OutdoorNebraska

5 Reproduction Most freshwater mussels are either male or female. The male produces sperm that he expels into the waterbody through the excurrent siphon. A female filters this sperm out of the water and uses it to fertilize her eggs. The fertilized eggs develop into a larval form called glochidea which are parasitic on fish. The ways that freshwater mussels can trick fish into range for the glochidea to attach are as many as there are mussel species. Now think about this for a minute. Most freshwater mussels live in flowing waters. While a few species can live in lakes or reservoirs, that is not where they evolved and it is not their preferred habitat. Buried in the bed of a flowing stream, the male releases its sperm. This sperm flows downstream with the current until it is picked up by a female. Now, if the female only released fertilized eggs or baby mussels, they would float some distance downstream before they could settle onto the stream bottom. In the long term, with this constant downstream movement, all of our freshwater mussels would end up in the ocean. Their survival depends on having some means of getting their progeny back upstream. Here is where those parasitic glochidia factor in. These are released into the water where they clamp onto the gills or fins of a suitable fish host. While not harmful to the fish, these glochidia will encyst within the fish's tissues and there develop into a juvenile mussel. After a few weeks, these juveniles drop off and bury themselves in the substrate where, if the habitat is favorable, they can now develop into adults and repeat the cycle. It is during those few weeks of parasitism that the fish has a chance to move upstream where the juveniles can recolonize upstream habitats. It is in this manner that freshwater mussels can sustain their populations in a watershed or colonize new watersheds. There appear to be a few species that are, or can be, hermaphroditic (both sexes). The literature indicates that three species found in Nebraska, the Paper pondshell, the Lilliput and the Creek heelsplitter, may be hermaphroditic. It has also been observed that in areas where mussel densities are very low, any species can self-fertilize.

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