Biology of Finfish and Shellfish (2+1)

4.10.4. Glochidium

The glochidium larva is enclosed by two valves, each edge of which bears a hook. The shell valves cover a larval mantle, which bears four groups of sensory bristles. A rudimentary foot is present, to which is attached a long adhesive thread, the byssal thread. There is neither mouth nor anus and which measures from 0.1 mm to 0.5mm depending upon the species.

The glochidia of anodonta, immediately clamp into the fins and other parts of the body surface of the fish. The glochidia require certain species of fish as host; others can tolerate a wide range of host species and the byssal thread aids in initial contact and adhesion (Wood, 1974). The larval mantle contains phagocytic cells that feed on the tissue of host and obtain nutrition for the developing clam. The parasitic period lasts from 10-30 days. In the mean time, the parasite is surrounded by the over growth of skin of fish forming a cyst. Some of the larger freshwater mussels may produce as many as 3,000,000 glochidia.