The sharks that glow in the dark: Scientists release first experimental study of bioluminescent predators

Kitefin shark has subverted normal use of ‘counter-illumination’, researchers believe

The glowing skin of the kitefin shark (Jérôme Mallefet, Darren W Stevens, Laurent Duchatelet)

Scientists have been left puzzled by a glow-in-the-dark shark after conducting the first experimental study of the deep-sea dweller.

The researchers, from Belgium and New Zealand, found that the kitefin shark possessed bioluminescent sites along its belly, sides, back and dorsal fins which were controlled by hormones.

Samples were collected from the Chatham Rise area of seabed east of New Zealand.

The 180cm kitefin has been spotted swimming at depths between 50m and 1.8km but usually inhabits a zone below 300m, experts said. It is the largest known bioluminescent vertebrate.

However, Dalatias licha does not have any predators so the usefulness of its camouflaged glowing belly – with which, it is thought, it can “counter-illuminate” itself against weak sunlight filtering from above – was not clear.

In constructing their hypotheses, researchers cited previous studies which said that although D. licha has one of the slowest “cruising speeds” known among sharks, it is believed to have a very high short-range burst speed. Evidence has shown it can eat fast-swimming fish as well as slower creatures that dwell near the sea bed.

Therefore, the scientists believe, either “luminescence might be used to illuminate the ocean floor while searching and hunting for prey, or to stealthily approach toward [fast] prey, using counter-illumination camouflage, before striking fast when close enough”.

They added: “In both cases, the principle of counter-illumination would have been distorted to serve as a predation tool instead of an avoidance mechanism, a hypothesis already proposed for the cookie cutter shark.”

Further studies of live specimens are required, the scientists wrote.

The study also looked at two other species, a pair of lantern sharks. These displayed a use of glowing skin that was more in line with previous knowledge of similar creatures, including for communication.

Researchers wrote: “The dorsal photophores, flank markings, and brighter pectoral fin and claspers are likely to be used for intraspecific communications, while the ventrally emitted light is likely to be used for counterillumination.”

The study was published in the journal Frontiers in Marine Science.