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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Sponges are colonial animals found growing on the river bottom or submersed objects. © Doug Stamm
Sponges are colonial animals found growing on the river bottom or submersed objects.

© Doug Stamm

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

Here, the habitat varies from quiet backwaters with lush vegetation to busy channels with constantly rushing currents. Natural conditions mix with manmade changes that created locks and dams dividing the Upper Mississippi River into a series of navigation pools. River reaches just below the dams are tangles of sloughs and side channels bordered by wooded islands, while the sections just above the dams tend to be wide-open shallow lakes.

This rich mix makes the Mississippi home to an incredible variety of plants and animals. Some are well known – like the migrating eagles in spring and tundra swans in fall. Others, like healthy populations of walleyes, catfish, bass and bluegills, attract their own followers. Still other interesting creatures and plants live quiet lives in peaceful oblivion far from the crowds of anglers, birders and summer tourists. These river residents range from the very small to the very large, but all are unexpected finds just waiting to be explored and discovered anew.

Freshwater sponges

Sponges not only mop up fresh water, they live in it! On the Mississippi, the most common sponges look like brown growths or fibrous algae on submersed objects. Other white ones with fingerlike projections resemble coral on the river bottom. They vary in size from one- to four-inch irregular shapes and are tough enough to be picked up without falling apart. Sponges are colonial animals, masses of cells embedded in a gelatin-like goo that is bound together by tiny, spiny structures of calcium, silica and organic fibers. Sponges filter large volumes of water through their pores, capturing tiny particles for food.

Glass shrimp

Hidden among lush vegetation in slow moving backwaters are small, delicate crustaceans that closely resemble marine shrimp. They grow to almost two inches long, but are easy to overlook because they stay hidden and their bodies are transparent except for the eyes. These glass shrimp, mainly found in the central and southern U.S., also inhabit the Mississippi, St. Croix and Wolf rivers at their northernmost range. Glass shrimp prefer the slowly moving parts of river where they can hide in dense vegetation to avoid becoming desirable morsels for small fish.

Just like marine shrimp, these freshwater prawns have a large spiny protrusion that sticks in front of the head and their bodies are flattened sideways. Glass shrimp can have a tinge of color in hues of pink, yellow or tan, which can give them an opalescent appearance. At times, their abdomens may appear green after eating plants. Freshwater shrimp walk on their first five sets of legs and use five sets of swimmerets for swimming and tightly tucking green eggs under their tails. They spawn from late May until August. Eggs are carried by the female for 12 to 16 days. After hatching, the young grow rapidly for about three months. They are active during winter and growth resumes in April. The adult shrimp population may die-off as year-old males are scarce by late June and few females are found by August.

Meat-eating plants

River shrimp are wary of another hungry floater in the water column – the carnivorous, rootless bladderwort. More than 200 species of terrestrial and aquatic bladderworts are found across every continent except Antarctica. They frequent marshy areas and calm waters with little current. Eight species of bladderwort are found in Wisconsin; three are species of special concern. The most widespread species on the Mississippi is, appropriately, the common bladderwort. Its most noticeable features are bright yellow flowers resembling snapdragons that rise above the water surface on slender stalks during the summer. Branching underwater stems have many finely-divided leaves and small buoyant sacs (bladders) with a unique adaptation for capturing prey. Each bladder has a trapdoor with special trigger hairs near the lower edge. When an aquatic bug brushes one of the trigger hairs, the plant pumps out creating a vacuum and the trapdoor swings open. Water rushes in pulling the bug with it. The trapdoor slams shut in less than half a second. The trapped insect is digested, then water is again pumped from the bladder and the trap is reset in as little as 15 minutes.

Bladderwort prey include water fleas, mosquito larvae, glass shrimp, scuds and tiny insect larvae. Common bladderwort can feed on fish fry or newborn tadpoles. A common bladderwort may have thousands of active bladders on a single plant and can thrive in nutrient-poor waters as long as there is insect life.

