Unholy matrimony: the nightmarish sex lives of anglerfish

Deep-sea anglerfish have one of the most bizarre and disturbing mating behaviours in the animal kingdom, known as sexual parasitism, where dwarf males bite into and then permanently fuse bodies with larger females. However as off-putting as this may sound, it is actually a remarkably effective strategy.

A deep-sea anglerfish lies in wait for any fish who fall prey to their famous lure

Sexual reproduction is one of the most important biological processes in the natural world, the combination of genetic material between two separate individuals to create entirely unique offspring. It is the driving force of evolution by natural selection and without it none of us would be here today. Yet it is also one of the most diverse and fascinating behaviours throughout the natural world, because no species does it quite the same. As a result just about any sexual scenario you could possibly imagine probably exists somewhere on earth, even those you might not want to think of. One such example is the sexual parasitism of deep-sea anglerfish, where the much smaller males seek out, bite and then permanently fuse to females, where they spend the rest of their lives producing sperm for their egg laying hosts. Yikes!

Big fish, little fish

There are over 300 known species of anglerfish, belonging to the genus Lophiiformes, all of which are extremely varied in size, shape and overall appearance. Of the 300+ species around 160 live in the deep ocean, spending their entire lives in complete darkness. Of those 160 deep-sea anglers only 25 are known to engage in what is scientifically known as sexual parasitism. This makes it an extremely rare behaviour, not just in the animal kingdom as a whole but also amongst anglerfish. What is so intriguing about these particular species is that, as a result there is a big size difference between genders, known as sexual dimorphism. In these species the male is much smaller than the larger females and in most cases looks like an entirely different species, lacking the female’s enormous jaw and characteristic lure.

anglerfish species
Anglerfish are one of the most diverse groups of fish in the oceans and only a small fraction of them reproduce by sexual parasatisim

Identifying a mate

The reason for this drastic difference in size and appearance is down to the very different lives the two genders live. Females must hunt in order to gather enough resources to produce large numbers of energy expensive eggs, whereas the males sole purpose is to find a female. This is easier said than done, especially when you live in total darkness and have to find a specific species of female out of a possible 160. To do this the male anglerfish have developed the largest nostrils compared to the size of their head of any animal on Earth. This allows them to locate the right female via a unique chemical trail that they can detect over vast distances.

male anglerfish
A male anglerfish is a fraction the size of a female and lacks many of its characteristic traits such as a law and large jaw

However, as much it is the male’s responsibility to seek out a mate, the females also have to help them out by confirming whether or not they are in fact the right species to match. To do this they simply light up their bioluminescent lure, which they would normally use to hunt. This helps the male make his decision because each anglerfish species emits their own special light, thanks to a unique arrangement of filaments and pigment patterns. This acts as a ‘double measure’ allowing the males to confirm what their nostrils told them and that they have found a familiar female. Unfortunately this is something only 1% of male anglerfish will ever achieve and most live short sexless lives. However for those ‘lucky’ males that find a female, life will never be the same again.

Female fusion

When they have successfully identified a mate, males begin the mating process by biting into the female. This usually on their underside (or belly), but not exclusively, and if that wasn’t kinky enough for you then things start to get really demented. Where his mouth comes into contact with the females flesh, their tissues fuse together in a disturbing and permanent union. Over time the males teeth, eyes and famous nostrils disappear, known as atrophy, but they don’t die. Instead they are kept alive via nutrients from the females blood and oxygen they extract from their own gills. From this point on they are essentially just an additional part of the females whose sole job is to create sperm, essentially like an attachable penis.

duel male attachment
Two separate males fuse onto a female, the three will remain connected until the female dies

This nightmarish form of bondage will last for the rest of their shared lives which can be up to 30 years, the average lifespan of the female anglerfish. However it gets weirder, because females can fuse with multiple males creating a mix of fused bodies that become one living orgy. When it actually comes to reproduction though, it gets even stranger still. When she is ready to expel her eggs the female does so in a ‘gelatinous sheath’ behind her body, which is designed to trap the sperm released by her attached males and can be up to 30 feet in length. The gelatinous mix of sperm and eggs then breaks off and floats to the surface where the eggs develop into larvae that grow until they are big enough to migrate back down to the depths.

Why so weird?

As fascinating and intriguing as this behaviour is, it begs the question – why reproduce like this at all? The other 275 odd species of anglerfish don’t mate like this and although sexual parasitism exists elsewhere in nature, it is never so extreme. “The idea is basically that it’s a deep-sea economy measure” explains ichthyologist James Maclaine, of London’s Natural History Museum, speaking to WIRED magazine in 2013. He thinks that the reason behind the sexual parasitism and more importantly the extreme dimorphism in size, is that as a couple the anglerfish require much less food. Finding enough to eat for one can be challenging enough in the pitch blackness of the deep ocean, let alone findings a mate. So by fusing together in this way the two are guaranteeing a mate for life and using half as many resources to do so.

The drastic difference in size and combined bodies allows an anglerfish couple to use half the resources it otherwise would

Regardless of exactly what drove these deep-sea creatures to start reproducing in this way, the sexual parasitism of anglerfish is yet another glaring example that the most unique and alien forms of life on our planet can be found at the bottom of the sea. As we continue to explore the ocean’s depths it is likely we will find some other behaviours and interactions that will leave us equally as dumbfounded and amazed. It is therefore important that we keep searching down there to see what else we might discover.

No turning back, a male anglerfish bites and then fuses into a female

To see an example of the sexual parasitism of anglerfish, check out this short video below.

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