National Park Service Mollusks are the second largest group of invertebrates after arthropods. They are defined by their soft bodies, which are usually enclosed in a calcium carbonate shell, but also include slugs, squid, and octopuses. Freshwater mussels, pea clams, and snails are important types of mollusks found in the Upper Delaware River. Relatively little is known about pea clams and snails in the Upper Delaware River; however, freshwater mussels have received significantly more research attention. Mussels are more formally known as bivalve mollusks, meaning they have two shells hinged together. The mussel life cycle takes place in distinct stages. Once the female mussel’s eggs are fertilized by the drifting sperm of the male, mussel larvae, called glochidia, develop in the female's gills. Once glochidia are released from the female's gills, they attach to a fish host. The host fish ensures the glochidia's survival and dispersal throughout their habitat. Some mussel species can only survive if they find the right species of host fish. Glochidia may be attached to the host fish for several days or several weeks. After the glochidia metamorphose into juvenile mussels, they drift to the bottom of the river. Mussels can become concentrated in large groups in portions of the river. These large groups of mussels living together are known as mussel beds and may contain many different species. Once they reach adulthood, many mussels can live for decades. Shells from dead mussels can be seen on the river bottom and along the banks. River Otters (Lontra canadensis) and Raccoons (Procyon lotor) prey upon mussels and often leave them in piles known as middens. These empty shells can remain for many years. Mussels play an important role in maintaining the high-water quality of the Upper Delaware River. They are filter feeders, meaning they remove algae, bacteria, and sediments from the water. One adult mussel can filter 20 or more gallons of water per day. Mussel beds can filter millions of gallons of water per day. Otherwise inaccessible particles are converted into nutrients that other organisms can use. It helps keep the water clear, allowing bottom-dwelling plants more to access sunlight. The benefits of mussel filtration reach beyond their own ecosystem; municipalities who rely upon the clean water source don't bear as much cost for water filtration as they otherwise might. The most common mussel in the Upper Delaware River is the Eastern Elliptio (Elliptio complanata), which makes up the largest biomass of animals in the river. Their larvae, or glochidia, are host specific and depend mainly upon the American Eel (Anguilla rostrata) for reproduction. The Eastern Elliptio can be difficult to distinguish from other mussel species, but some clues come from the shell shape and nacre color. The nacre is the interior layer of the mussel shell, sometimes known as the mother-of-pearl layer. The nacre color of the Eastern Elliptio varies but is never deep purple. Mussel species in the Upper Delaware River face threats from invasive species, pollution, and habitat modification. Some mussels are even in threat of extinction. The Dwarf Wedgemussel (Alasmidonta heterodon) is federally endangered and protected under the Endangered Species Act. Like the Eastern Elliptio glochidia, the Dwarf Wedgemussel's glochidia are host specific and rely mainly upon the Tessellated Darter (Etheostoma olmstedi) for survival. Other mussel species are listed as state endangered species. The Brook Floater (Alasmidonta varicosa) is a New York threatened species, and the Eastern Pearlshell (Margaritifera margaritifera) is a Pennsylvania threatened species. The free-flowing nature of the Upper Delaware River is essential to the survival of these mussel species. If the river were dammed, fish hosts would not be able to migrate and disperse glochidia throughout the river. |
Last updated: January 26, 2021