A small blue-blotched fish that lives among the tentacles of a siphonophore.
A slender fish reaching about 25 cm, though the animals met on blackwater dives are a few centimetres long. Juveniles are silvery to translucent with a bold pattern of deep blue blotches over the back and flanks and long pelvic fins; larger fish darken and lose most of the pattern. The eye is large and the body compressed.
The species is associated with the floating siphonophore Physalia, living among the long fishing tentacles that hang below the float. The relationship is one-sided in the fish’s favour: the tentacles deter most predators, and the fish takes shelter there and feeds partly on the siphonophore’s own tentacle tissue and partly on animals the siphonophore has caught. It is not immune to the stinging cells. Tolerance appears to rest on avoiding the most heavily armed tentacles and on a degree of resistance rather than on immunity, and fish held against the tentacles are killed by them.
On a blackwater dive the fish is met either alongside a drifting siphonophore or, more often, alone in open water after separation from one. Records from Indo-Pacific blackwater sites are irregular and depend on how much Physalia is at the surface, which in turn depends on the wind of the previous days.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
