From a review of The Deep by Claire Nouvian, a beautiful book from the University of Chicago Press on the creatures who live in the deepest parts of the ocean:
To understand the full extent of the constraints that the abyss places on life, consider the black seadevil. It’s a somber, grapefruit-sized globe of a fish–seemingly all fangs and gape–with a “fishing rod” affixed between its eyes whose luminescent bait jerks above the trap-like mouth. Clearly, food is a priority for this creature, for it can swallow a victim nearly as large as itself. But that is only half the story, for his description pertains solely to the female: the male is a minnow-like being content to feed on specks in the sea–until, that is, he encounters his sexual partner.
The first time that a black seadevil meets his much larger mate, he bites her and never lets go. Over time, his veins and arteries grow together with hers, until he becomes a fetus-like dependent who receives from his mate’s blood all the food, oxygen, and hormones he requires to exist. The cost of this utter dependence is the loss of function in all of his organs except his testicles, but even these, it seems, are stimulated to an action solely at the pleasure of the engulfing female. When she has had her way with him, the male seadevil simply vanishes, having been completely absorbed and dissipated into the flesh of his paramour, leaving her free to seek another mate. Not even Dante imagined such a fate.
Next time you are in a bookstore, go find this book. It has the most amazing photographs of these mysterious fish.