| Female anglerfish (also known as monkfish) are pretty widely known as the meter long, ugly fish with bioluminescent lights dangling from their heads. Male anglerfish are much less known because of their diminutive size, only five or ten centimeters long, and their relative inedibility. While the females eat fish attracted to them by their glowing lure, males subsist on plankton until they find a female to mate with.
You see, anglerfish are one of the extremely few fish that mate for life, but not merely by staying together through the thick and thin, the ups and downs of fishy partnership. Instead, the male anglerfish uses his very sharp teeth to bite into the side of the female, sort of like a lamprey. For a while the male stays this way, feeding on the female's blood and releasing sperm as needed to fertilize the female's eggs.
It doesn't end here, though. I said they mate for life, remember? First, the male's skin fuses to the female's, and becomes spiny to match her own, leaving two open holes to circulate water through. Next, blood vessels grow between the male and female through the attached skin, removing necessity of the male to do any active feeding at all. The male's testes start expanding, and other organs (now unneeded because of the shared bloodstream) atrophy to make room and energy for their expansion. Eventually the male's eyes and nervous system disappear entirely, and he becomes essentially a reproductive organ of the now hermaphroditic "female" anglerfish. Females with up to five attached males have been found.
There's a metaphor for human relationships lurking somewhere here, I'm sure of it :-)
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