Freshwater Mussels – Hitchhikers on Fish

Most of us are familiar with the edible blue mussels that cling to rocks in the ocean’s intertidal zone, and also, perhaps, with the ribbed horse mussel imbedded in the mud of salt marsh estuaries. But, what about fresh water? Are mussels found in rivers, streams and lakes as well? They certainly are! About 300 species of freshwater mussels are found in North America – the greatest diversity of any continent. Twelve native species occur in Connecticut, six of which are listed as endangered, threatened or special concern in the state.

Contrary to general perceptions that mussels are about as interesting as dead stones, these animals are truly fascinating. Belonging to the Phylum Mollusca, they are related to marine octopus and squid, but only mussels, their bivalve clam cousins, and snails (Gastropoda) possess freshwater species.

Mussels are filter feeders, pumping up to 40 liters of water per hour through their bodies in their quest for oxygen and food. Mussels may live from a decade to over a century, depending on species and habitat conditions.

Most fascinating is the fact that fish play a critical role in a mussel’s life cycle. Life begins when sperm, expelled by male mussels, is siphoned into a female and fertilizes her eggs. After two to ten months of brooding in her gills, the eggs develop into tiny (0.05 to 0.50mm diameter, smaller than a period on this page), strange-looking larvae called glochidia (glah-KID-ee-ah). When mature, hundreds of thousands of glochidia are released into the water, where a few will find their way to the correct host fish and attach themselves to its gills, fins or body. The glochidia soon become grown over, or encased, by fish tissues, forming a cyst.

musselsSome mussel species have one specific host fish, others may have up to three or more. For example, brook and brown trout are hosts to the Eastern Pearl Shell Mussel (swift, cool streams), while carp and white suckers, tolerant of lower quality streams, are host to the equally tolerant Eastern Floater Mussel.

After one to ten weeks in the parasitic cyst stage the juvenile mussel breaks out of its cyst on the host fish, falls to the bottom and burrows in. Thus, the fish host not only provides food and shelter for the developing larvae, it also is its means of dispersal, often moving the mussel larvae upstream where it could not possibly go otherwise. Remember, no fish, no mussels!

Pesticides, fertilizers and other chemicals used on land eventually make their way into our rivers and streams, affecting water quality. Siltation, caused by livestock or development too close to the water’s edge, is also a problem. Because adult mussels are essentially sedentary, and continuously filter and “sample” their water environment, scientists are using them to help monitor water and habitat quality. Mussel die-offs can be an indication that the host fish for that species has disappeared from that area, or that pollution has compromised the mussels’ ability to survive. Freshwater mussels are considered good “biological indicators” of what is happening in a river system. Connecticut’s Dept. of Environmental Protection is currently conducting surveys of these amazing, imperiled creatures in our state’s rivers, streams and lakes.

Submitted by David Houston