A few years ago, I was involved with an ecology exercise for elementary students. We waded out into a lake, scooped up some of the substrate with pie plates, and observed all the little critters that were hidden among the substrate and overlying detritus. As you might expect, there were lots of little invertebrates such as crustaceans and insects. We then explained how these invertebrates are important components of the aquatic food chain, and as such are important food items for the game fish we so enjoy.
One of the more conspicuous groups of these little critters was what are commonly known as scuds. These scuds have no connection with scud missiles. These scuds are small crustaceans, close relatives of crayfish, shrimp, and lobster. Some people also know them as freshwater shrimp or side-swimmers, or perhaps Gammarus, the genus to which scuds belong.
Scuds are smaller members of the group of crustaceans called amphipods. There are around 10,000 species in the group. Most are marine, but around 20% are freshwater species. They may be found in a variety of aquatic habitats, but generally are found at or near the bottom of the body of water where they feed on detritus (dead plant and animal material). As such, they are important detritivores, reducing the buildup of dead material on the bottom of the body of water and returning the nutrients back into the food chain to perhaps eventually become the tasty flesh of bluegill, perch, or walleye.
Scuds are generally between a fourth of an inch to three-fourths of an inch long. They are whitish in color with perhaps some darker markings. They have two pair of antennae, gills, and eight pair of legs, with the front legs modified for grasping, and function as accessory mouthparts. The rest of the legs function in swimming and creating water currents to flow over their gills. Some of these legs are oriented forward while those further back are oriented backwards.
Ice anglers occasionally will see these scuds come swimming up the holes in the ice. They are also occasionally observed when fishing during the other seasons. The abundance of these scuds is an indication that there are probably some happy, well-fed fish in the body of water. Moreover, if all goes well, the anglers will happily enjoy a meal of freshly caught fish.
Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Dakota College at Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.