Glochidium

Glochidium is the larval stage of freshwater mussels, primarily members of the families that comprise the unionoid clade. These larvae are parasitic and non-feeding, and their development depends on a fish host for nourishment and dispersal to new habitats. The adult mussel releases glochidia into the water as part of its reproductive cycle.

After release, glochidia must encounter a suitable fish host. They attach to the fish, often near gills or fins, using specialized structures that enable encystment. Once attached, they encyst and derive nutrients from the host during a period that can last from several days to weeks, depending on species and host compatibility. During this period the glochidium undergoes metamorphosis into the juvenile mussel, which then detaches from the fish and settles onto the substrate to grow into an adult.

Host specificity varies among species. Many glochidia infect only a limited number of fish species, while others are capable of using a broader range of hosts. This dependence on fish hosts makes the reproductive success of freshwater mussels closely tied to the presence and health of their host fish populations. Environmental changes that affect fish communities or water quality can thus have significant impacts on mussel populations.

In ecological terms, the glochidium stage is crucial for dispersal and population connectivity of freshwater mussels, helping to move individuals to new habitats and maintain genetic exchange across watercourses.