A plankton-feeding worm that uses bristles to swim and sports slender "arms" on its head, has been discovered nearly 3,000 metres below sea level off the Indonesian coast. The squid-like creature, which grows to 9cm or more, was captured by a robotic submersible in the Celebes Sea, a marine basin that plunges to 6,200 metres at its deepest.
Researchers, from Woods Hole Oceanographic Institute in Massachusetts and the Scripps Institute of Oceanography in California named it Teuthidodrilus samae – a new genus and species. They said: "This illustrates how much we have to learn about even the large, common inhabitants of deep pelagic communities."
In a series of exploration dives, researchers spotted the worm, slowly rising and falling in the water around 100m above the seafloor, where it feeds on passing plankton. The worm swims or treads water by waving hundreds of bristles that run along the length of its body on either side.
The 10 slender arm-like appendages that give the worm its unusual appearance are a combination of elongated gills and sensory organs. They are probably used to pick scraps of food from the "marine snow" of organic detritus that constantly falls to the seabed from above. Each is as long or longer than the whole of the worm's body.
The discovery has surprised scientists because the creature appears to be so common. In seven underwater missions the submersible spotted 16 individual specimens. "How could such an animal evade collection until now?" the team writes in the journal Biology Letters.
The Celebes Sea is thought to have formed more than 40m years ago between what is now the Philippines and Indonesia. The deepest parts of the sea are isolated from surrounding waters by shallow sills, so marine life there has evolved in a unique and largely undisturbed environment. The sea lies at the centre of the Coral Triangle, an area considered a conservation hotspot due to its diverse and unusual marine life.
The worm was discovered during an expedition in 2007, when the researchers collected seven specimens from depths ranging from 2,028m to 2,912m using traps and suction devices on the robotic submersible Max Rover Global Explorer.
While the juveniles are almost transparent, adults are dark brown and covered with a gelatinous sheath. A similar animal was spotted, but not captured, by another submersible off western India in October 2004.