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In the darkest depths of the ocean where the water is freezingly cold, the pressure is enormous and food is scarce lives the angry-looking deep sea angler fish. It is an incredible example of how living organisms manage to survive in even the most inhospitable environment. Even more incredible is their way of reproducing, which is one of the most bizarre in the animal kingdom.

The ferocious looking deep sea angler fish has one of the most extreme methods of reproduction

The deep-sea anglerfish is one of the deep ocean’s most fascinating residents. Living at the extreme depths between 1.5 and 2.5 km, their most peculiar appearance is a modified fin that acts as a lure, hanging above their huge mouths with needle-like teeth. The tip of this “fishing rod” on top of its head is bioluminescent (glowing in the dark) because of bacteria living inside it and attracts prey as moths to a lightbulb. By wiggling the glowing outgrowth, it lures prey close enough to devour them whole! Because of thin, soft bones and jelly-like skin, these scary looking fish can expand their jaws and stomach to such an extend that they are even able to actually swallow prey up to twice their own size. In the deep sea where food is very scarce, being able to eat large quantities at once is a great adapation.

The scary looking deep sea angler fish has long needle-like teeth

Despite its ferocious appearance, the deep sea angler fish is actually pretty small. Females will only reach a length of about 10 centimeters, while male are only a fraction of this can can be more than 10 times smaller. Males of one specific species of are even the world smallest fish, measuring only some 6.2 mm. However, because of the much larger size of females, it is not the smallest species of fish in the world.

Males of the deep sea angler fish are very very tiny and may only measure some 6mm!

Because individuals are so sparesly populated, finding a mate in the depths of the deep sea is problematic. To still be able to reproduce, the deep-sea anglerfish have one of the most bizarre methods of reproduction.

When scientists first discovered these fish, they kept wondering why all the fish they captures where females. However, after some time they noticed that small “growths” on the females skin, which upon closer inspection turned out to be males (see image below). The tiny males live solely to find a female. Upon encounter, a male bites into her skin after which the tissues of the male and female become completely fused together so that their blood vessels join as one. The male now assured of a life without the need to hunt for food, degenerates his eyes and all internal organs exept for the testes!

This 46 mm female Photocorynus spiniceps has what looks like a small nub in the middle of her back. Actually, that's a 6.2 mm long male, the world's smallest known, sexually mature vertebrate

A female can carry up to six males on her body at a time and is therefore assured of a fresh supply of sperms. She can reproduce any time or place, without worrying about meeting a male in the darkness of the ocean.

I am astonished species can evolve to such extremes that the male is becomes nothing more than a small lump on the females skin to assure her of her own lifetime supply of fresh sperm.

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Written by Jolle Jolles

Jolle Jolles is a zoologist working at the University of Cambridge with a love for natural history and animal behaviour and enjoys fascinating others about our weird and wonderful natural world. Follow Jolle on Twitter or sent him a personal message.

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