Fish
Tripodfish
Bathypterois grallator · also called tripodfish, tripod fish, spiderfish
The tripodfish does something no other fish does: it stands. On the muddy floor of the deep sea, it props its body up on three enormously stretched fin rays, like a camera on a tripod, turns to face the faint current, and holds absolutely still — waiting, nearly blind, for tiny drifting animals to brush against its outstretched fins.
Overview
The tripodfish lives on soft bottoms of the deep sea worldwide, mostly between about 800 and 4,700 m. Its body is 30–37 cm long, but the three stiffened fin rays it stands on can each be tens of centimetres more.
Standing and fishing
Two pelvic fin rays and the lowest tail ray are lengthened into rigid struts; the fish balances on the three tips, holding its belly clear of the mud. It then holds its forward pectoral fins up and spread like hands, feeling for prey — copepods and amphipods — carried past on the current and steering them into its mouth. It barely moves.
Nearly blind, and both sexes
Its eyes are tiny and probably sense little more than light; it hunts entirely by touch. And every tripodfish is a simultaneous hermaphrodite, able to fertilise its own eggs if no partner is found — an adaptation to how rarely animals meet in the vast, sparse deep.
Status
The tripodfish has not been assessed. It is widely distributed on the deep ocean floor, beyond most fishing, though deep-sea trawling and seabed mining are potential future pressures.
Tripodfish: common questions
How does the tripodfish stand on the seabed?
It has three fin rays that are much longer and stiffer than normal, one from the bottom of the tail fin and one from each of the two pelvic fins low on the body. The fish lowers these three rays to the bottom, spreads them apart, and locks its body up on top of them, so that it is standing rather than lying, with its belly a good distance above the mud. This keeps it out of the soft, oxygen-poor sediment and holds it steady in the current, and because the rays are stiffened struts rather than muscles doing work, standing there costs the fish very little energy.
Why does it face into the current?
The tripodfish is an ambush feeder that lets its food come to it. In the deep sea there is a constant, slow drift of water across the bottom, carrying suspended particles and small animals. By turning to face directly into that flow and holding its forward fins up and spread, the fish creates a sort of net into which prey is swept. It can then detect the contact and guide the item toward its mouth with minimal movement. Facing downstream, or sideways, would mean the current carried food away from it rather than toward it.
Why is a deep-sea fish a hermaphrodite?
Because finding a mate is extremely difficult where you live. The abyssal plain is enormous, food is scarce, and tripodfish are spread very thinly across it, so an individual may go a long time without encountering another of its species. Being a simultaneous hermaphrodite, with both male and female organs at once, doubles the chance that any meeting can result in reproduction, and, as a fallback, allows a completely isolated fish to fertilise its own eggs and still leave offspring. Several other deep-sea fish have independently arrived at the same solution.
How was the tripodfish discovered and studied?
Tripodfish have been known since the nineteenth century from specimens brought up in deep trawls, which is where the alternative name spiderfish comes from, since a preserved one with its long rays spread looks spider-like. Much more was learned once deep-diving submersibles and remotely operated vehicles began filming the sea floor in the late twentieth century, revealing the fish alive and upright on its stilts, oriented to the current, something that was not obvious from a dead animal in a net. It remains hard to study because it lives so deep and so sparsely.