In the darkest depths of the ocean where the water is freezing cold, the pressure is enormous and food is scarce lives the angry-looking deep sea anglerfish. This fascinating fish is not only an incredible example of how organisms manage to survive in the most inhospitable environments, it has one of the most bizarre ways of reproducing in the animal kingdom.
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 containing needle-like teeth. The tip of this “fishing rod” on top of their head is bioluminescent (glowing in the dark) because of bacteria living inside it and attracts prey in a similar way like 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 extent 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 adaptation.
Despite their ferocious appearance, the deep sea anglerfish is actually pretty small. Females will only reach a length of about 10 centimetres, while male are only a fraction of this and 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.
Because individuals are so sparsely 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 captured where females. However, after some time they noticed small “growths” on the females skin that, upon closer inspection, turned out to be males (see image below). The tiny males actually only live 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 except for its testes.
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 at any time or place without worrying about meeting a male in the darkness of the ocean.
It is astonishing that species can evolve to such extremes that males become nothing more than a small lump on the skin of a female to assure her of her own lifetime supply of fresh sperm.