Some sharks glow on the underneath of their body making themselves invisible to both prey and predators, according to a new study.
Light trickery permits the optical illusion, described in the current issue of the Journal of Experimental Marine Biology and Ecology.
The findings represent the first experimental tests of shark luminescence.
About 50 different shark species, or more than 10% of all known sharks, are luminous, says lead author Julien Claes, a researcher in the Laboratory of Marine Biology, Earth and Life Institute at the Catholic University of Louvain in Belgium.
This means the sharks can produce and emit light from their bodies.
Claes and colleagues chose to focus on one particular luminous shark, nicknamed "the phantom hunter of the fjords": the velvet belly lantern shark.
This shark's shimmer originates from light emitting organs called photophores from underneath its body, "effectively creating a glow from that region," says Claes.
"Since many predators have upward-looking eyes, it is a common method of camouflage in the mesopelagic zone (200 to 1000 metres below the surface), although it is the first time it is demonstrated in sharks," he adds.
For the study, Claes and his team collected male and female velvet belly lantern sharks in Bergen, Norway. The sharks were then brought to Espeland Marine Station, where they were maintained in cold, dark water tanks, replicating conditions of their natural habitat.
The scientists next measured the luminescence intensity of each shark. Measurements were again taken after an overhead light simulation, some days later, in order to test the sharks' response to light.
Immediately after being caught, most of the sharks produced a spontaneous and long-lasting luminescence, occasionally lasting over an hour. The spectrum of this light closely matched that of the shark's usual deepwater fjord home.
The sharks were able to adjust slightly their emitted light in response to external light changes.
This ability suggests that they use both their eyes and a small gland in the brain to monitor information on light shining down from above.
Like most sharks, the mouth of this species is on its underside, so the camouflage system allows the shark to grab prey, such as krill and pearlfish, with invisible ease.
Similar to how lipstick makes a woman's lips stand out more, the shark's light may also turn on members of the opposite sex.
"Communication is also a function of the luminescence, since some parts of the animal appear brighter at close range, such as the pelvic part containing the sexual organs," Claes suggests.
Dr Bernard Seret, a shark expert at the National Museum of Natural History in France, says he agrees with the new study. Seret hopes the research team will explore the many other possible functions of shark bioluminescence.
Dr Rui Coelho, a shark research scientist at the Florida Museum of Natural History, also supports the new paper's conclusions.
"I believe that what most surprised and excited me about this paper was the finding that the emission of light on the ventral surface of the sharks closely resembles the environmental light," says Coelho, "allowing the sharks to efficiently camouflage themselves by counter-illumination, remaining invisible to both possible predators and potential prey."
Luckily humans are not on the prey list for the velvet belly lantern shark. Even if we were, this glowing predator would probably pose little threat.