Glochidia: Freshwater Mussel Larvae on Fish

Glochidia are the larvae of freshwater mussels that briefly attach to a host fish's gills and fins. It is a natural life-cycle stage, not a disease to treat.

Occasionally an aquarist or pond keeper notices tiny specks attached to a fish's gills or fins after adding wild mussels or wild-sourced water, and fears a parasite outbreak. In most cases these are glochidia, the larvae of freshwater mussels, passing through a natural and temporary stage of their life cycle on the fish.

What glochidia are

The glochidium is a microscopic larval stage of some freshwater mussels, aquatic bivalve mollusks in the families Unionidae and Margaritiferidae. They are extremely small, typically 100 to 200 micrometres, roughly a third the size of a grain of salt, and may be rounded or carry hooks for attachment.

The parasitic stage in the life cycle

Attaching to a fish is an obligate part of mussel development. Released glochidia attach to the gills, fins and scales of fish for a period before they detach, fall to the substrate and take on the typical form of a juvenile mussel. Because a fish is active and free-swimming, this process helps distribute the mussel to areas of habitat it could not reach any other way. The relationship is often host-specific, and some mussels even produce a conglutinate or lure that resembles an aquatic fly larva or a fish egg, complete with a dark eyespot, to entice a fish to take it.

Does it harm the fish?

Usually not. Under normal circumstances, glochidia do not harm fish, and a light, temporary load is part of a natural cycle rather than a disease. The exception is overload: overexposure or heavy infections may greatly decrease the host's ability to respire, because affected gill tissue can turn to scar tissue and lose function. This matters mainly in confined settings where many glochidia meet few host fish.

Practical guidance

• Do not add wild-collected unionid mussels to a stocked fish tank if you want to avoid glochidia on your fish.
• Expect any light infestation to resolve by itself as the larvae mature and drop off.
• Watch heavily exposed or small fish for laboured breathing, the main sign of a heavy gill load.
• Treat wild mussels and wild water as a route by which glochidia, and other organisms, can enter.