Humor me for a bit: how can evolutionary theory explain anglerfish mating? Which mutation pushed the first anglerfish male to say "fuck it, i'm fusing for life"? How did one generation of non-fusing anglerfish evolve to be fusing in the next?

Basic articles for reference:
https://www.nationalgeographic.com/animals/article/anglerfish-mating-rare-video-spd
https://www.sciencealert.com/anglerfish-become-physically-fused-to-their-mates-and-we-finally-know-how-they-do-it