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Deep-sea dragonfishes have large fangs and dark skin. Their bioluminescent light organs (prominent in this image) are thought to be used for locating mates. Credit: Naturalis Biodiversity Center
From the front, dragonfish look about as terrifying as their mythological namesakes likely appeared to the knights in ancient tales. Sleek, slender, and quite small at about 20 inches long, they don’t really resemble dragons, but their gaping jaws full of fangs are scary enough to seem like a storybook monster. They also live deep in the dark twilight and midnight zones of the sea, where their dark skin allows them to remain unseen so they can ambush their prey.
Living in the light-less depths of the sea, it can be hard to find a mate, especially because creatures can be very spread out. Dragonfish have bioluminescent light organs on their head that are thought to be used to attract mates. However, the light organs of males are bigger and brighter than those of females. So a female might see a male’s glow, and he might not be able to see her dimmer response. But evolution has produced a partial fix for that problem: bigger eyes for the males, so they can take in more light and see females more easily.
Researchers from Boston College and Stockholm University discovered this adaptation in males by studying two different species of deep-sea dragonfishes. They measured eye size and calculated the amount of light produced by the light organs of each sex for Malacosteus niger and Photostomias guernei. Then they used those data to estimate the distances at which a fish would be able to see a member of the opposite sex. In both species, the females would see the males sooner—that is, when they’re further away—than males would see females. But males of both species had bigger eyes, and that shortened the distance at which males would see females by up to about sixteen feet.
The larger eyes of males still wasn’t enough to match the distance at which females could see males, leading to another question for biologists to investigate: why are light organs of males bigger in the first place?
