Saturday, January 19, 2008

Freshwater mussels


North America is home to almost 300 freshwater mussel species, with 71 on the Endangered Species List. Missouri, a land of clear, fast moving streams and rivers, hosts 69 species, with the highest diversity occurring in the Ozark Highlands. One of Missouri's mussels, Cumberlandia monodonta, the spectaclecase, is marching towards the federal list (they have feet!), what with the largest populations in the country found in two Ozark rivers-- the Meramec and the Gasconade.

Mussels live in a variety of habitats, from clear rivers to sluggish prairie streams. (When I lived in southeast Missouri, I found 8 species living in pesticide-rich ditches, partially buried in the silty substrate.) Some mussels require large boulders upon which they can attach, while others, like the heel splitter, can live in sandy soils without a rocky substrate. They feed on plankton and detritus which they are able to filter from the water. In fact, mussels have the ability to filter everything from the pesticide dieldrin to heavy metals, which makes them rather undesirable to the restaurant industry. It has been estimated that they can filter 100 gallons of water a day, keeping rivers free of bacteria, sediment and contaminants. When the sediment load in a river becomes too heavy, however, they suffer, unable to keep up. They are somewhat mobile animals, moving around gravels by implementing their foot, a strong muscle found at the base of the shell.

To reproduce, many mussels require the presence of a certain fish species. One, the winged mapleleaf (a federally listed animal), uses the channel catfish as a host. Others are generalists, using anything from bluegill to bass as a host fish. The reproduction of mussels is a fascinating process; they produce a lure that can look like a minnow, a crawfish, or some other invertebrate. Attracted by the lure, a fish will swim directly next to the mussel, at which time, the mussel releases thousands of glochidia, small immature mussels which attach to the fish's gills. There they will stay, not harming the fish, but growing to juvenile stage when they release their grip on the fish to settle to the bottom of the waterway. If you look to the right of your screen, you'll see a link to (the brilliant, fantastic, kind) Dr. Chris Barnhart's mussel webpage; on it, you can see videos of this process which he filmed in aquaria. Truly fascinating.

Considering that certain mussels require certain fish for breeding, the conservation of fish populations ties directly to conservation efforts for mussels. Clearly, without the fish, the mussel can't reproduce. With the disappearance of many darters in Ozark rivers, the mussels dependent on them will fade from the landscape.

Mussel populations were never in any real trouble until the end of the last century. Around 1889, a button manufacturer in Iowa harvested mussel beds near Muscatine, depleting a population. In the company's search for more raw material, the industry began traveling up and down rivers, harvesting other populations. As the button industry grew, so too did the demand for mussels. In 1896, 500 tons of mussels were taken from one mussel bed two miles long and a quarter mile wide.

Certain species were more valuable to the button industry than others: the ebony shell, mucket, slough sand shell, etc. These had a white, unblemished nacre and were of uniform thickness. By 1912, the Bureau of Fisheries determined that the profit afforded by mussels to the button industry totaled $6,173,486. The depletion of the mussel beds in American rivers was looming large on the industry; clearly, they were taking more than were reproducing. Unfortunately, with the increase in timber harvest and other land clearing operations, America's rivers were being bombarded with silt and sediment that choked mussel populations. Not to mention the overharvesting of certain fish species which mussels depend on and the general water pollution that comes with industrialization. All of these factors had immediate implications for the early 20th decline of mussels.

With the big river mussel populations harvested to near extirpation, the button industry moved their operations to smaller streams and rivers. In 1913, northern Missouri's Tarkio and Nodaway Rivers were dredged, leaving tons of mussels in the shallow backwaters. Today, no mussels are present in these rivers. It wasn't until the close of World War II when the introduction of plastics obviated the need for mussel collection by the button industry.

Mussel populations returned to some of the over-harvested rivers, thanks to the host fish populations which carried the glochidia. Many mussel populations, especially in the Mississippi River, have never recovered because of channelization, sedimentation and pollution. The best habitat for mussels remains free flowing rivers, free of sewage, feedlot runoff, mine tailings and other pollution. The construction of dams and lakes also destroys populations by cutting off the fish host source and reducing the food source. I think we're past the period of damming rivers and streams in Missouri, something which bodes well for our mussels. Too, our fish biologists now work directly with mussel biologists to identify hosts and implement protection measures.

Missouri is fortunate to have a stalwart group of malacologists (those who study mussels) who has dedicated their lives to the protection of these fascinating invertebrates. It's a small group, but each member is eager to get out into the field, to conduct research, to educate. Now, they're working diligently to protect the remaining populations of the spectaclecase. Historically, they were widespread, appearing even in the Niangua drainage. Knowing how talented and eager these men are to protect mussels, Missouri's populations have a chance for survival. If only other states could be so lucky.

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