When scientists first started capturing anglerfish for study, they were baffled at why all the specimens were female. Where were all the males? Though the males were nowhere to be found, female anglerfish were rarely discovered alone. Many came with tiny parasites attached to them.
After more careful examination, however, scientists reached a shocking realization: those tiny attachments weren't parasites; they were the male anglerfish!
It turns out the evolution of the male anglerfish has left them highly reduced. In some species, the males are not even capable of feeding themselves. Instead, they must quickly find a female to attach themselves to, or die. After attaching, their circulatory systems merge and she provides him with sustenance, while he provides her with sperm.