Up close, this tiny, pugnacious fish appears all head, mouth, and needle-like teeth. Its most outstanding characteristic is its enormous mouth, larger in males, that extends the full length of the head to the edge of the gill cover. It inhabits small abandoned burrows and shells, crevices among rocks, and is particularly fond of discarded cans and bottles. The fringehead rarely leaves its home and defends it fiercely, attacking anything that comes near, regardless of the intruder's size. When defending its home, whether a crack in the rocks or a beer can condominium, a fringehead can open its mouth so wide that its head all but disappears.
Adults are almost completely fearless and very ill-tempered.
Anything that approaches their territory can trigger animated threat displays, complete with flexing jaws, spreading of gill covers, and snapping of teeth. The display culminates in vicious, gaping-mouth attacks on the intruder. Spawning occurs January through August with females laying eggs, which are aggressively protected by the males until they hatch. Despite their ferocity, males have a tender side. They are devoted fathers, and will often collect and brood the eggs in their mouths to provide maximum protection.