The tripod fish 'stands' at 1960m depth. Image Credit: Bahamas Deep-Sea Coral Expedition Science Party, NOAA-OER

Tripod fish: a deep-sea fish able to 'stand'

  • March 20, 2014

This unusual fish's bony fins can extend up to 1m to help them stand

Contributor
Becky Crew

Becky Crew is a Sydney-based science communicator with a love for weird and wonderful animals. From strange behaviours and special adaptations to newly discovered species and the researchers who find them, her topics celebrate how alien yet relatable so many of the creatures that live amongst us can be.

THE THOUGHT OF what's lurking in the deep sea, so far down that our submersibles can barely catch a glimpse, only gets more exciting when we find a species like the bizarre tripod fish (Bathypterois grallator).

One of the ocean's deepest-living fish, tripod fish are found widely in the Atlantic, Pacific and Indian oceans. A couple have been identified off the coast of Western Australia, and one was discovered near Flinders Island, just off Tasmania's north-east coast.

They thrive in depths of up to 6000m below the surface, together with other deep-sea residents, such as the black swallower and giant squid.

Tripod fish, which grow up to 30cm long, are sometimes found sitting quietly on the ocean floor alongside another tripod fish, while others are happy towering over their close but shorter relatives, the spiderfish.

Tripod fish: why has it evolved to 'stand'?

Those long, bony fins can extend up to a metre from the tripod fish's underside and tail, and researchers have suggested that fluids are pumped into them when the fish is 'standing' to make them more rigid. When the tripod fish is swimming, the long fins float softly, and a little awkwardly, beneath it (see video below).

Exactly why this fish has evolved to sit on incredibly long fins appears to be as simple as why anyone would choose to prop themselves up on some kind of stilt-like device - to be taller. By sitting perfectly still at around a metre above the seabed, the tripod fish is perfectly positioned for tiny prawns, fish, and crustaceans to come hurtling along on the ocean's current right into its mouth. At the seabed level, the current is virtually non-existent.

The tripod fish won't see its prey coming - it's practically eyeless because of its super-dark habitat - but that doesn't matter, because its long fins can feel the vibrations made by approaching creatures in the muddy sediment.

And that pair of pectoral fins that sit erect just behind its head? They act like antennae, providing extra sensory information to the tripod fish about incoming prey, and can also usher wandering creatures into the vicinity of its mouth.

Video: tripod fish