I presume that’s the skull of a fang blennie, and one species injects an opioid venom to mess with their predators’ brains.
The fang blennies have, generally speaking, evolved into rascals. Their ancestor probably used its fangs to scoop flesh out of bigger fish, much like the non-venomous bluestriped fang blenny does today. It’s a visual mimic of a cleaner fish, which allows it to get in close without raising an alarm. Then it strikes. “A total asshole move, of course, since that bigger fish is going to take this out on the next innocent cleaner wrasse it encounters,” says biologist and study co-author Bryan Fry of the University of Queensland.