The fish family Psychrolutidae contains over 35 recognized species in 8 genera. This family consists of bottom-dwelling marine sculpins shaped like tadpoles, with large heads and bodies that taper back into small, flat tails. The skin is loosely attached and movable, and the layer underneath it is gelatinous. The eyes are placed high on the head, focused forward closer to the tip of the snout. Members of the family generally have large, leaf-like pectoral fins and lack scales, although some species are covered with soft spines. This is important to the species as the depths in which they live are highly pressurized and they are ambush/opportunistic/foraging predators that do not expend energy unless they are forced to.

The Cottidae are a family of fish in the superfamily Cottoidea, the sculpins. It is the largest sculpin family, with about 275 species in 70 genera. They are referred to simply as cottids to avoid confusion with sculpins of other families.