The spectrum of evolutionary adaptions is far wider than you can possibly imagine. Parasites and symbiotes provide some bizarre stories. But here's something you may not have expected from a vertebrate.
There's an order of fish called Lophiiformes. The most well-known species is the deep-sea Anglerfish, so-called because of the lure they use for attracting prey. But there's something stranger than that.
With the Anglerfish - as for most Lophiiformes - the male's digestive system degenerates when mature - so he cannot eat. With an extraordinary sense of smell, he locates a female by the pheromones she releases. The male then bites the female's side, and releases an enzyme that digests the tissue of her body and his mouth. The circulatory systems then fuse, and the male's organs all waste away - except the gonads. This chimera now has everything needed for reproduction.
Evolutionary oddity #2: the giant tube worm.