The cephalopod, which has spidery, seven-metre-long (20 feet) arms, is detailed in the journal Science.

The creature has been spotted in the Pacific, Atlantic and Indian Oceans, and in the Gulf of Mexico. One sighting was made at a depth of 4,734 metres (15,534 feet) - almost five kilometres (three miles) below the surface - in the western Atlantic, off the coast of Brazil.

The researchers behind the discovery say the find indicates how little we know about life in the Earth's largest ecosystem.

The squid's arms are longer than those of any known squid species and held in an unusual position: spread outward from the body and then bent anteriorly.

The scientists speculate that the squid may be an adult member of the recently identified family called Magnapinnidae (which means "big fin"). Only juvenile squids in this family have been seen before. More research will be required, however, before the animals can be properly classified.

"I call it a mystery squid,'' said Mike Vecchione, a National Oceanic and Atmospheric Administration (Noaa) researcher and first author on the Science paper. "It's unlike any other squid I've ever seen.''