Jan Jeffrey Hoover
US Army Engineer Research and Development Center
Shrimp are typically thought of as marine animals, characteristic of Gulf waters, but there are also species inhabiting the fresh waters of the Mississippi River. The largest of these (60-100 mm) is the Ohio River shrimp, or simply “river shrimp” (Macrobrachium ohione).
Like its saltwater relatives, the river shrimp has well-developed walking legs with claws (chela) and a laterally compressed abdomen with well-developed swimming legs. It is distinctive, however, because of its elongate second walking leg bearing a slender claw (Fig. 1). This long-armed crustacean, usually unseen by the average river visitor, is common in the lower river and readily collected by those with special interests in aquatic invertebrates, including fishermen, conservationists, and aquarists.
The river shrimp once supported important artisanal food fisheries and is still fished locally for bait. Fishermen along the river used to harvest the fish as a source of “seafood” and the species was a “market staple” in Louisiana and as far inland as Illinois and Indiana. Fishing methods varied, but one technique was setting tied clusters of willow or cottonwood branches in shallow water near shore, then returning later with dip-nets to remove shrimp that had colonized the vegetation. Commercial fisherman Bill Lancaster has collected river shrimp for more than 20 years to bait his trotlines. To catch them, he uses traps made of hardware cloth, consisting of a wide entrance ramp narrowing into a 1-meter long dead-end funnel (Fig. 2). Shrimp are removed from the trap by opening a metal clip at the rear end to empty it. The shrimp are sufficiently abundant in the Lower Mississippi River that a few weeks of trapping can provide Bill with a year supply of bait or more.
River shrimp are also collected by biologists using trawls to document their abundance and geographic range which have declined in some parts of the country. They have been rare or absent from the Ohio River for 50 years and are no longer as common in the Middle Mississippi River as they were historically. Decreasing range and declining numbers are due, in part, to their unusual life history, which has only recently been fully understood by biologists who collect them by trawling. Eggs are carried by the females on their undersides of their abdomens, but the hatchlings, like those of their marine relatives, must develop in salt water. This means that the adult females must migrate long distances downriver, possibly hundreds of miles depending on their location. Modifications to rivers (like locks-and-dams) and to hydrology (natural rise and fall of water) have impaired migrations in parts of the country but the shrimp maintain a stronghold in the Lower Mississippi River. River shrimp may be seen on occasion negotiating these obstacles en masse (Fig. 3).
River shrimp make colorful but cannibalistic aquarium pets. A group of uniformly tan adults collected from the river, when placed in a decorated tank, will, in about a week, diverge in a spectrum of color – clear, green-brown, blue-violet, frequently with red highlights. This is due to their prominent pigment-dispersing structures called chromatophores (Fig. 4). Unfortunately, shrimp will also feed on their tank mates. After a shrimp sheds its skin, which it must do to grow, it is soft for a while, and the walking legs are tempting targets for other hungry shrimp. Cannibalism can be avoided, however, if plenty of hiding places and caves are provided, like small pieces of PVC, and if shrimp are well fed on a varied diet, like fish food pellets and flakes and frozen blood worms. When housed appropriately, the river shrimp is a hardy, lively, and entertaining symbol of the colorful but often unappreciated denizens of the lower river.