Behind the Science: Sharks that Glow in the Dark
Author: Natasha Hynes
Cookiecutter sharks (Isistius brasiliensis) are infamous for their circular bites but did you know that they also glow in the dark? Cookiecutter sharks have special light organs on their bodies called photophores. Photophores use compounds called luciferases and photoproteins to emit light, but we still don’t know exactly which ones glow-in-the-dark sharks are using. Although we know the cookiecutter shark emits a blue-green light from its underside, researchers from Belgium set out to map the photophores on the shark’s body and take a closer look at the photophores themselves.
Figure 1. Bioluminescence of Isistius brasiliensis. (A)Ventral side of I. brasiliensis. The dark-collar or black band is visible on the anterior part. (B) The ventral luminous pattern observed on the ventral side. An absence of bioluminescence is observed on the dark collar. The dark region in the middle of the shark is artifactual and corresponds to the area where the specimen was stabilized for the picture. Credit: Delroisse J, Duchatelet L, Flammang P and Mallefet J (2021)
The sharks used in this study were bycatch from a longline fishery in La Réunion Island. There was also one individual used that was kept alive aboard a vessel for the study! Researchers used high-sensitivity photography in the dark to capture different areas of the shark’s body. They also looked at patches of skin from different parts of the body under the microscope to observe the photophores more closely.
Cookiecutter sharks are thought to use the bioluminescent photophores on their belly for counterillumination. Counterillumination is like camouflage for these sharks: the light they emit blends in well with the slightly lighter water above them, so an animal looking up at them can’t see them very well. This is especially helpful for avoiding predators. In the case of the cookiecutter shark, this counterillumination may have another benefit: luring in prey! The cookiecutter glows pretty much all over its underside, except for a distinct black band between their jaw and their pectoral fins. In this band, there are very few photophores and the area does not glow. Scientists think that this black band can look like prey to larger animals and when they come to investigate, the cookie cutter can strike with its signature circular bite!
As for the photophores themselves, the authors of this study found them to be very similar to other glow-in-the-dark sharks. The photophores consist of a photocyte (a special cell that catalyses the reaction used to produce light), inside of a pigmented sheath. On top of this sheath is an iris-like structure with lens cells. When you look at the shark’s skin in the light, they appear as tiny black dots on the skin in between their scales.
If you liked learning about cookiecutter bioluminescence, make sure you check out our species synopsis on our Cookiecutter Shark Blog
Delroisse J, Duchatelet L, Flammang P and Mallefet J (2021) Photophore Distribution and Enzymatic Diversity Within the Photogenic Integument of the Cookie-Cutter Shark Isistius brasiliensis (Chondrichthyes: Dalatiidae). Front. Mar. Sci. 8:627045. doi: 10.3389/fmars.2021.627045