[Caitlin]

http://www.wnrmag.com/stories/2006/aug06/sponge.htm

August 2006

Curious creatures on the Big River

Sponges, shrimp and meat-eating plants dwell in the Mississippi.

Ruth Nissen

Freshwater sponges | Glass shrimp
Meat-eating plants | Mussels
Monsters of the deep | Paddlefish
American eels | Native lamprey

Mussels

Hickorynut. Deertoe. Threeridge. Pocketbook. Mapleleaf. Threehorn. Pimpleback. Pink Heelsplitter. These odd sounding names all belong to the 35+ species of freshwater mussels, frequently called “clams,” found in the Upper Mississippi River. The names describe the outer appearance of the shells, which range from smooth to bumpy and rippled with ridges. The pearly inside of the shells can range from pink to purple to white. This pearly covering, called nacre, played a major role in a long, rich history.

Records show that Native Americans ate mussels and used the shells for tools, utensils, tempering pottery and trade. European settlers “clammed” for freshwater pearls and cut the shells for pearl buttons. It took a phenomenal number of shells to supply the country with buttons. With the advent of plastic buttons, mussel populations started to recover until people discovered that small plugs of mussel shells were a perfect medium for seed pearls for the Japanese cultured pearl industry. In 1995, more than one million pounds of shells were removed from Wisconsin waters along the Mississippi River and shipped to Japan.

Mussels are equally woven into the web of river life. Freshwater mussels continuously pump water through their bodies to obtain food and oxygen. In the process, they filter and clean the water. Small mussels provide food for fish; older ones are a food source for mink, muskrats, otters, raccoons and turtles. Groups of mussels gather in mussel beds and can form a hard “cobble” on the bottom of the river that supports fish, aquatic insects, benthic algae and worms.

Fish and mussels have a relationship that extends beyond the food web. Mussel larvae or “glochidia” released into the water attach to fishes’ gills and remain implanted for a few weeks or a month before dropping off to colonize a new area. Certain mussel species have raised this hitchhiking to an art form. The pocketbook family of mussels, which includes the endangered Higgins eye, has developed body parts to look like a minnow complete with an eyespot. The mussel “fishes” this structure with an undulating motion that mimics a small struggling fish. When a bigger fish grabs for it, the female mussel expels her larvae into the face and gill area of the fish. Many mussel larvae only attach to specific fish species, others will hook up with a widespread variety of fish. Where host species are rare, mussel species in the river are jeopardized. Mussel populations are also threatened by overharvest, declining water quality and effects from converting the free flowing river into an impounded system of locks and dams.

At one time, mussels were so common in the upper Mississippi River that an 1867 report to Congress stated, “In places protected from moving sands, the mussels grew so abundantly as to accumulate beds of their shells at least two or three feet thick.” These beds were quite extensive. In 1896, nearly 500 tons of shells were taken for the button trade from a bed two miles long and a quarter-mile wide. More recently, one remarkable site, the East Channel mussel bed near Prairie du Chien, was described as the mother lode of mussels on the Mississippi River. It extended bank to bank over an entire 3.5 mile length and contained as many as 30 different species of freshwater mussels until it was decimated by zebra mussels.

Most people don’t realize the United States is a heavyweight of mussel diversity. All of Africa contains 56 species and most countries can claim only 20. The U.S. weighs in with about 300 different kinds of mussels. Sadly, they are also the most threatened order of animals in this country.