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DescriptionRhizostoma octopus has a solid appearance. It varies in colour from whitish pale or yellow to shades of green, blue, pink or brown. The umbrella margin is divided into a number of semi-circular lobe like extensions (marginal velar lappets). Beneath the bell is a large manubrium, consisting of a short pillar-like basal stem, followed by sixteen, three winged epaulets with frilled mouth openings. It then divides into four pairs of oral arms that consist of three winged portions, followed by three winged elongated terminal appendages (Russell, 1970). Mature males have blue gonads whereas, ripe females are reddish brown. When exposed to air their nematocyst warts give the umbrella surface a matt like appearance. The large crustacean Hyperia galba can be found throughout the body of the jellyfish and more commonly in its gastric or gonad pouches.Rhizostoma octopus is regarded by some authors to be a variety of Rhizostoma pulmo rather than a distinct species (Russell, 1970). The latter occurs in the Mediterranean and adjacent seas whereas, Rhizostoma octopus is found in northwest Europe. Russell (1970) states that the only valid character by which they can be distinguished is by examining the number of velar lappets on the umbrella margin. Rhizostoma pulmo average ten velar lappets in an octant, while Rhizostoma octopus have eight velar lappets per octant although other studies have shown Rhizostoma pulmo to also have eight lappets per octant, and further research is required (Russell, 1970).
It is sporadic in occurrence from year to year but occurs in 'swarms' in summer and autumn. It is believed that some specimens probably live in deep water during winter.