Nomeus gronovii (Bluebottle-fish)

From fishes of australia: The striking pelagic juveniles shelter amongst the tentacles of floating Bluebottles (Physalia or Portuguese Man-o-War) which are colonial animals called siphonophores (not jellyfish). Juveniles are silvery-white with a dark blue back, dark blue markings on the sides, large dark fan-like pelvic fins and a deeply forked tail. Adults live near the bottom and are mostly dark brown with large dark pelvic fins.