These sharks glow bright green in the dark

A glowing catshark.

(CNN)Glowing green sharks swim in the dark depths of the ocean thanks to an entirely different kind of biofluorescence than other marine animals.

Previously, researchers knew that some shark species produce a green glow that is only visible to other sharks.
Researchers discovered that previously unknown small-molecule metabolites are the cause of the green glow. The glow helps sharks identify each other and even fight against infection on a microbial level.
    The study, focusing on swell sharks and chain catsharks, published Thursday in the journal iScience.
    "Studying biofluorescence in the ocean is like a constantly evolving mystery novel, with new clues being provided as we move the research forward," says David Gruber, study author and professor at City University in New York.
    "After we first reported that swell sharks were biofluorescent, my collaborators and I decided to dive deeper into this topic. We wanted to learn more about what their biofluorescence might mean to them."'
    The two types of sharks in the study have two skin tones, light and dark. During their research, Gruber and his colleagues realized the molecule responsible for the fluorescence was only in the light skin.
    "The exciting part of this study is the description of an entirely new form of marine biofluorescence from sharks--one that is based on brominated tryptophan-kynurenine small-molecule metabolites," Gruber said.
    A glowing swell shark.
    The metabolites responsible for the fluorescence are active in the light skin the same way that similar metabolites activate in the central nervous and immune systems in other animals.
    This is completely different from other bioluminescent ocean creatures like jellyfish and coral. They use green fluorescent proteins to turn blue light in the ocean into other colors.