(species Nomeus gronovii), small marine fish of the family Nomeidae (order Perciformes; sometimes placed in family Stromateidae), noted for living unharmed among the stinging tentacles of the Portuguese man-of-war jellyfish (Physalia). The man-of-war fish is usually found in the open sea, near its protector. It is striped or mottled, with large, black pelvic fins and is about 7.5 cm (3 inches) long.