Cetomimidae is a family of small, deep-sea beryciform ray-finned fish. They are among the most deep-living fish known, with some species recorded at depths in excess of . Females are known as flabby whalefishes, Males are known as bignose fishes, while juveniles are known as tapetails and were formerly thought to be in a separate family, dubbed Mirapinnidae. Adults exhibit extreme sexual dimorphism, and the adult males were once thought to be exemplars of still another family, Megalomycteridae. They are distinguished from other whalefishes by their loose, scaleless skin and lack of photophores.
thumb|240px|[[Ditropichthys storeri]]
Description
alt=Long reddish-orange fish with a very distinct dorsal line, as well as dorsal and anal fins that are set very far back on its body. |thumb|Female Cetomimidae, also known as a flabby whalefish
Living at extreme, lightless depths, adult females have evolved an exceptionally well-developed lateral line system. Their eyes are either very small or vestigial and instead this system of sensory pores (running the length of the body in a distinct lateral line) helps the fish to accurately perceive its surroundings by detecting vibrations. Named after the baleen whale-like bodies of adult females (from the Greek ketos meaning "whale" or "sea monster" and mimos meaning "imitative"), Cetomimidae have large mouths, and their dorsal and anal fins are set far back of the head. All fins lack spines, and the pelvic fins are absent. The fish also lack swim bladders.
Adult female Cetomimidae are a red to orange-brown color in life, with the fins and jaws, in particular, being brightly colored. Longer electromagnetic wavelengths (such as red and orange) do not penetrate this depth of the ocean. Animals which have evolved at this depth cannot see these longer wavelengths, rendering the fish effectively black.
Their stomachs are highly distensible, allowing adult females to pursue prey otherwise too large for them to eat. Adult males do not eat at all, their jaws having fused shut during the transformation from their juvenile phase. Males retain the shells of prey consumed while still in the juvenile form and continue to metabolize these shells throughout the remainder of their lives. Both traits may have evolved due to extreme food scarcity in the ocean depths. In early 2009, the Royal Society published an article detailing the discovery "that three families with greatly differing morphologies, Mirapinnidae (tapetails), Megalomycteridae (bignose fishes), and Cetomimidae (whalefishes), are larvae, males, and females, respectively, of a single-family, Cetomimidae." Apparently, "morphological transformations involve dramatic changes in the skeleton, most spectacularly in the head, and are correlated with distinctly different feeding mechanisms. Larvae have small, upturned mouths and gorge on copepods. Females have huge gapes with long, horizontal jaws and specialized gill arches allowing them to capture larger prey. Males cease feeding, lose their stomach and esophagus, and apparently convert the energy from the bolus of copepods found in all transforming males to a massive liver that supports them throughout adult life."
The tapetails are named for their caudal fins, which include a narrow streamer that may be longer than the fish's body. The genus Mirapinna, known as the hairyfish, lacks the streamer, but has multiple hair-like growths on its body. All mirapinnids lack scales and fin rays. Mirapinnids are all small fish, less than in length. They feed on small crustaceans.