Many of us got our first glimpse of the terrifying creature that is the deep sea anglerfish (Melanocetus johnsonii) in the movie Finding Nemo, when our hero, Marlin, is lured to the terrifying jaws by a fluorescent lantern attached to the fish. Perhaps, if the makers of the movie knew the mating behavior of this menacing-looking fish, they would have realized it is not fit for a children’s movie, but rather for a horror film.

The Unusual Deep Sea Anglerfish

All anglerfish belong to a group of fish called Lophiiformes, however the most unusual anglerfish are those from the suborder Ceratiodei, which consists of 160 species. These fish are found at depths below 984 feet throughout the world’s oceans.

Ceratioid anglerfish are unusual as they show extreme sexual dimorphism – meaning the females are much larger than the males. The males are so small, that males from the species Photocorynus spiniceps are considered one of the world’s smallest vertebrate species, and female ceratioid anglerfish can be more than 60 times longer, and half a million times heavier than the males.

Males Do All The Effort

When they are young, males and female anglerfish look similar, but go through metamorphosis when they develop into adults. Females take on the characteristics that we associate with anglerfish – they develop large teeth and a fleshy, glowing lure to attract prey. Males grow large, developed eyes and nostrils. Males also lose their teeth and grow a set of denticles – toothlike projections that look like pincers that sit at the front tips of their jaws, and are necessary for mating.

Males spend all their lives swimming through the deep dark ocean looking for females. Their large nostrils and excellent sense of smell help them locate female pheromones. The species that have less well-developed nostrils use their large eyes to locate the bioluminescent lures of the female fish.

A Horrific Mating Spectacle

In the 1920s, an Icelandic biologist named Bjarni Saemundsson discovered the body of a female ceratioid anglerfish with smaller fish attached to her belly by their snouts. He assumed it was her babies, and wrote about the discovery “I can form no idea of how, or when, the larvae, or young, become attached to the mother. I cannot believe that the male fastens the egg to the female…This remains a puzzle for some future researchers to solve”.

What Mister Saemundsson did not know was that the fish attached to the large female was not her offspring, but rather her mate. Once a male finally locates a female, he uses his specialized denticles to latch onto her belly region. Strangely, he is upside down when he does this. After latching, the tissues of the male and female anglerfish start fusing, and their circulatory systems connect – a phenomenon that researchers are yet to understand and explain the mechanisms of.

After fusing, the male is fully dependent on the female for nutrients transported through her blood. Organs the male used to survive, such as his eyes, fins and internal organs, degenerate and wither away. The female is suddenly a self-fertilizing hermaphrodite as she now has the ability to fertilize herself through the fused male. While the male is attached to the female, he grows substantially larger than any free-swimming male. He remains alive, and is able to reproduce as long as the female is alive. Sadly, free-swimming males will die if they do not find a female within the first few months of their lives.

Mating in ceratioid anglerfish is a form of sexual parasitism. Unlike with many other animal species, the female ceratioid anglerfish has no choice in who her mate is, and it has been observed in some anglerfish that the female hosts up to eight males. When a female is sexually mature and ready to reproduce, fertilization takes place externally: males and females simultaneously release their sperm and eggs into the surrounding water. Scientists suspect that this synchrony of external fertilization is done through hormonal communication.

Caught In The Act

Naturalist William Beebe described the mating of anglerfish perfectly in 1938 when he wrote in Ceratias – Siren of the Deep: “But to be driven by impelling odor headlong upon a mate so gigantic, in such immense and forbidding darkness, and willfully to eat a hole in her soft side, to feel the gradually increasing transfusion of her blood through one’s veins, to lose everything that marked one as other than a worm, to become a brainless, senseless thing that was a fish – this is sheer fiction, beyond all belief unless we have seen the proof of it.

Although corpses of fused anglerfish had been found by scientists, the mating ritual was finally caught on film for the first time in 2016 by deep sea explorers Kirsten and Joachim Jakobsen. When the pair was exploring the waters around Portugal’s Azores islands in a submersible, they spotted an odd shape floating through the water at a depth of 2600 feet (800 meters). Turns out this was a ceratioid anglerfish pair. The Jakobens were able to film the pair for 25 minutes, resulting in a video that captures a unique and never before spectacle. The video allowed anglerfish researchers to observe the fish (belonging to the Caulophryne jordani species) in their natural habitat.

This video also allowed researchers to closely observe the unusual body structure of this species of anglerfish. Anglerfish have filaments and fin-rays on the outside of their bodies, which they used to locate prey, and when prey touches one of the filaments or fin-rays, the anglerfish can turn towards it and get a meal. However in the video, researchers were shocked to discover that each fin-ray and filament moved independently of the other (in other fish species these structures move as a single unit), meaning each has its own set of muscles and nerves.

It is important to note that extreme sexual dimorphism and parasitic mating are not found among all anglerfish species. In other suborders, the males remain free-swimming all their lives, hunting on their own, and only attach to females temporarily for reproduction.

Why Not Observe Anglerfish In The Lab?

Unfortunately for scientists, anglerfish cannot survive in laboratory conditions. Despite the best attempts of the scientists to mimic natural conditions, anglerfish are unable to adapt to different temperatures and pressures compared to that in their natural environment in the deep sea.

Anglerfish are an incredibly diverse group, with a large variety of species consisting of different shapes and sizes. Due to their inability to survive in a laboratory, and their home thousands of feet below the surface of the ocean, they are incredibly hard to study. Luckily for researchers and anglerfish enthusiasts, there are ongoing advances and development of deep-water exploration technologies, and researchers are hoping that soon more footage will arise of these terrifying creatures in their natural habitat.