Fish
Anglerfish
Melanocetus johnsonii · also called humpback anglerfish, black seadevil, deep-sea anglerfish
The deep-sea anglerfish is one of the most extreme animals in the ocean. It lives in permanent darkness kilometres below the surface, it fishes for its food with a glowing lure grown from its own body, and its reproduction involves the male dissolving into the female.
Overview
Melanocetus johnsonii, the humpback anglerfish or black seadevil, is one of around 170 species of deep-sea anglerfish. They inhabit the bathypelagic zone — the open water column between roughly 1,000 and 4,000 metres, where no sunlight reaches, the water is a few degrees above freezing, and food is extraordinarily sparse. Anglerfish do not swim actively in search of prey; they hang almost motionless and let the prey come to them.
The lure
The anglerfish’s signature is the illicium, a thin, mobile rod projecting from the top of the head — an evolved and relocated fin spine. At its tip is the esca, a light-producing organ that glows because it is colonised by bioluminescent bacteria. The fish cannot make light itself; it farms the bacteria and controls the glow by regulating their oxygen supply. In the black midwater, a small pulsing light is irresistible to many animals, and when one approaches the anglerfish’s mouth, a huge gulp draws it in.
Built for a hungry ocean
Because meals may be weeks apart, the anglerfish is built to make the most of every one:
- Enormous jaws studded with long, needle-like, hinged teeth that fold inward.
- A distensible stomach and loose body wall that let it swallow prey larger than itself.
- A dark, non-reflective skin that hides it from the few other animals that produce their own light.
- A low-energy, sedentary lifestyle and reduced muscle and bone.
Sexual parasitism
Finding a partner in the bathypelagic zone is nearly impossible, and many anglerfish have solved it drastically. Males are tiny — a fraction of the female’s size — and equipped mainly with large olfactory organs to sniff out a female’s pheromones. On finding one, the male bites onto her flank and his tissues fuse with hers, sharing a blood supply. He then degenerates into little more than a pair of testes, permanently attached and ready to fertilise her eggs whenever she spawns. Some females carry multiple attached males.
Conservation
Deep-sea anglerfish are not commercially fished and live in one of the least disturbed habitats on Earth, so they are presumed secure and listed, where assessed, as Least Concern. The longer-term concerns for the deep pelagic realm as a whole are climate-driven changes in oxygen and food supply, and the possibility of future deep-sea mining and fishing.
Anglerfish: common questions
How does an anglerfish's glowing lure work?
The lure is a fishing rod evolved from the first spine of the dorsal fin. Its bulb-shaped tip, the esca, is hollow and packed with bioluminescent bacteria that the fish acquires from the surrounding water. The anglerfish controls the light by widening or narrowing pores that let seawater — and oxygen — reach the bacteria, so it can flash, glow steadily, or go dark. Hanging motionless, it dangles and twitches the light in front of its mouth, and smaller animals that come to investigate are engulfed.
Is it true the male anglerfish fuses to the female?
In many deep-sea anglerfish species, yes. Males are dwarfed, with huge nostrils for tracking a female's scent and pincer-like teeth. When a male finds a female he bites onto her body and never lets go — his skin and blood vessels merge with hers, his eyes, fins, and internal organs degenerate, and he becomes a permanent attached sac of testes drawing nutrition from her bloodstream. A large female may carry several fused males. It solves the problem of finding a mate in a vast, near-empty ocean.
How can an anglerfish eat prey bigger than itself?
It has hinged, inward-pointing teeth, jaws and a stomach that expand dramatically, and a flexible body. When prey is engulfed, the teeth prevent it from backing out and the stomach stretches to accommodate it. In the deep sea, meals are rare and unpredictable, so being able to swallow anything that comes within reach — even something twice your length — is a major advantage.
Do anglerfish live in the deep sea or shallow water?
Both, depending on the group. The famous glowing, lure-bearing anglerfish of nature documentaries are deep-sea species in the suborder Ceratioidei, living 1–4 km down. But anglerfish as a broader order also include shallow-water bottom-dwellers such as monkfish (goosefish), which use a fleshy lure on the seabed, and the walking frogfishes of coral reefs.