World’s Largest Bioluminescent Vertebrate Is a Shark With a Sparkling Belly

Many species dwelling in the ocean’s dark waters rely on bioluminescence to deter threats or attract mates. In the case of the kitefin shark, these “light outfits” turn out to aid predation.

Offensive Camouflage

Among the largest bioluminescent vertebrates in the world, the kitefin sharks (Dalatias licha) measure about 1.8 meters in length and weigh around 8 kilograms as adults. The skin on their bellies, fins, and flanks features millions of tiny organs called photophores, which emit a characteristic bluish halo.

While for many deep-sea creatures such a glow helps them to blend into the background and thwart potential predators at greater depths, the D. licha uses this strategy to surprise its prey, as recent experiments have shown.

Detailed in the journal iScience, the experiments involved eight kitefin sharks accidentally captured, which were placed in tanks submerged in darkness. Different optical instruments allowed precise measurement of the luminous profile of the sharks’ skin, which was then compared to that of the waters where they typically swim (between 480 and 540 meters below the waves).

The team found that the glow emitted by the sharks closely matched the intensity of solar radiation reaching such depths. The fact that the maximum emission wavelengths observed in these sharks lean more toward green than dark blue suggests that such optical camouflage would be suited to waters with high concentrations of suspended chlorophyll particles.

A Question of Angles

These studies also illustrate the importance of the angle at which the photophores of D. licha emit their glow.

“In animals that camouflage themselves for defensive purposes, it corresponds to the downward light field in all angular directions from which a predator could observe them,” the authors of the new study write. “When the kitefin shark attacks from above, an emission angle of light that is exclusively vertical will essentially allow it to mask its silhouette.”

According to the researchers, the new optical data corroborate the spotlight hypothesis, with lateral brightness helping the kitefin shark illuminate the zone where its prey is located.

In 2021, two other species of bioluminescent sharks had been discovered in the New Zealand waters.