by Bruce Mundy, NOAA Pacific Islands Fisheries Science Center
November 1, 2018
During Dive 2 of the expedition, we observed this fish, which is a stripefin brotula, Neobythites marginatus. The species is in the brotula family Ophidiidae, commonly known as cusk eels, but they are neither true eels or cusks (relatives of codfish). The stripefin brotula is found in the western Atlantic, Caribbean Sea, and Gulf of Mexico, from North Carolina to northern Brazil, at depths of 75 to 935 meters (245 to 3,068 feet). They are small fish, growing to about 22 centimeters (8 2/3 inches) in length, that feed on small crustaceans.
Like many brotulas, the stripefin brotula has modified pelvic fins (the paired fins on the chest) that are simple filaments. Those are the "barbels" mentioned in the video. These fins probably have taste buds and are used to feel for potential food on the bottom, with the same function as the barbels of catfishes, but structurally different (an example of convergent evolution, where different structures evolve to have the same function in unrelated species).
Little else is known about the biology of the stripefin brotula. They have been observed by submersibles diving into holes on the seafloor. Shallow-water brotulas that live at depths where sunlight penetrates the ocean are very shy, usually active at night, and hide in shelter during the day or when exposed to light. Other Neobythites species seen during NOAA Ship Okeanos Explorer expeditions in the Pacific Ocean were seen sheltering in crevices and under ledges.
The one seen in this video is unusual because it was using a glass bottle as shelter. We don't know why it chose that, but the beginning of the video shows that the bottle may have been the only protective shelter for the brotula in the flat, sedimented area of the observation, and in the dim [or no] light at the site, it may have provided protection as good as an opaque structure. The solid walls of the bottle may have felt like safe shelter to the fish.
Aquarium hobbyists sometimes provide glass or clear plastic tubes as shelter for similarly shaped freshwater knifefishes (Gymnotiformes) that have cryptic, light-averse habits like brotulas. The function of that, however, is very different from what was observed for this fish in the bottle during Dive 2. Freshwater knifefishes (in the same group as the electric eel) produce mild amounts of electricity that they use as a "sixth sense" to detect objects and other animals around them. Many knifefishes have poor vision and a clear tube seems the same to them as an opaque one, from their electrical senses.
That doesn't apply to the brotula, though, because salt water is such as good conductor of electricity that marine fishes cannot use electricity that they produce as an extra sense. Some marine fishes, like sharks and rays, have sense organs that can detect minute changes in electric fields produced by muscle activity of other animals, and a few, like torpedo rays and some stargazers can produce strong electric shocks, but those are different functions than the weak electricity produced by freshwater knifefishes for communication and electro-detection.