This beautiful bright-blue member of the jelly-fish family is an immigrant from more tropical waters and is often washed up in large numbers on our northern west coast beaches. Unlike the common jellyfish, it is not a single individual, but a colony of organisms up to 6 in. long, the whole group floating by means of an air-filled bladder on the upper surface. From the under surface hang a number of long tentacles armed with powerful stinging cells which can paralyse quite large fish, and can also give human beings a very painful sting. Above the water surface projects a diagonally placed crest, very like the sail of a yacht and serving the same purpose. The sail may be either left- or right-handed or in yachtsman's terminology the man-of-war may be on either the “starboard or the port tack”. It is believed that the direction of the sail is related to the prevailing wind; thus it will usually be in opposite directions north and south of the equator. A similar but smaller species is the “By-the-wind-sailor” (Velella) which also has a raft and sail.
by Richard Morrison Cassie, M.SC.(N.Z.), D.SC.(AUCK.), Senior Lecturer in Zoology, University of Auckland.