Flabby whalefish are small, deep-sea cetomimiform fish of the family Cetomimidae. They are among the most deep-living fish known, with some species recorded at depths in excess of 2.17 mi (3.5 km). Within the family are nine genera and 20 species.
Thought to have a circumglobal distribution throughout the Southern Hemisphere, Flabby whalefish are the most diverse family of whalefish. The largest species, Gyrinomimus grahami, reaches a length of 15.75 in (40 cm). They are distinguished from other whalefish by their loose, scaleless skin and lack of photophores.
Living at extreme, lightless depths, Flabby whalefish have evolved an exceptionally well-developed lateral line system: as the eyes are either very small or vestigial, this system of sensory pores (running the length of the body) helps the whalefish to accurately perceive its surroundings by detecting vibrations. Named after their whale-like bodies (from the Greek ketos meaning “whale” or “sea monster” and mimos meaning “imitative”), whalefish have large mouths with the dorsal and anal fins set far back of the head. All fins lack spines, and the pelvic fins are absent. The fish also lack swim bladders.
Flabby whalefish are a red to orange-brown color in life; the fins and jaws are brightly colored. This is explained by the fact that longer electromagnetic wavelengths (such as red and orange) do not penetrate into the whalefish’s realm: animals which have evolved at this depth cannot see longer wavelengths, rendering the whalefish effectively black. Their stomachs are highly distensible, allowing the whalefish to pursue prey otherwise too large to swallow; an important adaptation in the abyss where food is scarce.
Little is known regarding the life history of flabby whalefish. They are known to feed primarily on small crustaceans such as decapods, copepods and euphausiids. All males captured have been much smaller than females at a maximum of 1.38 in (3.5 cm). This suggests a strong sexual dimorphism similar to anglerfish: resources are at a premium in the deep ocean, making it sensible to limit size in males. As the male’s only purpose is to supply sperm, only the female need grow large in order to reproduce effectively.
Like many deep-sea fishes, Flabby whalefish are thought to undergo nightly vertical migrations: they feed within the upper 2296.59 ft (700 m) of the water column by starlight and retreat back to the abyssal depths by daybreak. Judging by the latest studies, the younger whalefish seem to frequent shallower water than adults.