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To all except marine biologists, the well-known Portuguese man-o'war Physalia is likely to be regarded as the only British siphonophore. This is doubly unfortunate because Physalia is not a regular member of the British fauna and neither is it typical of the Siphonophora in general as it straddles the air/water interface (i.e. is pleustonic). Most of the fragile, beautiful and anatomically complex siphonophores are members of the plankton, seldom seen as complete organisms but nearly always as transparent fragments in plankton samples.
Included in this field-guide along with the 56 species of siphonophores is the only British "chondrophore", the 'Jack Sail-by-the-Wind' (Velella) – another member of the pleuston blown occasionally to our shores. Once considered to be related to the siphonophores, it has long been recognised as a somewhat p parallel branch of the hydrozoan cnidarians with only a superficial resemblance to the Siphonophora.
This field guide contains keys and descriptions for all 56 species.
