Sea devils are a group of anglerfishes from the deep seas that are known for their fearsome appearance and their use of lures to catch prey. Both elements were used to great effect in Finding Nemo, where Marlin and Dory are nearly eaten by a sea devil after being entranced by its lure (which it produces by cultivating luminescent bacteria at the end of an elongated fin spine). Though their huge mouths and inflatable stomachs are certainly awe-inspiring, I’d like to point to another aspect of their lifestyle: sea devils have perfected the art of sexual reproduction.

It has often been noted that sex is rather wasteful, as only the females can actually carry offspring. The sea devils have therefore reduced the role of the male to its bare essentials: sperm production. At maturity, the males are only a fraction the size of the females (typically no larger than the females’ pectoral fin). At this point they stop eating and look for a female, which they bite, never to let go. Their blood vessels fuse, allowing the male to take nutrients from the female, and the male gets rid of most of its body, except for the testicles, which swell. The male then spends the rest of its life as an appendage to the female’s body, so she can draw sperm from him whenever she needs it.

Female Krøyers angler fish (Ceratias holboelli) with a tiny male attached to her belly.
Female Krøyers angler fish (Ceratias holboelli) with a tiny male attached to her belly.

A little swimming scrotum searching the deep seas in the hope of finding a mate. Now isn’t that just lovely.

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