or-miss proposition. The male simply flushes out sperm through its out-going siphon and the female must draw in the sperm through an intake siphon. It's no easy trick considering this usually goes on in flowing water. The odds for successful fertilization are naturally increased if the male and female are, pardon the pun, in the same bed.

What happens next is even more random and haphazard. The female, following fertilization, will release many miniature bivalve larvae called glochidium. These larvae enter the water, sink to the bottom, and remain there with their valves opened upward.

When fish stir up the river bottom, larvae attach themselves to the fishes' gills, fins, or other tissues. Only certain species of fish are acceptable hosts, so if the far, vae attach themselves to the wrong species of fish they soon drop off. If the right fish doesn't happen along within a few days, the larvae usually die.

If a larval clam is successful in attaching itself to the right species of fish, it becomes parasitic and the tissue of the fish grows over it, forming a nodule or lump. This parasitic state can last anywhere from 10 to 190 days depending on the species of mussel. After the parasitic stage is over and the larva has grown and developed, the young mussel falls off the host fish and drops to the bottom.

Then, as nature would have it, the little clams become an important part of the food chain, even providing food for the very fish it once parasitized. If not eaten, clams will continue their growth and as they get larger, their soft inner tissues become an important food source for mink, raccoons, snapping turtles, great blue herons and other predators. Clams can live for decades. Recent studies in Manitoba have found freshwater mussels more than 70 years old. These long life spans are critically important to the survival of the species because it can take 10 to 20 years for some clams to become sexually mature.


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