Up to 5,000 feet below the surface, a tube-shaped creature with a huge jaw and a projection hanging from its chin swims through the ocean. This is a dragonfish (aka stomiidae aka the sea moth) - one from a family of animals that live in the deep sea. When I first saw one of these on the Australian Museum website (Australia and its surrounding waters are a haven for weird creatures) I thought it looked like a CGI alien or something out of a fantasy novel - more of a dragon than a fish. When I started to consider, though, how different their dark ocean world is from ours, a place where few humans have ever been, their unusual adaptations began to make sense.
First: the jaws. Dragonfish are the only type of creature in the deep sea that can unhinge their mouth bones to ingest prey even bigger than their heads! Most dragonfish can open their mouths like a broken pair of scissors, flexing a joint between their skull and spine so that they can eat other fish headfirst. In the picture below, you can see a lanternfish chilling in a dragonfish’s belly - swallowed whole from the cranial hinge.**
If the gaping jaws weren’t enough to scare you off, don’t worry. Dragonfish also have multiple rows of razor-sharp invisible teeth! These small creatures spend most of their time lurking in the depths of the ocean with their jaws open, waiting for other organisms to come along and get chomped. To make sure they go undetected, the dragonfish have developed fangs that are completely transparent due to tiny crystals made of hydroxyapatite in their enamel. These teeth are so durable that some researchers are using them as inspiration for new types of glass and ceramics. Their fangs are stronger than a piranha’s and have about the thickness of a needle, arranged in rows to catch and gobble up prey.
Another thing you might notice when staring at the dragonfish is the little goatee hanging off its chin. This is known as a barbel, which has a bioluminescent organ at the end that can act like a lantern for the fish. Dragonfish often use these barbels like fishing lines to attract prey. Many unsuspecting lanternfish latch onto the orbs, thinking it's a piece of food, but instead they get snapped up and swallowed by the dragon.
Each dragonfish species has barbels of different lengths and patterns unique to their type. It’s been theorized that the barbel orbs could also be mating signals - a light the animal turns on to let other dragonfish know they are open for mating. These creatures live at such great depths, though, that little research has yet to be done on this front.
The last feature of the dragonfish that made me question how we live on the same planet as them is the way that they emit light. Most bioluminescent creatures (which are common in the deep ocean where the sun’s rays can’t reach) give off an eerie blue glow. The stoplight loosejaw - a species of dragonfish named for its red light and loose jaw - is different. These are one of the only fishes in the world that bioluminesce the color red. At the depths where dragonfish live, the wavelengths of red light are too long to reach and nearly all creatures can’t detect the color. In fact, it’s been determined that the loosejaws don’t use their red barbel orbs as bait, but rather to illuminate their prey while going undetected by other creatures. This red invisibility cloak is produced by a photophore (the bioluminescent organ referenced earlier), a pigmented sac with a shiny inner lining and a mass of gland cells where light is reflected out through an aperture. These pigments come from a bunch of bacteria that the loosejaw eats that allow it to both detect and produce that odd red light.
Even though the dragonfish seems so different from us, some 400 million years ago a creature from which we've both descended swam in the oceans. The case of the dragonfish is one where form follows both function and evolutionary pressure. In this unique world without light sources, with heavy pressure, and few means of finding food at 5,000 feet under, a creature as bizarre as the dragonfish can survive and even flourish.
**Extra fact: Dragonfish have pitch-black stomach walls so that their bioluminescent food can’t be seen from the outside. This adaptation keeps the dragonfish safe from its predators, making sure all lights are out when it has to go undetected in the deep ocean.
I find it so cool that dragonfish have developed transparent fangs to allow them to stay undetected.
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