Most gobies are extremely small; in fact, the smallest known vertebrate is a goby from Japan, no longer than 10 mm at sexual maturity. The largest, Gobioides broussenetii from the Caribbean, may reach 50 cm TL. Gobies are usually recognized by their small size, the existence of two dorsal fins (the first with eight flexible spines and the second soft), and a blunt round head with large eyes. Some gobies have prominent head barbells as well. Most gobies, and all freshwater species, have pelvic fins united to form an adhesive or sucking disc. However, some reef species have separated pelvic fins although the degree of separation is highly variable. The   scales may be cycloid, ctenoid, or absent and the lateral line is absent. (Click here to see a   fish diagram).

Coloration in gobies ranges from vivid, especially in reef species like the brilliantly marked neon gobies, to drab, as in many estuarine species (Bathygobius). Still others may be pallid or translucent (Coryphopterus). Although most reef gobies are sexually monomorphic in terms of permanent coloration and gross morphology, temporary sexual dichromatism (color differences between the sexes) has been observed during courtship and spawning on reefs and other habitats. When permanent   sexual dimorphism does occur, it may vary even within a genus. For instance, males in some genera, Lythrypnus and Coryphopterus, have longer dorsal and/or anal spines than females, but other species within these genera lack any morphological differences. Permanent sexual dichromatism also exists in some species but investigators have been unable to explain why there is such variation within genera.

Many gobies have evolved unique physical adaptations for life in tidal or estuarine environments. For instance, mudskippers, which span the genera Boleophthalmus, Periophthalmus, Periophthalmadon, and Scartelaos, are essentially amphibious. The skin contains numerous blood vessels enabling them to take up atmospheric oxygen and a muscular tail helps them to skip over the mud. Additionally, their eyes are perched high on the head to allow them to forage effectively and avoid predation. Another goby, Gillichthys mirabilis, has evolved a highly vascularized buccopharynx, which allows it to gulp air from the surface when the waters it inhabits become depleted of oxygen.

Other Physical Features: ectothermic ; bilateral symmetry

Sexual Dimorphism: sexes alike; male larger; sexes colored or patterned differently; male more colorful; sexes shaped differently; ornamentation