FRESH WATER MUSSEL REPRODUCTION



Most mollusks set their eggs and sperm free, but the fresh water bivalves of the Family Unionidae have a peculiar mode of reproduction. In a female the ripe eggs pass from the ovaries to the suprabranchial chambers, and there are fertilized by sperm discharged from a male and brought in water entering the female (A). The eggs become attached by mucus within the water tubes of her gills, which enlarge as brood chambers (marsupia) (B). In some species all four gills are used, in others only the outer two or parts of these. Each egg, by total but unequal cleavage, develops into a minute larval glochidium (C), 0.1 to 0.4 mm wide, with two valves closed by an adductor muscle and a long larval thread; in some (Anodonta, etc.) the valves bear hooks ventrally, but others (Unio, Quadrula, Lampsilis) lack hooks.

The larvae are shed into the water through the female's exhalent siphon, then sink to the bottom or are scattered by water currents. They can open or close their shells but cannot move independently. Hooked glochidia attach to soft exterior parts of fresh water fishes (D) resting on the bottom;

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.