Some Anglerfish 'Permanently Mate' and Become a Single Being

The carnivorous female anglerfish are ambush predators who use unique strategy to obtain prey.

They wait patiently in the depths of the sparsely populated deep sea to lure in their next meal. They wiggle and "angle" a fishing pole-like extension of their dorsal fin that protrudes from their head and emits light.

However, the deep sea anglerfish doesn't create this light on its own. Its lure is actually full of nutrients that sustain a colony of bioluminescent bacteria known as photobacterium. The exact process by which the bacteria gets inside the lure is currently unknown.

Once her prey comes close, the female strikes and snags them, using her large, sharp teeth to swallow prey, even if they are twice her size.

And in a scientifically groundbreaking 2018 video, scientists captured images of a female with numerous thin filaments extending from her body in addition to her main dorsal appendage.

These filaments also emit light, creating a bioluminescent web of whiskers to entice prey and ensnare it.

Some species of anglerfish (sometimes called sea toads) live in shallow, tropical waters near the surface, but the ones that pique the interest of scientists are the deep sea ceratioid anglerfish that live in the murky depths of the ocean — some as deep as 16,400 feet (5,000 meters).

Ted Pietsch, a professor at the School of Aquatic and Fishery Sciences at the University of Washington and author of Oceanic Anglerfishes: Extraordinary Diversity in the Deep Sea, has been studying the elusive deep sea anglerfish almost his entire career.

He reports by email: "There are about 166 species so far, but new ones are still coming up. They live so deep that we don't really have a good idea of how big they actually get. We send nets down to collect them, and the deeper we go the larger specimens come up."

But how do they manage to stay so close to the ocean floor at such depths?

Pietsch shares that most anglerfish, along with some other deep-sea fish, don't have a swim bladder (a gas filled sac that helps fish stay afloat without the need to constantly swim). The lack of a swim bladder not only helps them stay near the bottom, it also conserves energy — energy that's at a premium, given the difficulty of finding a meal so far down.

Female anglerfish are definitely running the deep-sea show.

Pietsch says, "Most females aren't much larger than your fist, but other species (most notably the Certias species) are close to 4 feet (1.2 meters) long. A male anglerfish on the other hand is significantly smaller. Usually an inch (2.54 centimeters) or so long. In the most extreme cases, the larger females are 60 times the length and about half a million times as heavy as the male."

The male, who has no way to feed itself, must rely completely on the female for survival. "They (the males) have tiny, pincher-like teeth on the tip of their snout, and they bite on to the female," says Pietsch. Afterward, the two actually merge and become one in a symbiotic relationship.

Strange Breeding Behavior

But it's not because they're in love. "The blood flows from the female to the male provides the nutrients. If they don't find a female, they're toast," says Pietsch. Scientists believe the female emits alluring pheromones that the male can sniff out with his proportionately large nostrils.

Their mating ritual really is quite unique, Pietsch says. "These are the only animals in the world that attach permanently to mates and exchange fluid."

Some species of deep sea anglerfish mate for only a short time, during which the male will drink blood from the female and pump sperm into her body, and detach once reproduction is complete. Other anglerfish species mate permanently: The male's head "melts" into the skin of the female and it gives its life entirely, becoming little more than a nub along the larger angler fish's body.

Scientists note that the size difference is a survival mechanism that allows them to thrive on the limited deep sea menu. If they were both large, it would take a whole lot more food and energy to keep them alive and their unique reproductive cycle going. By staying tiny, the male is able to pass on its genes without expending energy which would be required to hunt for prey.

These creepy-looking deep sea dwellers take sexual parasitism to a whole new level.

While the female has to carry the little guy around and keep feeding him, she's also getting a pretty good deal out of it too. There's no need to put herself out there to attract a loyal partner — she's got a sperm bank (or two or three or even six) available 24/7.

Even though the male is small, he's always got his testes ready and available to fertilize her eggs.

Anglerfish Spawning

Once fertilized, the female anglerfish is able to produce hundreds of thousands of eggs, which bundle together in strings and float their way up to the surface of the water. A huge amount of eggs is necessary because anglerfish do not look after their young, and the larvae will be vulnerable to predators all through the incubation process.

Once hatched, the tiny deep sea anglerfish immediately begin their journey down into the ocean's depths.

Some types of anglerfish are considered a delicacy in Japan, but Pietsch says the fatty and oily composition of other types doesn't make for a very tasty meal.

Well, unless you're talking to a sperm whale. Anglerfish remains have been found in whales' mouths and stomachs, and they seem to be the main predator of the larger anglerfish species.

While it's extremely challenging to find anglerfish ,and their lives are still somewhat of a mystery, scientists are entering a new realm of studying their behavior. Their scary-looking teeth and angry face may not look appealing to most of us humans, but they continue to light up the deep sea, attracting an oh-so-lucky male anglerfish right along with their next deep sea dinner.