The greenside darter (Etheostoma blennioides) is a species of freshwater ray-finned fish, a darter from the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches. It inhabits swift riffles in the eastern United States and southern Ontario.
The greenside darters have an elongated body with a long and rounded snout. The dorsum is greenish-brown, with six or seven dark quadrate saddles and the sides with five to eight dark green, typically U- or W-shaped blotches. The nape, cheeks, opercle, and belly are completely scaled, with the breast naked. The anal fin has six to 10 rays (usually eight) and 13-16 pectoral fin rays, and both are bright green in breeding males; caudal fins are yellowish to clear; dorsal fin rays number 12-15, with red basal bands; breeding males have intensely bluish-green nasal and oral areas and sometimes black on the head.
The E. blennioides subspecies are believed to have diverged in separate drainage systems and glacial refugia during the Pleistocene ice ages, which destroyed older connections and shaped new river systems.
Greenside darters inhabit gravel riffles of large creeks to medium rivers and often are found in swift waters over large boulders and large rubble.
Greenside darters typically live for three to five years, grow to a maximum of 5.2 inches (132 mm) standard length and sexually mature at one to two years of age. Greenside darters are reproductively active from February to April in the Midwest and Southeastern United States. Spawning occurs over algae- or moss-covered rocks in deep, swift riffles that are guarded by males that vigorously defended against intruders. Females linger in pools below the riffle and move into a male's territory when ready to spawn. Eggs are viscous and stick together in small clumps on green algae (Cladophora), moss (Fontinalis), and riverweed (Podostemaceae). E. blennioides is insectivorous, with analyzed gut contents containing Nematocera larvae, more specifically black fly and midge (Chironomidae) larvae. Significant numbers of mayfly (Ephemeroptera) and caddisfly (Trichoptera) nymphs were also found.
