Not one animal but a colony of specialised clones strung together, swimming and hunting as a single body.
A siphonophore is a colony. It grows by budding genetically identical individuals that never separate and instead specialise: some become swimming bells, some become feeding polyps with a stinging tentacle, some become reproductive units, some are reduced to protective shields. They are arranged in order along a central stem, and the whole assembly is coordinated well enough to behave as one animal. Whether to call it an organism or a colony is a real question rather than a semantic one, and it is the reason the group is interesting.
On a blackwater dive the usual encounter is with a physonect: a small gas float at the top, a compact stack of transparent swimming bells below it, and then a long trailing stem carrying the feeding polyps, which may extend many metres past the edge of the torch beam. The animal fishes by hanging that stem out as a curtain of stinging tentacles and waiting.
This is the group that includes the Portuguese man o’ war, which is why the sting has a reputation. The species commonly met on a night drift in the Indo-Pacific are far less severe, but the practical warning holds: the visible part is not the whole animal. Divers are stung on the forearms and the back of the neck by a stem they never saw, having approached the bells with a camera. Working the head end from the up-current side, with the rest of the animal accounted for, is what avoids that.
They are also fragile. Backwash from a fin or a hand is enough to break the stem, and a broken colony does not reassemble.
Identification from photographs is possible to family in some cases and rarely below it. Log the form — physonect, calycophoran, or the flat-swimming Praya-like types — and leave the name at Siphonophorae.
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.
