What is "man-of-war fish"

Nomeus gronovii, the man-of-war fish, is a species of fish in the family Nomeidae, the driftfish. It is native to the Atlantic, Pacific and Indian Oceans, where adults are generally found at depths from . It is notable for its ability to live within the deadly tentacles of a siphonophore, the Portuguese man o' war, upon whose tentacles and gonads it feeds. The fish is striped with blackish-blue blemishes covering its body, and the caudal fin is extremely forked. It can reach a length of . It is of minor importance to commercial fisheries. This species is the only known member of its genus.