INTRODUCTION: THE FATHEAD MINNOW

General Description, Biology, Habitat, Distribution:
The fathead minnow Pimephales promelas Rafinesque is a member of the fish family Cyprinidae, the largest family of fish with more than 2,000 species worldwide and nearly 300 extant in North America (Jenkins & Burkhead, 1994). Etymology of the minnow's name derives from the shape of the male's head (Pimephales - "fat head") and it's color in breeding males (promelas - "forward" and "black") (Mettee et al., 1996). The "fat head" during breeding is primarily attributable to the proliferation of epithelial cells along its anterior dorsum. Adult minnows have a moderately compressed body, a short, blunt snout, and a slightly subterminal mouth. Colors are olive to brown on the upper body and silvery-white on the lower body with a dark midlateral stripe. Adults range from 40 - 100 mm TL and display strong sexual dimorphism. Nuptial males tend to be larger than females with horny tubercles on the snout and a prominent pad of spongy rugose tissue on the nape (Jenkins & Burkhead, 1994; Mettee et al., 1996).

Fathead minnows typically inhabit the pools and backwaters of small creeks but are occasionally found in larger streams, ponds, and lakes. They generally stay near the shoreline or close to weedy cover. Adults are omnivorous eating insects, algae, detritus, and microcrustaceans (Jenkins & Burkhead, 1994). Fatheads originated throughout the Midwest and upper Mississippi River drainage, west to Utah, north to Canada, and east to Maine. Popularity as a bait minnow has led to fathead introductions throughout the United States resulting from bait-bucket release or hatchery escapement.

Maturity is reached in four to six months at a size of approximately 40 mm TL for females and 48 mm TL for males. Spawning occurs naturally from May through August (15EC to 32EC) in calm shallows of streams and along shorelines in ponds. Males select and prepare the nest site defending it from intruders and only allowing persistent females to enter and spawn. Nests are typically located on the undersurface of submerged stones or branches. Females spawn fractionally every 2 to 16 days producing up to 10,000 eggs in a three month breeding season (Gale and Buynak, 1982). Males protect eggs from predation and cannibalism and fan egg masses with caudal and dorsal fins to increase oxygen availability. The spongy nape pad, used for scrubbing the nest site, has taste buds capable of chemosensory assessment of egg condition and is believed to produce substances with fungicidal properties (Jenkins & Burkhead, 1994). Eggs hatch in 4 to 10 days depending on temperature. The combination of high fecundity and parental care stands in contrast to normal aspects of the r- and K-selection model (Jenkins & Burkhead, 1994).

The fathead minnows is unrivaled in toxicological research concerning the effects of pollution on freshwater resources. Tolerance of adverse conditions and ease of spawning make the fathead minnow ideal for laboratory culture. Brood stock can be maintained in spawning condition year-round ensuring a constant supply of larval fish for toxicity testing purposes. Standardized tests of acute (USEPA, 1993), short-term chronic (USEPA, 1994), early life-stage (ELS)(ASTM, 1993), and life-cycle (Benoit, 1981) toxicity have been developed for assessment and regulation of effluent discharges.

Female (left) and male (right) fathead minnow, Pimephales promelas.
The male is illustratedin breeding condition with rostral tubercles.
Illustrations by Joseph Tomelleri (with permission).