The freeloading lifestyle of fresh water mussels

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Conglutinates of the Ouachita kidneyshell – Chris Barnhart

I study oystery things. In my little myopic scientific snowglobe, I know a few things about shellfish, and I know a few more things about oysters. Then I know the most things about oyster filtration. So there’s still plenty of room for surprise.

This very thing happened this year during a local American Fisheries Society (AFS) meeting where I heard a talk about freshwater mussels. Probably because I always seem to be mucking about in briny water rather than its fresher counterpart, I was rather taken aback learning that many of these species have parasitic larval stages.

After females collect sperm that males eject externally, they fertilize their eggs and stow them in their gills where they develop into a minuscule larval stage called glochidia. These juvenile mussels cannot fully develop however until they somehow reach a host fish. They will encyst themselves into the host’s tissue where they will stay until more fully formed, at which point they will drop off and settle on the river floor. The host fish has graciously and perhaps unknowingly provided the small creature with protection and dispersal.

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The strangest detail of this whole process seems to be the intricate tactics mussels have developed to get their little parasitic spawn into hosts. Some species concentrate their glochidia into structures called conglutinates that they then release into the water. Many resemble prey items attractive to fish like in the video  and picture below:

Others, like mamas in the Lampsilis family keep their little ones closer to them while dangling parts of their mantle tissue to the same affect at the conglutinates described above.

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Lampsilis’ display attracts host fish – Paul L. Freeman, Nature Conservancy

For most species of freshwater mussel, the choice of host fish seems to be relatively specific. In some cases, the species of host has yet to be discovered, which provides fertile ground for research into the topic such as the work that Florida Wildlife Commission’s Blackwater Research and Development Center in Holt, Florida has done.

As with other symbiotic relationships within nature, freshwater mussels are incredibly dependent on the health of their host and the system around them. This has further increased the need for continued research and conservation, and in some instances agencies and institutions have fostered cultivation and propagation efforts.

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