Anglerfish live deep in the ocean, in the darkest abyss. They are known for having scary fangs and skeletal faces, blind eyes, and dangly lures hanging from appendages in front of their mouths. But what you may not know is that some anglerfish have strange breeding methods.
At birth, male ceratioids are already equipped with extremely well developed olfactory organs that detect scents in the water. When it is mature, the male's digestive system degenerates, making him incapable of feeding independently, which necessitates his quickly finding a female anglerfish to prevent his death. The sensitive olfactory organs help the male to detect the pheromones that signal the proximity of a female anglerfish. When he finds a female, he bites into her skin, and releases an enzyme that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level. The male then atrophies into nothing more than a pair of gonads, which releases sperm in response to hormones in the female's bloodstream indicating egg release. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available.
Did you catch that? Basically the male anglerfish bites onto a female and then atrophies into nothing more than a pair of gonads! And this is so that when the female is ready to breed she's got sperm immediately available. It's like having portable testicles, "pocket nuts" if you prefer.
This reminds me of two things: The first is that song by King Missile called "Detatchable Penis." Ten points to anyone who remembers that song. The second is what it feels like to be a man when your mate is trying to get pregnant. . .
. . . reduced to little more than a pair of gonads.