Clarke Garry is a professor of biology at the University of WIsconsin - River Falls.

It's typically a warm fall day when I'm leading my entomology class down to the Kinnickinnic, D-nets in hand, for the collecting portion of our riffle-based biotic indexing lab series. Many of these students have heard about the mayflies, stoneflies, and caddisflies they might encounter, but they don't yet appreciate the fact that the greatest percentage of any single species or group they will be collecting will be scuds. The idea that scuds can be ". . . unbelievably abundant" (Pennak 1978) or ". . . startlingly abundant" (Borger 1980) is accurately descriptive for a large part of the Kinni watershed year around.

This uniquely common macroinvertebrate in the Kinnickinnic River and its tributaries is Gammarus pseudolimnaeus (Crustacea:Amphipoda), commonly known as the scud, sideswimmer, or freshwater shrimp. The species was named by Bousfield when he revised the freshwater amphipods inhabiting previously glaciated regions of North America (Bousfield 1958). G. pseudolimnaeus is widely distributed throughout the Great Lakes region and it is likely that our location in west-central Wisconsin is near the northernmost edge of its distribution.

G. pseudolimnaeus appears to be the single species of scud in the Kinni, as supported by examination of ~800 specimens acquired in my 1999 macroinvertebrate survey of the river. All 17 river collection sites sampled over a nearly complete calendar year from Kinnickinnic State Park to north of Interstate 94 yielded specimens of this species. Additionally, in biotic indexing sampling that I carried out in the South Fork between March and May of 2000, this scud composed up to 90.2% of macroinvertebrates collected (Garry, unpublished data). By the way, for use in his biotic indexing (BI) protocol, Hilsenhoff (1987) provides G. pseudolimnaeus with a tolerance value of 4. On a ten point scale (0=excellent, 10=very poor) a hypothetical BI collection of 100% of this species would rate as "Very Good" and be representative of water with only "possible slight organic pollution."

As an amphipod crustacean, this creature appears quite different from the many and diverse insects of the Kinni. The shrimp-like body is arched (when not swimming) and laterally-flattened. Two pairs of antennae extend forward from the head, and nineteen serially-arranged, paired appendages are apparent. These include six pairs of mouthparts, two gnathopods (grasping legs), five pereopods (crawling legs), three pleopods (swimming legs), and three uropods (more-or-less-fixed abdominal legs). Technical identification of this species requires, among other things, enough magnification to see the concave posterior margin of the base of the 5th pereopod (crawling leg) and long setae (bristles) also along this margin (Holsinger 1972). Scuds appear in a variety of colors based on gray or tan; sometimes they are subtly orange, purple, blue, or green. Pennak (1978) proposed that color may be dependent on diet, temperature, or age of the individual.

Scuds are fast swimmers often seen dashing from place to place in shallow water. The very name "scud" (v. i., to move or run swiftly) suggests this quick movement. Scuds inhabit benthic vegetation and debris in shallow water and are therefore readily collected by kick sampling. Pennak (1978) comments that as a group, ". . . the amphipods are cold stenotherms, strongly thigmotactic, and react negatively to light." This translates as adaptation to a narrow, cold temperature range, an instinctive need to be in contact with a substrate, and hiding behavior in vegetation, debris, and stones during daylight. When collected live and brought into the lab, they readily position themselves under any cover present, and only occasionally are observed darting from one hiding location to another. They are active at night in the stream and their presence as part of the nocturnal behavioral drift phenomenon has been well documented (Waters 1972). Scuds are occasionally observed with the males carrying the females on their backs, a behavior recognized as pre-copulatory pairing. This is, of course, a great conversation starter when samples are brought in to a local 4th grade class as part of an aquatic biodiversity demonstration!

Scuds are omnivorous scavengers. They are, in turn, preyed upon by insects, amphibians, birds, and fish with the latter being their chief predators. McCafferty (1981) notes that scuds are an important food source for fish and Borger (1980) comments, in reference to their large numbers, that it's ". . . little wonder they're fed upon by trout."

References

 

Borger, G. A. 1980. Naturals, A Guide to Food Organisms of the Trout. Stackpole Books, Harrisburg, PA, 223 pp.

 

Bousfield, E. L. 1958. Fresh-water amphipod crustaceans of glaciated North America. Canadian Field-Naturalist 72:55-113.

 

Hilsenhoff, W. L. 1987. An improved biotic index of organic stream pollution. Great Lakes Entomologist 20:31-39.

 

Holsinger, J. R. 1972. The freshwater amphipod crustaceans (Gammaridae) of North America. U. S. Environmental Protection Agency, Water Pollution Control Research Series 18050 ELD, 89 pp.

 

McCafferty, W. P. 1981. Aquatic Entomology, The Fishermen's and Ecologist's Illustrated Guide to Insects and Their Relatives. Science Books International, Boston, 448 pp.

 

Pennak, R. W. 1978. Fresh-water Invertebrates of the United States. John Wiley & Sons, New York, 803 pp.

 

Waters, T. F. 1972 The drift of stream insects. Annual Review of Entomology 17:253-272.