Stoplight loosejaw

About the Stoplight loosejaw

With red and green “stoplights” around its eyes and a trap-like jaw, the stoplight loosejaw both earns its moniker and defies evolutionary expectations.

Like other twilight zone dwellers, this dark-colored fish sports bioluminescent organs around its eyes and bluish lights along its scale-free body. A large red patch under each eye, however, puts the stoplight loosejaw in an exclusive club of creatures capable of both emitting and sensing red light. It hunts and hides under the cover of long-wavelength red, a color that’s invisible to most other fish. This deep-sea ninja actually derives its ability to see red light from its favorite snack, copepods. In turn, these tiny critters feast on bacteria containing pigments from chlorophyll, which can absorb red light. That might explain why, despite their fearsome jaws, this deep-sea dragonfish feeds opportunistically on copepods rather than more filling meals of fish. You are what you eat, after all.

Now, about that loose jaw:  a lower mandible, which makes up a quarter of the fish’s length, also contains several hinges that let it open to incredible proportions. Curved, needle-like fangs pierce the flesh of its prey, and a gap where other fish might have a floor to their jaw ensures that no meal is too large for this fish’s gullet.

In another departure from the dragonfish norm, the stoplight loosejaw doesn’t migrate to the surface to feed. It prefers to hang out in depths ranging from 500-2,000 meters (1,500-6,000 feet) where it can continue its never-ending game of Red Light, Green Light in the deep.


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Quick Facts

Common NameStoplight loosejaw
Scientific NameTwo species in the Malacosteus genus (M. niger and M. australis)
Other NamesLightless loosejaw, rat-trap fish, black loosejaw, black hinged-head
SizeLargest recorded specimen 25.6 cm (10.1 inches)
DiscoveryWilliam Orville Ayres, 1848
Eats what?Crustaceans, amphipods, polychaete worms, squid, larvae, smaller fish like hatchetfish, fangtooths, and other lancetfish
Eats how?Lower jaw has no floor and is hinged in several places, making it possible to open very wide and skewer large prey with needle-like teeth before swallowing whole.
Is eaten by?[Not known]
BioluminescenceYes, with the added distinction of being from the only family of fishes that produce red light