Sorry for the clickbait, but this is fishy. Published this week in Science, here is a non-paywall summary
Krøyer’s deep-sea anglerfish, Ceratias holboelli, does not spawn, copulate, or do anything a fish would ordinarily do to mate. Instead, the male—just a few inches long—clasps onto the comparatively gigantic female’s body and never lets go. Slowly, his body morphs into hers, his cells becoming hers, including his testicles, which are used to make offspring. As he vanishes, two individuals become one—taking the concept of monogamy to a new level.