Marine biologists examining Chatham Rise bycatch have discovered that the kitefin shark, the world’s largest glowing vertebrate, uses its bioluminescence not just for camouflage, but as a biological searchlight to illuminate the seafloor and detect prey in deep, dark waters.
Chatham Rise Bycatch Reveals Unusual Glow Patterns in Kitefin Sharks
In the murky depths of the open ocean, lighting up usually serves a defensive purpose. Many marine creatures rely on counterillumination to survive, blending their own bioluminescence with the faint residual daylight filtering from above to erase the dark shadows that might otherwise attract predators. Yet the kitefin shark (Dalatias licha) operates under a different set of rules.
As the world’s largest glowing vertebrate, the kitefin shark has no known natural predators. That stark biological reality has long baffled scientists trying to understand why the species evolved to glow across nearly its entire body, including areas like the dorsal fin where standard camouflage offers little utility. To solve the puzzle, a team led by marine biologist Julien Claes of Université catholique de Louvain in Belgium examined eight live sharks caught as bycatch during a survey of Chatham Rise, to the east of New Zealand.
Measuring the Wavelength and Direction of Deep-Sea Bioluminescence
The research team set out to measure three distinct properties of the shark’s bioluminescence: brightness, color, and directional shine. Laboratory measurements revealed that the glow emitted from the underside of the shark peaks at a bluish-green wavelength of 491 nanometers. That specific hue matches the residual daylight found at depths between 480 and 540 meters, placing the luminosity precisely within the creature’s natural habitat range.
However, the directional analysis uncovered an unexpected twist. Rather than strictly mimicking downwelling daylight, the shark’s illumination spreads outward laterally with exceptional breadth, particularly around the nose, jaw, and pectoral fins. Compared to other counterilluminating species, this sideways spread is significantly wider.
“One parsimonious explanation is that this excess lateral emission functions as a biological searchlight, illuminating the seafloor to the sides of the shark and potentially enhancing prey detection in the otherwise dark benthic environment.”
Julien Claes and research team, via Sciencealert
Balancing Stealth Illumination and Slow-Speed Hunting Strategies
This dual-purpose light distribution helps explain how the kitefin shark sustains itself on swift prey. Recognized as the slowest-swimming shark on record—cruising at a sluggish pace of just 13 centimeters (5 inches) per second—the creature routinely hunts fast-moving, visually capable animals like cephalopods and lanternsharks.
Researchers suggest that the shark’s biology accommodates two complementary tactics at once. The downward-facing glow conceals the predator from any potential observers lurking below, while the sideways emission acts as a functional beam that highlights bottom-dwelling organisms. By pairing stealth camouflage with an active searchlight, the slow-moving predator manages to bridge the gap against much quicker prey.
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