Which deep-sea fish lures prey with a glowing rod?

The anglerfish, whose lure is a modified dorsal spine tipped with a bulb full of bioluminescent bacteria - the fish does not make the light itself, it farms a colony and appears to control the brightness by regulating the blood supply.

The reproduction is the part that stops people. In many species the male is a fraction of the female's size, has no functional gut, and exists only to find her - whereupon he bites on, their tissues fuse, their circulatory systems join, and he degenerates into a pair of gonads attached to her body. Females have been found carrying several. It is parasitism as a mating strategy.

That fusion should be immunologically impossible, and working out why it is not has been genuinely useful. A 2020 study found that anglerfish have lost large parts of the adaptive immune system - the machinery that would reject foreign tissue - which raises the question of how they survive infection at all, and makes them an unexpected model for thinking about transplant rejection.