Bluegill

The bluegill is noted for the large black appendage (the "ear") on each side of the posterior edge of the gill covers as well as the base of the dorsal fin. The sides of its head and chin are commonly a dark shade of blue, hence the name "bluegill". The precise coloration will vary due to the presence of neurally controlled chromatophores under the skin. The fish usually displays 5–9 vertical bars on the sides of its body immediately after being caught as part of its threat display. It typically has a yellowish breast and abdomen, with the breast of the breeding male being a bright orange. The bluegill has three anal fin spines, ten to 12 anal fin rays, six to 13 dorsal fin spines, 11 to 12 dorsal rays, and 12 to 13 pectoral rays. They are characterized by their deep, flattened bodies. They have a terminal mouth, ctenoid scales, and a lateral line that is arched upward anteriorly.

The bluegill occurs naturally in the United States east of the Rocky Mountains from coastal Virginia to Florida, west to Texas and northern Mexico, and north to western Minnesota, New York and southeastern Ontario. They have been introduced widely in North America and Europe, South Africa, Zimbabwe, Asia, South America, and Oceania. Bluegills have also been found in the Chesapeake Bay, indicating they can tolerate up to 1.8% salinity.

In turn, bluegill are prey to many larger species, including largemouth bass, smallmouth bass, striped bass, trout, muskellunge, turtles, northern pike, yellow perch, walleye, catfish, and even larger bluegill. Herons, kingfishers, and otters have also been witnessed catching bluegill in shallow water. However, the shape of the fish makes them hard to swallow. Raccoons are also believed to be among their predators.

Young bluegills' diet consists of rotifers, copepods, water fleas, and insects (mainly chironomids). The adult diet consists of aquatic insect larvae (mayflies, caddisflies, dragonflies), but can also include terrestrial insects, zooplankton, shrimp, crayfish, leeches, other worms, snails, and other small fish (such as minnows). If food is scarce, bluegill will also feed on aquatic vegetation and algae, and if scarce enough, will even feed on their own eggs or offspring. As bluegill spend a great deal of time near the surface of water, they can also feed on surface bugs. Most bluegills feed during daylight hours, with a feeding peak being observed in the morning and evening (with the major peak occurring in the evening). Feeding location tends to be a balance between food abundance and predator abundance. Bluegill use gill rakers and bands of small teeth to ingest their food.

Spawning season for bluegills starts late in May and extends into August. The peak of the spawning season usually occurs in June in waters of 67 to 80 °F (19 to 27 °C). The male bluegills arrive first at the mating site. They will make a spawning bed of six to 12 inches in diameter in shallow water, clustering as many as 50 beds together. The males scoop out these beds in gravel or sand. Males tend to be very protective and chase everything away from their nests, especially other male bluegills. Some bluegills, regardless of their small size, will even attack snorkelers if they approach the edge of the nest. As a female approaches, the male will begin circling and making grunting noises. The motion and sound of the males seem to attract the females. Females are very choosy and will usually pick males with larger bodies and "ears", making larger size a desirable trait for males to have.

The bluegill is the state fish of Illinois.

At Lake St. Helen, Michigan, an annual "Blue Gill Festival" is held in July.