Mollusc Larva: Glochidiasis

Volume 4, Issue 3
March 2002

Figure 1. Glochidia look like small mussel or clam shells attached or beneath the gill epithelium.

The larvae of most, but not all, freshwater clams and mussels must go through a parasitic stage on the gills or fins of fishes (Coker et al 1921). The larvae become attached in the epithelium of the gills or epidermis of the fins. Close examination of the larvae reveals them to look like tiny clams, and some have large hooks that they use to attach to the fish. The larval clams use the fish as a means of transportation, and as a food source by absorbing organic molecules and plasma from the fish. Some species of molluscs use specific species of fish to transport their offspring, others have very little host specificity. Molluscs that use a specific host species may time their spawning with fish migrations. The glochidia remain attached to the fish for usually 10-30 days during which time they undergo metamorphoses to their adult anatomy. The glochidia then drop off the fish and colonize the benthos.

Most glochidiasis is of only minor concern to the fish, however, sometimes when glochidia burdens are too numerous, fish may succumb. The glochidia are irritating to gill tissue causing hemorrhage and excessive mucous excretion. In Maine, American shad Alosa sapidissima can have heavy burdens of glochidia during the spring spawning migration. Sometimes it can be fatal.

Heavy infestation of glochidia

Figure 2. American shad gill hemorrhaging and slime covered because of heavy infestation by tens of thousands of glochidia.

As in other parasitic species, glochidia are shed in huge numbers to ensure the maximum potential for host contact and attachment. Fecundity in many molluscs can range from 200,000—17,000,000 glochidia/female/breeding season. Young and Williams 1984 studied glochidia survival. They concluded that in a natural population of Megalonaias margaritifera, 1 out of 100,000,000 glochidia lived to become a settled juvenile.

Some female molluscs also attract fish with vermiform (i.e.., worm-like) mantle flaps, luring fish closer. Fish that grasp the flap mistaking it for an easy meal get a mouthful of glochidia.

Special points of interest:

Fish with glochidia cannot transmit the
disease to people.

Glochidia infest many species of fish.

For more information read: Hoffman. 1999. Parasites of North American Freshwater Fishes.

Images were made possible by a grant from the Maine Outdoor Heritage Fund.