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It’s a Glow-in-the-Dark-Shark!!!

Velvet belly lanternsharks, Etmopterus spinax, picture Seapics

We’ve written about the cookiecutter shark before, that has a network of light producing organs called photophores on its stomach, that produce a green glow, with a dark patch resembling a smaller fish. This lures fish from the depths and will get bitten by the shark once they get too close. In sharks, bioluminescence occurs in two families, the Dalatiidae (kitefin sharks) and the Etmopteridae (lantern sharks). More than 10% of currently described sharks are luminous.

Lateral spontaneous luminescence from (a) Squaliolus aliae and (b) E. spinax. Arrows indicate photophore markings not involved in counterillumination: Pe, pectoral; Ca, caudal; Do, dorsal; Ic, infracaudal; La, lateral. Scale bars, 2 cm. (c) Etmopteridae lateral photophore markings (blue color) diversity illustrated by (from left to right): Centroscyllium ritteri, E. spinax, E. gracilispinis, E. lucifer and E. pusillus. Photographs by Mallefet. Drawings by Claes.

One example from the kitefin sharks is the tiny smalleye pigmy shark, that grows to a maximum of 22 centimeters, use their photophores on the stomach as camouflage. As predators swimming underneath them look up into the light, their glowing bellies make them invisible against the downdwelling light from the sun and the moon. Researchers found that the hormone melatonin (regulates the circadian rhythm) caused the sharks skin to glow and prolactin (hormone involved with many aspects of reproduction) caused the glow to fade. The hormones work by moving pigments in front of the light-emitting organs (covering them up) or retracting them to expose the glow.

The splendid lantern sharks take it a bit further and have 9 distinct luminous zones, not only used for camouflage, but also for communication and schooling. Additionally to their glowing belly, their dorsal spines glow to warn off predators. Their sexual organs also glow, to signal that they are ready to mate and a better candidate than another shark that glows less brightly. To aid in schooling, their flanks, tails and pectoral fins emit light too.

Recently a study in the Cayman Islands found that over 180 species of fish exhibit biofluorescence, including some rays and catsharks. With biofluorescence, the skin of the sharks absorbs light and it gets reflected in colours such as red and green, which is highly noticeable in the blue environment. As if now, the function is not yet known.

 

 

 

Text: Dorien Schröder, dorien@dutchsharksociety.org

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