Scuds (North American Aquatic Invertabrate)

Scuds

Although ecologically important and common, the term “scud” is better known as a series of Soviet ballistic missiles perhaps just as significant in a different way. Scuds, or side-swimmers are freshwater invertebrates belonging to the order Amphipoda, with approximately 150 species found in North America (Voshell 248). They are primarily found in all kinds of shallow waters (McCafferty 389), but many live underground. Scuds are very important to the ecosystem (Voshell 248).

Without the tail or antennae, scuds are 5-20 mm in length when fully grown, and are more or less flattened from side to side (Voshell 248). The body is segmented and divided into three groups: the cephalothorax, thorax, and abdomen. The head and first segment following makes up the cephalothorax. Two pairs of antennae, mandibles (mouth parts used for eating), and two fixed compound eyes are located on the head. There are 7 more segments on the thorax and 6 on the abdomen, each with a pair of appendages (Voshell 249). Gnathopds are the pair of walking legs on the cephalothorax and first pair on the thorax; they are hinged and enlarged at the ends (Shimek). They are used for feeding and to assist in crawling (Voshell 250). Five remaining pairs on the thorax end with a simple pointed claw. The last three pairs of legs extend outward laterally (Shimek) and downwards, making them longer when viewed from the side. Gills are flat oval shaped sacs found at the base of each leg. Each abdomen segment also has a pair of shorter appendages. Scuds vary in color, but most are a creamy light gray or brown. (Voshell 248).

Scuds are benthic and primarily crawl as movement. This is done mostly with the simple legs, while the gnathopods assist in pulling, and abdominal appendages in pushing forward. They also swim just above the bottom and roll over while doing so. Side-swimmers keep away from bright light and when disturbed, will hide under loose stratum. The main source of nutrition comes from detritus, but scuds also feed on films of algae and bacteria. They'll eat any recently dead organism, and “readily feed on all types of plant and animal matter” (Voshell 250). Like all crustaceans, they have mandibles to chew food when eating (Goodman 494). Scuds usually reproduce in the spring in temperate climates and year round in continuously cool streams. The female will hold 15-50 eggs in a marsupium (pouch) under the thorax for one to four weeks. By this the time, the eggs will have hatched and larvae developed, so when released, look like small adult scuds. Release happens when the mother sheds her skin the first time after mating. It takes 8-9 moltings to reach adulthood. Most kinds reproduce once, and die within a year (subterranean one live for 4-6 years). (Voshell 251).

“Because of their abundance and voracious appetite, scuds are often important in the breakdown of organic matter” (Voshell 251). Scud habitats include streams, springs, seeps, lakes, ponds, and subterranean water. They live in tangles of plants, detritus, stones, and loose sediment. However, the