Anglerfish don’t just mate- they bond for life, literally. The tiny male bites into a female, and never lets go. He eventually becomes fused with her body.
She provides nutrients to keep him alive, and he provides sperm for her eggs. The scary-looking anglerfish lives so deep in the ocean that they very rarely survive being brought to the surface. What we know about them is mainly from studying dead specimens.
But in 2016, a mating pair was captured on video for the first time.