As a biology major, I learned about the life cycle of various parasites. The worms present in the tail section of amberjack seem to be live worms and not dormant cysts (like trichinosis). What are they, and what is their life cycle? Do they have an intermediate host? Would they eventually invade the entire fish? Do they impede the fish? And if eaten uncooked, would they infect humans?
Several cestode (tapeworm) parasites have been described from greater amberjack (Seriola dumerili), and cestodes as a group infest many different fishes, utilizing them as either intermediate or final hosts. Based on the number of parasites and their location in your catch, you appear to have encountered Pseudogrillotia zerbiae, one species of what are collectively referred to as spaghetti worms. P. zerbiae infestations appear to be fairly specific to jacks (family Carangidae), and especially to members of the genus Seriola, although this worm has also been reported from black marlin and pilotfish. The parasite’s plerocercoid stage, which you observed, is most commonly found in the musculature of its host’s tail region and adjacent to its spine. Happily, mammalian experiments have shown that P. zerbiae is harmless to humans if ingested. Amberjack serve as intermediate hosts for these spaghetti worms; similar cestodes usually mature in sharks, and this is also likely the case for P. zerbiae. — Ray Waldner