Commensalism and Symbiosis
A small fish, Nomeus gronovii (the man o' war fish or shepherd fish), is partially immune to the poison from the stinging cells and can live among the tentacles. It seems to avoid the larger, stinging tentacles, but feeds on the smaller tentacles beneath the gas bladder. The Portuguese man o' war is often found with a variety of other marine fish, including clownfish and yellow jack. The clownfish can swim among the tentacles with impunity, possibly owing to their mucus, which does not trigger the nematocysts.
All of these fish benefit from the shelter from predators provided by the stinging tentacles, and for the Portuguese man o' war the presence of these species may attract other fish to feed on.
Read more about this topic: Portuguese Man O' War