Surprise! Clams are living in our local rivers, creeks and lakes. These unassuming animals are commonly overlooked, but when they are noticed they are often referred to as pet rocks (or less flattering terms). North Carolina is home to 65 species of native freshwater mussels or clams. They are distributed unevenly in North Carolina’s 17 river basins: 22 species live in rivers that drain into the Tennessee and Ohio rivers; 43 are found only in rivers draining into the Atlantic.
Some freshwater clam species can live up to 60 years – very long lives for invertebrates. They have an unusual life cycle: As tiny larvae, they must spend from a week to several months attached to the gills or fins of a fish. At the right time during their development, the clams drop off their hosts and begin to live in the river or lake bottom.
So how does the female clam attract a fish so she can place her larvae on their gills? Freshwater clams are sneaky. Some species pack their larvae into gelatinous packets that look like food for fish. When a fish eats the packet, the packet explodes – filling the fish’s mouth with clam larvae. As larvae pass over the fish gills, some attach to the gill filaments.
Other clam species have structures on their mantles (the layer of tissue that produces the shell) that work like lures, attracting fish to the female clam. When the female clam has larvae to release, these structures expand from the edge of her mantle and begin to resemble minnows, darters, crayfish or caterpillars. When a predatory fish, like a bluegill or bass, arrives to make a quick meal of this tasty-looking treat, it receives a face full of clam larvae instead, some of which attach to the unwitting host. Still other clam species attract host fish using a long, transparent gelatinous tube (up to about 75 inches) filled with clam larvae. This long tube waves in the current, resembling a sick minnow.
Because freshwater clams require these fish associations during their development, declines in native fish populations have a devastating impact on clam reproductive success. This illustrates a larger lesson: Species found in a given ecosystem can be connected in unexpected ways, and as a result, conservation efforts targeting single species might not be as effective as those that protect entire habitats and natural systems.
Arthur Bogan is research curator of aquatic invertebrates at the N.C. Museum of Natural Sciences.