GLOCHIDIAL SHELL OF PYGANODON CATAROCTA. NOTE TWO VALVES, EACH WITH SPINY HOOKS FOR ATTACHMENT TO FISH HOST (SEM photo by C. Bradford Calloway). Scale = 35 microns.
hookless forms clamp to the gill filaments of fishes (E), being carried in the respiratory water of the latter. In a few hours either type is covered with a capsule formed by migration and mitosis of cells of the host's epithelium. The parasitic larvae feed and grow by absorbing nutrients from the host's body fluids. Later the cyst weakens, the young mollusk opens and closes its valves, extends its foot, and escapes to the bottom to become free-living. In some clams (Quadrula, Unio) breeding occurs in summer (May to August); and the glochidial stages last 10 to 70 days depending on the species and water temperature. Others (Anodonta, Lampsilis) produce eggs in late summer, and the glochidia are retained in the female until the following spring. Individual fishes in nature may carry up to 20 glochidia, but a fish 3 or 4 inches long mayu be artificially infected with several hundred that will grow to metamorphosis. The glochidial stage serves to disperse the young over a wide area. At times glochidia in hatcheries cause losses among trout.
Excerpted from: Storer, T.I. 1951. General Zoology, 2nd edition. McGraw-Hill Book Company, Inc., New York.