Why Is That Shark Lying Motionless on the Bottom? It Does Not Need to Swim
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A slow bottom dwelling shark that rests in piles during the day and sucks prey out of crevices at night breaks most of the assumptions people hold about sharks.
What the animal is
The species is a large, heavy-bodied, brownish shark reaching three metres or more, found on shallow reefs and sandy bottoms in warm Atlantic and eastern Pacific waters. It has a blunt rounded snout, small eyes, a pair of fleshy whisker-like projections beside the mouth and a small mouth positioned well underneath the head. The tail is long and the body is built for manoeuvring slowly rather than for speed. It is one of the sharks most often encountered by divers, largely because it spends the day lying still and lets itself be approached.
How it differs from a typical shark
Several familiar shark facts do not apply to it:
- •It pumps water over its gills, so it can rest on the bottom
- •Many other sharks must keep moving to breathe at all
- •It is nocturnal, hunting at night and sleeping by day
- •It rests in groups, sometimes piled on top of each other
- •Its teeth are flattened crushing plates, not cutting blades
- •It feeds by suction rather than by biting and tearing
How it feeds
The hunting method is unusual and effective for the prey it targets. The small mouth and a large throat cavity allow it to generate strong suction, and it puts its snout against a crevice and pulls the contents out, which works on octopus, crustaceans, snails and small fish sheltering in reef structure. The whisker-like projections are sensory and used to locate prey by touch and chemical traces in the dark. Prey too hard to swallow is crushed by the flattened teeth. The suction is powerful enough to pull a conch from its shell, which few predators manage.
The name and its confusion
The common name has nothing to do with nursing and its origin is genuinely uncertain, which has produced several competing stories. One derivation traces it through older English words for a seabed-dwelling fish, altered over centuries until it happened to land on a familiar word. Another points to the sucking noise the animal makes while feeding, which divers do report hearing. The scientific name of the commonest species has a different confusion attached, since it refers to a rasp-like file, describing the skin rather than anything about behaviour. Several unrelated sharks in other oceans carry the same common name.
Why they still bite people
The species has a reputation for being harmless that is not quite accurate and the pattern of incidents is instructive. It is not aggressive and does not hunt anything human sized, and almost every recorded bite follows a person grabbing, sitting on or pulling the tail of a resting animal. The bite that results is a defensive clamp rather than an attack, and the crushing teeth combined with a refusal to let go make it a genuinely serious injury that has occasionally required the animal to be removed surgically. The lesson is about the behaviour of divers rather than about the shark.
The takeaway
A heavy slow shark that pumps water over its gills can rest motionless on the bottom, which most fast sharks cannot, and it spends the day piled with others and hunts at night. Flattened crushing teeth and a small mouth generating strong suction let it pull prey out of crevices and shells. Almost every bite on record follows somebody grabbing a resting animal.