Life on the seabed gives fish migrating eyes and oil-filled livers
A young flounder hatches with an eye on each side of its head, like any ordinary fish. As it grows, one eye travels across its skull until both stare upward from the same side, letting it lie flat and hidden on the sea floor. It is one of many odd solutions among fish that live on the bottom.
Such bottom dwellers are called demersal fish or groundfish, from the Latin demergere, to sink. They live on or just above the beds of seas and lakes, across the continental shelf and down the slopes beyond it, but seldom on the deepest abyssal plains. Their fillets hold only one to four percent oil, whereas fish of the open water above can reach 30 percent.
They split into two camps. Strictly benthic fish are denser than water and rest on the bottom effortlessly, usually with reduced or missing swim bladders and flattened bodies. Many have downward-pointing mouths for food buried beneath them and pump swallowed sand out through their gills. Flatfish such as sole, turbot and halibut, along with stingrays and dragonets, can bury themselves and wait in ambush, while stargazers, with upturned mouths, grab prey swimming overhead. Spiderfishes, almost blind, perch on long fin rays facing the current to snatch drifting plankton.
Most demersal species, though, are benthopelagic: neutrally buoyant, hovering just above the floor. Deep-sea sharks and deep-water stingrays achieve this with huge oil-filled livers, and sharks scavenge dead whales on slopes down to around 2,000 metres, although their appetite for energy keeps them out of the food-poor deepest water. The cusk-eel Acanthonus armatus takes the opposite approach, a flabby ambusher whose body is 90 percent water and which has the smallest brain relative to body size of any known vertebrate, alongside the largest ear stones.
Robust swimmers such as orange roughy and Patagonian toothfish patrol seamounts with strong currents. Predators have their own tricks for finding bottom dwellers: the great hammerhead sweeps its broad head over the sand to sense the electrical signals of buried stingrays, then pins them down with it.
Source: Demersal fish
