Physical description varies widely with age and habitat. In general, threespine sticklebacks tend to be streamlined and less than 10 cm long (usually from 3 to 8 cm). Freshwater populations vary in body shape, depending on the habitat they occupy. Limnetic ecotypes tend to have slender bodies with narrow mouths, long snouts, and large eyes. Benthic ecotypes tend to be deep-bodied, with a wide, terminal gape.
The fish can have a robust set of spines, a pelvic girdle, and numerous lateral bony plates (up to thirty or more on each side), but the extent of these features varies by population. Dorsal and pelvic spines vary in number, placement, and length, and the spines tend to be longer in populations that co-occur with predatory fishes. The pelvic girdle consists of a bilateral structure with an anterior process that has an ascending branch on each side, a posterior process and a spine and fin ray. The abdomen is ringed in bony armor. Marine fish almost always possess a fully developed pelvic girdle and a full complement of bony lateral plates. However, many freshwater populations have reduced armor plates and pelvic girdles, and some populations have lost these features entirely.
Although body color also varies among populations, threespine sticklebacks are generally cryptic, with brown-to-green barring above and paler coloring below. As males approach reproductive condition, they become less cryptic, and their eyes become an iridescent blue. In some populations, red coloration may expand onto the flanks behind the pectoral fin.
Range length: 3 to 8 cm.
Average length: 5 cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes alike; sexes colored or patterned differently; male more colorful