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.

Monsters of the deep

Big waters breed big whoppers about the lunkers that lurk below, but sometimes the tall tales are true! Just ask Kyle Schauf of Onalaska. While fishing in mid-March for walleye below Lock and Dam 7 near La Crosse, one very big fish hit his minnow.

"We were in a 13-foot boat and that fish pulled us wherever it wanted to go," Schauf said. He was pretty sure he'd hooked into a big catfish, but after battling 30-40 minutes to beach the behemoth, Schauf discovered he had latched into a five-foot lake sturgeon. Heidi Kuehler, a U.S. Fish and Wildlife Service fisheries resources biologist at Onalaska aged the fish at 35-40 years old. As happy as Schauf was to land it, he was just as glad to see the big fish swim off upon release.

Lake sturgeon is but one ancient fish species that cruised the Mississippi unchanged for more than ten million years. Other members of this elite club of odd looking fish include shovelnose sturgeon, paddlefish, bowfin, and the long- and short-nosed gar.

Sturgeon populations have not recovered on the Mississippi River from overharvest in the early 1900s. Pollution, habitat loss, accumulated silt on gravel spawning bars and the construction of dams that block seasonal movements have taken their toll. There are many unknowns about the Mississippi River population including details of their abundance, movement, spawning areas and whether the population is self-sustaining.

Shovelnose sturgeon, the smaller cousin of the lake sturgeon, are doing much better surviving the changing river conditions. Named for their long, spade-shaped snout, the shovelnose reach a maximum length of 38 inches. Their other common name, "hackleback," refer to a dorsal ridge of bony plates. Though found throughout the drainage basin, this species is most common south of Pool 7. Their population is large enough to support an open fishing season and a commercial fishery. "There is a demand for delicious smoked sturgeon, but the prize is their roe that is sold as hackleback caviar, selling for $15 an ounce," said Patrick Short, DNR Mississippi River Fisheries Manager at Prairie du Chien. The jet black eggs of medium size have a superb sweet, buttery, nut-like flavor. Prices in the last few years have risen rapidly as the supply of Caspian Sea caviar has diminished.

Paddlefish

Paddlefish, aka spoonbill catfish, duckbill catfish, and spadefish, all refer to the unique paddle-like rostrum that extends from the fish's head like a nose. It's used to sense food, which for the paddlefish is zooplankton, a microscopic meal. The rostrum and sides of the paddlefish's head have electro-receptors that can sense living organisms in dark murky water. To feed, the paddlefish merely swims with its mouth open and filters or strains zooplankton through comb-like gill rakers. The paddle helps the fish keep its balance while swimming with an open maw through river current. These docile, toothless giants grow to more than five feet and weigh more than 60 pounds. Paddlefish don't begin reproducing until they are at least seven years old and the females only spawn every two to three years. Paddlefish require a gravel or rubble bottom in six to 40 feet of flowing water with temperatures between 50 and 68 degrees to begin spawning.

Populations of docile paddlefish have declined since the early 1900s because of demand for their caviar. © Mark Steingraeber
Populations of docile paddlefish have declined since the early 1900s because of demand for their caviar.

© Mark Steingraeber

Populations have drastically dropped since the early 1900s, due to habitat changes and harvest for their prized caviar. Paddlefish are still found in the tributaries and channels of the Mississippi and the Missouri rivers. The largest populations in Wisconsin live in the Wisconsin River below the Prairie du Sac dam and in the lower Chippewa River. They also inhabit the Mississippi, Black, St. Croix, and Minnesota rivers. In both Wisconsin and Minnesota the paddlefish is listed as a threatened species.

