Fathead minnows are characterized by deep, compressed bodies, typically five to eight centimeters in length, and a short head that is dorsally flattened with a blunt snout, round lateral eyes, and terminal, upturned mouth. With the exception of a dark blotch at the rostral end of the dorsal fin, their fins are generally clear. Fathead minnows are dark-olive colored with a dusky, dorsal and lateral stripe, and yellow to white underbelly. They have an incomplete lateral line, 8 dorsal rays, 7 anal rays, 14 to 17 pectoral rays, 7 to 8 pelvic rays, pharyngeal teeth, and smaller scales along the nape. Males and females vary in size, banding, and secondary sexual characteristics, however, males and females are virtually indistinguishable until reproductive maturity. Adult males range from 3 to 5 g, while adult females are slightly smaller, ranging from 2 to 3 g. Additionally, males have dark heads with 2 white to gold vertical bars posterior to the head and dorsal fin. Males also have a fleshy, dorsal pad and 16 nuptial tubercles on the lower jaw. Although females do not undergo such obvious changes in morphology, they do develop a fleshy ovipositor approximately a month prior to spawning.
Range mass: 2 to 5 g.
Range length: 5 to 8 cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: male larger; sexes colored or patterned differently; male more colorful; ornamentation