(figure by Chris Todd)
(Click on the picture to get a live photo of the various stages)
The reproduction of freshwater mollusks from adult to an adult occurs in a variety of stages. The first stage is the water column in which the sperm flows with the water current until a female siphons the water, collects the sperm and fertilizes her eggs.
After fertilization takes place, it may take 1 to 10 months for the larva, glochidia, to fully develop within the female. When the glochidia are fully developed in early spring or fall, they are released into the water where they drift until they find a suitable fish host. Unionaceans are divided into two distinct behavioral groups depending on when the female disperses the glochidia, bradytictic or tachytictic (Watters 1995). Bradytictic (long term breeders) hold their larvae throughout the winter until the following spring or summer (Watters 1995). The tachytictic (short term breeders) release the larvae later the same year, usually by July or August (Watters 1995).
Timing of the dispersal of glochidia from the female's body for finding a fish host is a factor for the survival of the glochidia primarily because the larvae cannot survive a long period of time outside of the female's body. Some species of freshwater mollusks have adapted to this problem in that they help to ensure a fish host by evolving an enlarged mantle. This enlarged mantle resembles a food source, i.e. worm, insect larvae, or even a small fish; the mantle attracts a predatory host fish towards the freshwater mollusks. As the predatory host fish goes closer to investigate the attractant, the freshwater mollusk will blow its contents, the glochidia, at the predatory host fish. Thus, the glochidia will attach to the predatory host fish. Other species bind a number of glochidia into long mucus matrices called conglutinates (Watters 1995). These conglutinates also resemble a food source for the fish host and are eaten (Watters 1995). As the conglutinates are eaten, the glochidia attach themselves to the host's gills.
The purpose for the fish host apparently arose as a means of dispersal for the unionids (Watters 1995). The larvae act like a parasite to the fish host; they attach themselves to the fish's gills or fins, and feed on them. This causes the cells on the host tissue to lyse (Watters 1995). The fluid from the lysed cells provides an essential part of the nutrients needed for the glochidia to survive (Waters 1995). This does not cause any problems or stress to the host fish. Some freshwater mollusks depend on a single fish host while others need many different species throughout their transformation from a larvae to a juvenile mussel. This transformation from glochidia to a juvenile mussel takes approximately one to four weeks.
After the glochidia have grown and transformed into a juvenile, the juvenile falls off of the host fish and eventually attaches itself or burrows into the substrate in some fashion and begins to filter feed.
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