The fish with the light on its head is the anglerfish, one of the most recognizable creatures in the deep ocean. That glowing “lightbulb” dangling above its mouth is actually a modified piece of its dorsal fin tipped with a sac of bacteria that produce light. The anglerfish uses this built-in fishing rod to lure prey straight into its enormous jaws in waters so deep that sunlight never reaches them.
How the Light Works
The anglerfish’s glow comes from a partnership between the fish and bacteria living inside a bulb-shaped organ called the esca. The esca sits at the tip of a long, flexible spine (the illicium) that projects from the top of the fish’s head, essentially forming a tiny fishing pole. Inside the esca, bioluminescent bacteria from the family Vibrionaceae produce a faint blue light. The bacteria get shelter and nutrients; the fish gets a lure that works in total darkness.
The bacteria can’t turn on by themselves at will. The anglerfish controls blood flow to the esca, which regulates oxygen supply to the bacteria and lets the fish “switch” the light on and off. A complex set of muscles along the illicium allows the anglerfish to jerk and wiggle the glowing tip, mimicking the movement of small prey. Anything curious enough to investigate drifts right into the fish’s gaping mouth.
Where Anglerfish Live
Most people picture the deep-sea variety when they think of anglerfish, and those species (the suborder Ceratioidei) do live in the bathypelagic zone, the pitch-black layer of ocean roughly 1,000 to 4,000 meters below the surface. But the name “anglerfish” covers more than 200 species in the order Lophiiformes, and they occupy a surprising range of habitats.
Frogfish, for example, are anglerfish that live on tropical coral reefs in shallow water, using vibrant colors to blend in with their surroundings. Goosefish sit camouflaged on sandy continental shelves, where they ambush passing fish and have even been known to grab seabirds. Sea toads and batfish live on the deep seafloor. All of these are ambush predators that use some version of a lure, but only the deep-sea species produce light. Shallow-water anglerfish rely on other tricks: batfish release chemicals from their lure, while frogfish wave a lure shaped like a shrimp or worm.
What Deep-Sea Anglerfish Look Like
The deep-sea species are the ones burned into pop culture: round, dark bodies with oversized heads, wide mouths full of needle-like teeth, and that signature glowing lure. They don’t really swim. They drift through the water column, conserving energy and waiting for prey to come to them. Some species, like the wolftrap angler, have evolved elongated bodies, while close relatives like the footballfish keep the classic “blobby” shape. Both body plans trace back to a recent common ancestor, meaning the shift happened relatively quickly in evolutionary terms.
Size varies dramatically depending on the species and sex. In the humpback blackdevil (Melanocetus johnsonii), one of the best-known deep-sea anglerfish, females grow up to 18 cm (about 7 inches) long. Males top out at just 2.9 cm, barely an inch. Males lack the glowing lure entirely and look like an entirely different animal.
One of Nature’s Strangest Mating Systems
That size difference hints at something unusual. In some deep-sea anglerfish species, males don’t just find a female and mate. They bite into her body and never let go. Over time, the male’s tissues fuse with the female’s, their circulatory systems merge, and the male becomes a permanent parasite, receiving nutrients directly from her bloodstream. He essentially becomes a sperm-producing organ attached to her side.
This kind of tissue fusion would normally trigger a violent immune rejection, the same response that makes organ transplants so difficult. Anglerfish that permanently attach have solved this by losing key parts of their immune system. Species that form permanent bonds have lost the genes responsible for building the adaptive immune system that would otherwise attack foreign tissue. They’ve also lost much of their diversity in the molecules that cells use to identify “self” versus “not self.” It’s an extraordinary evolutionary trade-off: give up immune defenses to make reproduction possible in a vast, dark ocean where finding a mate is rare.
Not all anglerfish species go this far. Some have males that attach temporarily, and their immune changes are less extreme. The degree of immune system loss tracks closely with how permanently the male attaches.
Other Fish That Glow
Anglerfish are the most famous bioluminescent fish, but they’re far from the only ones. Lanternfish have rows of light-producing organs along their bellies that may help them blend in with faint light filtering down from above, making them harder for predators to spot from below. Dragonfish produce red light, which is invisible to most deep-sea animals. This lets them illuminate and hunt red-colored prey without alerting anything else nearby.
What makes the anglerfish stand out is how it uses light as a weapon. While many bioluminescent fish glow for camouflage or communication, the anglerfish dangles its light like bait on a hook. The strategy is so effective that bioluminescence in ray-finned fishes has evolved independently many times, with origins stretching back roughly 150 million years to the Early Cretaceous period.
New Species Still Being Found
Scientists are still discovering anglerfish species. In 2024, researchers from the University of Hawaiʻi described a new species called Gigantactis paresca, collected from the Clarion-Clipperton Zone in the deep Pacific during a research cruise assessing the potential impacts of proposed deep-sea mining. This species made the World Register of Marine Species’ top 10 remarkable new marine species list for the year. What makes it unusual, even among anglerfish, is a secondary lure that doesn’t produce light, paired alongside the primary glowing one. No other anglerfish has been found with this dual-bait setup, which is how it got its name: “paresca” means “pair of bait” in Latin.
Discoveries like this highlight how little of the deep ocean has been explored. The anglerfish’s habitat, thousands of meters below the surface, remains one of the least-studied environments on Earth, and many species likely haven’t been documented yet.