An extensive study in the 1990s examined the movements and range of this declining species. Spawning locations, wintering waters, age studies, mating patterns and seasonal migrations were plotted. "Both the Wisconsin and Chippewa river fish move downstream from wintering areas to suitable spawning sites." said Ann Runstrom, fisheries biologist with the U.S. Fish and Wildlife Service. "After spawning, the Wisconsin River paddlefish return upstream to just below the dam at Prairie du Sac." The Chippewa River fish, however, keep moving down to Lake Pepin in Pool 4 of the Mississippi River, which has an ample food supply. In late June, they begin to move back up the Chippewa River.

American eels

These eels are the Mississippi River's true long-distance travelers, moving thousands of miles to get here. These fish, which look like a snake with ribbon-like fins, are catadromous, meaning they spend most of their lives in freshwater but return to the ocean as adults to spawn – just the opposite pattern of salmon.

Eels hatch in the Sargasso Sea between Bermuda and the Bahamas. Eel larvae are tiny, transparent and look more like willow leaves than fish. They gradually drift to the edges of the Sargasso Sea and are caught up in the currents. After drifting for a bit more than a year, the larva, called glass eels, arrive off our coastal waters of the Eastern Seaboard and Gulf of Mexico. As "elvers," they rarely grow more than 3.5 inches long. Only the females journey up the Mississippi River. Males remain near the mouth of the Mississippi south of New Orleans where it empties into the Gulf of Mexico There is no evidence that elvers imprint to any particular stream.

To reach our part of the river, the small eels have to get past at least 20 locks and dams that limit the movement of other migratory fish like skipper jack herring and smallmouth bass. The eels migrate upstream at night, climbing and crawling up the sides of the dam. Young eels get around fast currents by leaving the water and crawling over wet grass until they have passed the swift sections, sometimes traveling over flooded or dewy fields, and turn up eventually in a pond or lake with no apparent access. This upstream movement may take years. This "yellow eel" phase can last 5-20 years. Adults can grow to three feet long and weigh three to four pounds. As eels sexually mature, they undergo internal changes to survive the salty ocean environment. Their yellow-green colors change to a metallic sheen with purplish black backs. Called "silver eels" at this stage, they are better camouflaged from oceanic predators.

The females migrate back down the Mississippi River to the Gulf of Mexico, generally in late summer to November, joining the males at the Gulf. Together, they begin a final three-month journey back to the Sargasso Sea. American eels spawn in the western part of the Sargasso Sea, peaking between February and April. Soon afterward, the silver eels die where they were hatched. Their offspring then begin this cycle anew.

While many Americans view them as "trash" fish, eels are highly prized food in many parts of the world. Once abundant in the Mississippi River and the Great Lakes Basin, this fish is now considered uncommon.

Native lamprey

Lamprey are generally viewed as repulsive or loathsome. The adults, resembling snakes, are coated with slime and their sucking discs rasp into the sides of fish. They are not true eels, which have jaws, a mouth and scales. Lamprey are scaleless, have cartilaginous bones and their sucking discs are armed with horny teeth. Lamprey species are distinguished by their teeth patterns. Five species occur in Wisconsin, of which three are parasitic. The most notorious, the sea lamprey, is an exotic species that invaded the Great Lakes in the 1930s. Fortunately, they are not found in the Mississippi River, but the parasitic silver and chestnut lamprey do occur here.

Adult lamprey deposit eggs in stream riffles, then die. Larvae burrow into silt and sand in quiet water, filter-feed, and grow for three or more years. Parasitic lamprey migrate downstream to prey on large fish. They are not viewed as a threat to Mississippi River fish survival due to their small size and small populations. Some of the river fish will jump out of the water, falling with a crash to try and dislodge the irritating lamprey.

The fascinating adaptations of these little known species only make them more distinctive members of the Mississippi River community. Changes people have made during the last 150 years for navigation and flood control have not been kind to species that were better adapted to a free flowing river, and some species face threats from exotic species. We hope through river restoration that we can conserve more of these vulnerable species and they will continue to be a part of the Mississippi's future.

Ruth Nissen is stationed at DNR's La Crosse office on the Mississippi River.