Bony fish · Perciformes · Nomeidae

Man-of-war Fish

Nomeus gronovii

Least Concern

The species is abundant and widely distributed; it faces no significant short-term risk. See full glossary

Nomeus gronovii is a fish of the family Nomeidae. It is the only member of the genus Nomeus. It is native to the Atlantic, Pacific and Indian oceans, where the adults are generally found at depths of 200 to 1,000 m. It is notable for its ability to live within the deadly tentacles of a siphonophore, Physalia physalis, on whose tentacles and gonads it feeds. The fish is striped with blackish-blue markings covering its body, and the caudal fin is extremely forked. It can reach a length of 39 cm. It is of minor importance to commercial fisheries. This species is the only known member of its genus. Nomeus gronovii is very scarce in the Caribbean Sea. It is very rare to see one, since it lives very deep in the sea.

Size

15 cm – 39 cm

Diet

Carnivore

Feeds mainly on other animals.

Activity

Diurnal

Active mainly during the day.

Social structure

Solitary

Lives and hunts alone, joining others only to breed.

Sighting difficulty

Easy

OceanCarnivore Diurnal Solitary Fish Ocean

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Fun facts

It tolerates doses of Portuguese man o' war venom that would be lethal to other fish of its size, though it is not immune: it moves carefully among the tentacles and can die if it becomes entangled.
It devours its host's tentacles, so the relationship is more parasitic than symbiotic.
The juveniles shelter among the shorter and more dangerous tentacles, and the adults stay beneath the float.
It lives among the tentacles of the Portuguese man o' war, and it does not merely shelter there: it eats its tentacles and its gonads.
Its resistance to the venom is around ten times greater than that of other fish, and it has at least one antigen against the toxin, but it is not immune: it actively avoids the large tentacles, the dactylozooids.
And it does not manage this with mucus, as the clownfish does with its anemone, but by dodging them. It has 41 vertebrae, an extremely high count that gives it extraordinary flexibility, and it swims mostly with its pectoral fins, something typical of fish specialised in manoeuvring in tight spaces.
Each man o' war can carry five to ten adults, with numerous juveniles around it.
It changes its way of life completely as it grows: the juvenile is pelagic, showy and lives at the surface, while the adult moves to live near the bottom, at great depth, with a far more subdued greyish appearance.

Life cycle & biology

Skeleton type: Vertebrate - internal bony skeleton (fish)
Habitat & environment: Surface waters of the open sea, associated with floating siphonophores

Sources consulted: https://es.wikipedia.org/wiki/Nomeus_gronovii