Why Do Anglerfish Have a Light? The Glowing Lure Explained

How the anglerfish lure works

An anglerfish’s “fishing rod” is a modified fin spine. In all anglerfish, the first spine of the dorsal fin has moved forward onto the head and become a movable rod called the illicium. The tip carries the lure, or esca. Small muscles at the base let the fish raise, lower, wiggle and twist the rod, making the lure dart about like a small living animal.

In the deep-sea anglerfish, a group of about 170 species, the esca is a glowing bulb. The humpback anglerfish, for example, holds its light just above its huge, toothy mouth. When curious prey comes close to inspect the glow, the fish opens its jaws and sucks it in. Its stomach can stretch to hold animals as big as or bigger than itself.

Hunting this way saves energy. Food is scarce in the deep sea, so instead of swimming after prey, the anglerfish simply waits and lets the prey come to it.

Where does the light come from?

The anglerfish does not make the light itself. The esca contains a dense colony of luminous bacteria, in the genus Enterovibrio, relatives of the Vibrio bacteria common in seawater. This kind of partnership, in which two species live together and both benefit, is called symbiosis. The fish gives the bacteria a sheltered home, nutrients and oxygen; the bacteria give the fish its light.

Genetic studies show that these bacteria have lost a large share of their genes compared with free-living relatives, a sign of a long and close relationship. Even so, young anglerfish do not seem to inherit them. Larvae hatch without glowing bacteria, and scientists think each fish picks them up from the surrounding seawater, possibly through a tiny pore in the esca, as it grows.

Because the bacteria need oxygen to glow, the anglerfish can probably control the brightness by changing the blood flow to the lure. Many species can also fold the rod down into a groove on the head.

Why does a light attract prey?

In the deep sea, light means life. Most animals in the open ocean below about 200 m (650 ft) can make light, and many use it to find food or partners, so a small glowing spot is something worth investigating. A lure might look like a tiny glowing shrimp, a jellyfish or a drifting scrap of food.

Most deep-sea glow is blue-green, because that color travels farthest through seawater and is the one most deep-sea eyes are tuned to. Anglerfish lures are thought to glow mainly in these colors too, although observers have reported different shades in different species. Some lures have extra flaps, threads or branches that may make them more attractive, and their shapes are so distinctive that scientists use them to tell species apart. Learn about other glowing animals in our list of bioluminescent animals.

Why do only females have lures?

In deep-sea anglerfish, the two sexes live very different lives. Females are relatively large, hunt with their glowing lures and produce eggs. Males are dwarfs, often less than 3 cm (1.2 in) long, with no fishing rod at all. Their job is not to hunt but to find a female in the vast darkness, using large nostrils to follow her scent and, in some species, large eyes to spot her.

In males, the bones that form the rod in females instead develop into toothlike hooks on the snout, used to grab hold of a female when they meet. In some families the male then fuses with the female for life. Read more in how do anglerfish mate?

Do all anglerfish lures glow?

No. Only the deep-sea anglerfish have glowing lures. Their many relatives in shallower water also fish with a rod and lure, but they rely on movement and disguise instead of light.

Other lights in the deep sea

Some deep-sea anglerfish carry more than one light. Female leftvents (genus Linophryne) also have a branched chin barbel with light organs, and warty seadevils have two or three small glowing bumps on the back called caruncles. Scientists think that the males of some species may recognize the right female partly from her lights.

Other deep-sea animals use light for very different jobs. Many fish and squid glow on their bellies to hide their outline against the faint light from above, a trick called counterillumination. Some jellyfish flash to startle predators, and a few dragonfish produce red light that most other deep-sea animals cannot see, like a private searchlight. For more strange creatures of the depths, see our list of deep-sea animals.