Scott Salyers
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Anglers who spend a lot of time in clear blue waters offshore – especially when drifting or slow trolling – have likely seen the stunning little blue-and-silver fish drifting this way. . Seen from above they resemble small flying fish thanks to their large pectoral fins. Miami resident Scott Salyers provided this photo, after it fell to the deck amid “a passel of pogies” from a castnet cast off Port Canaveral, Florida.
“We don’t know what could happen,” he said. The answer: a juvenile man-of-war fish, Nomeus Gronovius. It is a family member Nomeidaethe driftfishes.
The concept of niche evolution describes this species well, common to all warm oceans. It is called the man-of-war fish because, even when young, it lives in symbiosis with the Portuguese man-of-war, characterized by its bright pink-and-blue sail or float and its long string of tentacles is full with stinging nematocyst cells. Any angler or swimmer who gets nailed by them can be forgiven for shouting “ouch!” or worse.
The man-of-war fish takes up residence under these animals, which drift in the ocean. (Misleadingly called jellyfish, man o’ wars are siphonophores — each a floating community of organisms). In venturing about the stinging tentacles, the small fish get little protection from predators, though they are always on the lookout because they are not immune to the tentacles’ poison.
So how do they live with poisonous, sticky strings? Apparently, it has adapted to avoid the tentacles by being alert and agile. The species also has more vertebrae than usual, making its body more flexible, and relies more on its large pectoral fins, an adaptation typical of species that must be particularly agile. Beyond coexistence, there is evidence that the man-of-war fish can get some sustenance from its partner, which bites into the smaller tentacles.
Symbiosis suggests a two-way street, so the man o’ war probably got something from the man-of-war fish that swims freely in its tentacles, probably attracting other fish with the wrong sense of security to err until their death.
All of this describes the youth, and that’s all we see Nomeus Gronovius. This is because as they grow to adulthood — up to 15 or 16 inches long — they descend to live near the bottom in up to 3,000 feet of water. At that stage, the man-of-war fish bears little resemblance to the colorful, elegant little form it took on as a juvenile, looking more like a gray, slightly elongated bluefish.
