Biologists working with deep-sea sharks have discovered a fascinating survival trait in two different species. According to a report from Phys.org, researchers from the University of Hawaii – Manoa (UHM) and the University of Tokyo have identified six-gill and prickly sharks’ ability to remain positively buoyant, which means that if they were to stop moving, they would float toward the surface.

The majority of sharks are classified negatively or neutrally buoyant. Sharks’ bodies are usually less dense than other marine animals’, largely due to their skeleton made completely out of cartilage. To compensate, they generate buoyant force with a large, oil-filled liver. The liver helps the sharks a little bit, but most still tend to sink if they stop moving through the water. The majority of sharks can never stop moving if they want to avoid sinking to the bottom.

The authors of the study didn’t expect to find shark species that had positive buoyancy. They conducted two additional experiments to make sure their results were accurate.

Carl Meyer of UHM fitted sharks from both species with an instrument that measured acceleration to track their movements throughout the deep ocean. The  device also collected data about the sharks’ speed, direction, alignment, and even the rate at which they beat their tails back and forth. They also deployed the world’s first shark-mounted camera to collect video information.

Deep-sea sharks spend most of the day at unimaginable depths, only venturing into the shallows when the water has cooled off at night. The scientists believe that positive buoyancy may give these shark species an evolutionary advantage in the deep, cool sea floor environment where access to food is limited. They think that the buoyant force could either help the sharks approach their prey from below, or it could even help them conserve energy as they make their nightly migration back to shallow waters.

The scientists say that they will continue to investigate these peculiar creatures, and hope to deploy more shark-mounted cameras in upcoming studies.