A six-gill shark equipped with an instrument package is shown returning to deep water. Researchers found that some shark species were surprisingly buoyant, which could help them survive in deep-ocean environments. University of Hawaii/Mark Royer

It’s pretty dark in the ocean’s so-called twilight zone, but new research has shed some light on the biology and behavior of the deep-sea shark species that live there. Little is known about the elusive creatures that dwell in near-darkness and only visit shallower waters to feed at night. Researchers from the U.S. and Japan studying two deep-sea shark species near Hawaii have found that the animals are surprisingly buoyant, contradicting the common notion that sharks sink if they stop swimming.

Scientists said they were astonished by the discovery. “Our findings are preliminary, and more data are required to determine whether this is a widespread phenomenon among all life-history stages of these deep-sea sharks, or whether positive buoyancy is widespread in other deep-sea organisms,” researchers wrote in their study, published this week in the journal PLOS One. They said the characteristic might be helpful for “exploiting deep-sea environment.”

The sharks studied were the six-gill and prickly sharks. The six-gill shark can grow up to 16 feet in length and is something of a living fossil – its relatives date back 200 million years, according to the Marine Detective.

Prickly sharks are bottom dwellers that can reach 13 feet in length and, like six-gill sharks, travel into shallower waters only to feed. The sharks were captured and tagged in Kane’ohe Bay near Oahu, Hawaii, and were pulled from depths of nearly 1,000 feet. Researchers fitted the sharks with devices similar to flight recorders that tracked the sharks' every movements.

The ocean zone that extends from 650 feet to roughly 3,300 feet is known as the twilight zone. Only a small amount of light reaches the twilight zone, which means photosynthesis is not possible, so there’s no phytoplankton there.

Scientists speculated about the evolutionary advantage of the sharks’ buoyancy. "One hypothesis is that these six-gill sharks will use their positive buoyancy as a way of sneaking up and catching prey from underneath,” Carl Meyer, from the University of Hawaii’s Institute for Marine Biology, told KITV Channel 4. "It might be that if there is a fish or something up in the water column that a six-gill wants to eat, that shark can stop swimming and then glide and ambush in the water.”