To non-scientists, amphipods are known as “scuds.” These small, segmented animals belong to a group called crustaceans - the shrimp, lobsters and crawfish of marine and fresh-water systems. However, the most closely related crustaceans to “scuds” include isopods (sow bugs) and mysids (opossum shrimp).

Scuds are identified by their numerous segments and laterally compressed sides (the sides of their body are squeezed together). Particularly interesting to us is the genus Diporeia.

Scientists discovered that Diporeia within the Great Lakes was a new genus, evolved from Pontoporeia. The illustrations show the male Pontoporeia and female (bottom). Illustrations from Zooplankton of the Great Lakes by Mary Balcer, Nancy L. Korda and Stanley I. Dodson.

Diporeia is best described as fossarial - designed for digging and burrowing through the fine-grained sediments at the bottom of our northern lakes. Diporeia has relatively long antennae equipped with organs to detect vibrations that predators make while searching the sediments for food. Its legs are broad and powerful, which aids efficient digging.

This innocuous creature is usually less than 10mm in size. However, its concentration in the benthos (lake bottom) can be substantial. Surveys in Lake Erie in 1979 reported concentrations of 1,844 individuals per square meter of surface area.

Similar surveys around the Great Lakes confirm the same potential for great densities, but there are large variabilities in density. Remarkably, in recent years, several locations in Lake Michigan, which normally reported rich populations, have been found to be devoid of Diporeia.

There is concern that the absence of this detritus feeding organism may be a factor in the absence of yellow perch, predators that favor Diporeia for food. Researchers hypothesize that the exotic, introduced zebra mussels are consuming the majority of food that falls through the water column. Food then doesn’t get mixed into the fine benthic sediments, home of Diporeia.

While Diporeia is common to Lake Superior, forming an important part of the food web for many bottom-grubbing fish like burbot, suckers, spottail shiner and slimy sculpin, it is also important for some of our favorites, like young lake trout, cisco and bloaters. Even old squaw ducks rely heavily on Diporeia. The Lake Superior Binational Forum’s Ecosystem Principle and Objectives. Indication and Targets for Lake Superior identify Diporeia hoyi as a critical indicator of a healthy Lake Superior aquatic system.

Since Diporeia is a nonselective benthic feeder, it has the ability to concentrate toxins gathered in the sediments and pass them up the food chain.

Yet, probably the most distinctive feature of the genus Diporeia is that this little known group of animals is endemic to the more northern aquatic system of North America. It evolved into eight species found in our area without contact with the outside world. This was discovered in 1989, when the amphipod genus Pontoporeia was reviewed by taxono-mists (scientists who study organism relationships and their names).

Their conclusion is that the Great Lakes Pontoporeia was a new genus that evolved over a considerable period of time and not a relatively recent relic of a marine forefather trapped during the ice age. The new species was named Diporeia. Whatever the name, this almost microscopic and reclusive animal, Diporeia, is an important part of the Lake Superior and Great Lakes ecosystem and an important friend.

David D. Lonsdale is executive director of the Great Lakes Aquarium at Lake Superior Center, coming to that position from Chicago’s Shedd Aquarium. He regularly contributes a column on the science of Lake Superior for this magazine.The Great Lakes Aquarium is under construction and scheduled to open in mid 2000.