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A mature female sea devil. Museum of Comparative Zoology, Harvard University

Life is hard in the bathyal zone - 300m down, pitch black, temperatures hovering around 4ºC and besides a scattering of sea stars and sponges, not much other life besides the occasional passing whale or squid.

A few slimy fish do manage to survive on the detritus sinking into the twilight zone and the meagre pickings of deepwater plankton.

Sea devils are one of those to scrape a living in this dismal environment. They are a family of deep-sea anglerfish known as Ceratiidae, with more than 160 known different species and can be found throughout the world's oceans.

One of the most peculiar features of these lonely anglerfish is the sexual dimorphism they exhibit.

Sexual dimorphism refers to the phenomenon where males and females are different in size and appearance, which is common in the animal kingdom. But these anglerfish take it to the extreme. Female sea devils are big, reaching up to 70cm and are frightfully viscous with rows of teeth and a tangle of bioluminescent lures.

Males, on the other hand, only grow to three centimetres and have no large teeth or even a jaw to help them catch and breakdown food. He is reliant on the snow of organic material falling from above which can barely sustain him.


A free-swimming male. Museum of Comparative Zoology, Harvard University

From birth, his sole purpose is to find a mate and fast. He has no means to survive long on his own and without a mate, the male anglerfish will never reach full maturity and will likely die within a month.

Once ready to mate the males stop feeding. This mechanism helps them to survive in the darkness by staying small and unnoticeable. They are born with a large liver used to sustain them while they scour the deep for a potential partner.

In these depths it is impossible to see other fish, the male can locate his date by her shimmering bioluminescent light show. If that doesn't work, they also have a strong sense of smell. The olfactory cells found in the male nose are large and are remarkably efficient in tracking down the pheromone which the females release.

Once the male finds a potential female, his large eyes will determine whether she is of the correct species.

If he is one of the one per cent of males lucky enough to find a mate, the strange ritual of their mating begins.

He bites on to her flesh with his tiny, but sharp teeth, tearing his way into her. His body releases a chemical that causes the skin around his mouth to grow towards the female. He then fuses his mouth to her body.

Her blood vessels will grow into this new skin and attach to the blood vessels inside him, grabbing them and connecting him to her circulatory system, a match made for life.

shutterstock 771961648

Two parasitic males attached to a mature anglerfish

He loses all autonomy as his body fuses to the females. He will get considerably bigger, compared to free-living males and, henceforth will be a parasite, unable to obtain nutrients on his own.

He is not dead. However, his only source of food will be from the female's blood and the nutrients she gains from eating.

The organs he has no use for such as the dorsal fins, eyes, nose and internal organs such as the digestive system will waste away leaving behind his reproductive organs for sperm production and his gills. His circulatory system will then fuse with hers joining their bodies together.

Food sources are scarce below 100m because photosynthesis is impossible - and even rarer at 300m. By sharing her body and the nutrients she gains, she improves her chances of producing viable offspring.

The male will remain attached to her for the remainder of her lifetime, which is around 30 years. A parasitic tumour, never leaving her side for the rest of his life until he dies with her.

The males do not become entirely useless. They become a source-and-demand reproductive mechanism. He is in effect a reproductive organ for the female, allowing her to impregnate herself when she is ready to lay eggs. He becomes permanently dependent on her. She becomes a self-fertilising hermaphrodite.

The production of eggs and sperm become synchronised. She gives the signal to her attached sperm producer by releasing a hormone in her bloodstream. Once it reaches what is left of the male, he will begin to produce sperm which can fertilise the eggs that she releases.

This relationship is not monogamous. Over her lifetime the female anglerfish can mate with up to eight males, all attaching themselves to her. They provide her with a lifetime supply of sperm for her offspring and she lets the males pass on their genes. Better than a short, hungry life in the dark.

The males will compete for the chance to fertilise the eggs by all producing sperm at once. Her eggs are released in a pool of buoyant gelatinous goo. As she releases them, the hormones she produces will travel to each male and they will all release sperm. The fastest male and the direction of the current will determine whose genes are passed on.





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