Herrings and shads, as a group, are silvery, flat-sided fish, easily recognized by the row of sharp-edged, spiny scales (or scutes) along the midline of the belly. These scutes are readily apparent when you rub your fingers forward along the fish’s belly.
The gizzard shad is a silvery, moderately deep-bodied herring with a large lustrous dark or purple spot (may be faint in adults) just behind the upper end of the gill cover. The tail is deeply forked, the belly keel-shaped. The last ray of the dorsal (top) fin is prolonged into a long, slender filament. Anal fin rays usually number 29–35. The mouth is small, the lower jaw not projecting beyond the tip of the snout. Lateral line scales usually number 55 or more. In living specimens, the tail fin is not yellow.
Upperparts are silvery blue, grading to silvery white on the lower sides and belly. The upper sides have several horizontal dark streaks. There is a large, lustrous purple spot just behind the upper end of the gill opening, but this spot may be faint or absent in large adults. The fins are dusky, without prominent yellow colors.
Similar species: Four species in the herring family are recorded for Missouri:
- The threadfin shad (D. petense) is most similar to the gizzard shad. Both have the last ray of the dorsal fin elongated into a long, slender filament, and both have a dark spot behind the upper end of the gill opening. However, the threadfin shad has the lower jaw projecting beyond the tip of the snout; its anal fin rays usually number 20–25; its lateral line scales number 50 or fewer; and in life, its tail fin is bright yellow. Also, threadfin shad are generally smaller, rarely exceeding 6 inches in length.
- Both the skipjack herring and the Alabama shad may be separated from the gizzard shad and threadfin shad by their lack of an elongated, filament-like last dorsal fin ray and their lack of a dark spot behind the upper end of the gill opening. Also, the principal rays of the dorsal fin usually are 16 or more.
Members of the herring family might be mistaken for the mooneye and goldeye (which are in a different family), but herrings have the following key characters: The dorsal fin is far forward of the anal fin. The head is without scales, but the body is covered with thin, smooth-edged (cycloid) scales that are easily dislodged. The lateral line is absent. A small, triangular projection (an axillary process) is present just above the base of the pelvic fin, and the eyes are partly covered by transparent membranes (adipose eyelids).