Reproductive Biology of the Flat Floater Mussel, Anodonta Suborbiculata, Say, 1831 (Mollusca: Unionidae)
The flat floater mussel, Anodonta suborbiculata , is a large, distinctive freshwater bivalve that formerly occurred in shallow lentic habitats throughout the Mississippi River Drainage. The flat floater is presently rare due to habitat loss. Several aspects of the reproductive biology of this species were investigated in a population occupying a small impoundment (Shannon Lake) near the Marais des Cygnes River in Linn County, KS. Flat floaters in Shannon Lake appear to be gonochoristic (separate sexes). Spawning occurred between mid-September and mid-October in 1995, and most glochidia were released before the end of February in both 1995 and 1996. Drop-off of most juveniles from fish hosts occurred between March 17 and April 18 in 1996. Natural glochidial cysts of A. suborbiculata were observed on the fins of gizzard shad, white crappie, largemouth bass, bluegill, golden shiners, and freshwater drum, but not on common carp, nor black bullhead. Host suitability was confirmed by observing the transformation of glochidia experimentally placed on white crappie, largemouth bass, warmouth, golden shiners, and mosquitofish. The fate of flat floater glochidia during the parasitic phase was monitored at 10, 15 and 20 °C on golden shiners. Mortality of attached glochidia was significantly higher at 20 °C than at 10 or 15 °C. Mean transformation time decreased from 42.6 days at 10 °C to 9.3 days at 20 °C. Flat floater glochidia transformed readily in cell culture medium and survived several weeks in the laboratory after transfer to water. Transformation success in vitro was tested in relation to temperature, pH, PCO₂, and the presence of HEPES buffer. Mortality of glochidia in vitro was significantly higher at 10 °C than at 15 or 20 °C. Transformation success was significantly higher at lower levels of pH and higher levels of CO₂. However, reasonably high transformation success was observed in the absence of elevated CO₂. HEPES buffer had a negative effect on transformation success.
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