The family of freshwater mussels known as Unionidae represents the largest group within the order Unionida. These bivalve mollusks, sometimes called river mussels or pearly mussels, are found in freshwater habitats across the globe. While their distribution is worldwide, the highest diversity of unionids is found in North America, which is home to nearly 300 distinct species.
Physical Characteristics and Habitat
The external appearance of Unionidae is defined by a two-part, hinged shell that protects the soft body inside. These shells are diverse, varying in shape, size, and coloration among the many species. The interior of the shell has a pearly, nacreous layer. Key anatomical features include a muscular foot for burrowing into the riverbed substrate, and incurrent and excurrent siphons used to draw water in for respiration and feeding, and to expel it.
Unionids are adapted to permanent freshwater bodies, including rivers, streams, and lakes, and are most often found in flowing waters at depths of less than two meters. Their survival depends on the riverbed, as they must anchor themselves into substrates like sand, gravel, or mud. The chemistry of the water is also a factor, with mussels favoring alkaline conditions that support the development of their calcium carbonate shells.
The Parasitic Larval Stage
The reproductive cycle of unionid mussels is distinct among bivalves. It begins when males release sperm into the water, which is then drawn in by females downstream through their incurrent siphons. Fertilization is internal, and the eggs are brooded in the female’s gills, where they develop into microscopic larvae known as glochidia. This stage requires a parasitic relationship for the larvae to mature.
To complete their life cycle, the glochidia must attach to a vertebrate host, almost always a fish. Females have developed various strategies to ensure their larvae find a host. Some species release clouds of glochidia when a fish passes nearby. Others have evolved specialized mantle tissue that acts as a lure, mimicking a small fish or insect to attract a host. When the fish strikes the lure, the female expels the glochidia, which then attach to the fish’s gills or fins.
Once attached, the glochidia become encysted in the host’s tissue, where they act as temporary parasites. During this phase, which can last for several weeks, they draw nutrients from the fish’s body to transform into juvenile mussels. After this metamorphosis is complete, the young mussels detach from the host, fall to the riverbed, and burrow into the sediment to grow into adults.
Ecological Significance
Freshwater mussels are important to the health of aquatic ecosystems as powerful filter feeders. A dense population of unionids can filter substantial volumes of water, removing suspended particles like algae, bacteria, and detritus from the water column. This filtration activity improves water clarity and quality. Their ability to accumulate pollutants in their tissues also makes them effective bioindicators, as a decline in a mussel population can signal deteriorating water conditions.
Beyond filtering water, these mussels function as ecosystem engineers. Their shells, from both living and dead individuals, create structural habitats on the river bottom, offering shelter for algae, insects, and other small invertebrates. The act of burrowing into the substrate helps to mix and aerate the sediment, a process known as bioturbation. This activity supports nutrient cycling and the health of the benthic, or bottom-dwelling, environment.
Conservation Status and Threats
Unionidae are considered one of the most threatened groups of animals globally. Their populations have faced significant declines, and many species are now at risk of extinction, reflecting the vulnerability of the freshwater ecosystems they inhabit. This precarious status underscores their sensitivity to environmental changes.
A primary threat to freshwater mussels is habitat destruction and alteration. The construction of dams and other impoundments fragments river systems, transforming flowing waters into stagnant reservoirs and altering downstream conditions. These barriers also block the movement of host fish necessary for mussel reproduction and dispersal. Water pollution from agricultural runoff, industrial discharge, and municipal waste degrades water quality. The introduction of invasive species, particularly zebra and quagga mussels, has had a severe impact, as these non-native species outcompete unionids for food and space.