(Linnaeus, 1758)
Description
Pneumatophore huge and asymmetric and lies horizontally on the sea surface. The cormidial elements are attached on one side of the float, at its oral end. The tentacles can stretch out to many metres below the float and can inflict a painful sting on an unsuspecting bather. A longitudinal crest runs along the top of the float and can be raised to act as a sail. The effect of the wind on this sail is to move the left-handed specimens to the right of the wind direction and vice versa for the right-handed ones (dimorphism in the direction of the crest in respect to the pneumatophore). Colour of pneumatophore is bluish.
Size
Pneumatophore up to 30 mm in length.
Depth range
In the warmer waters on the surface.
Remarks
The species is the only pleustonic siphonophore.
Distribution in the North Sea
Unlikely to occur in the North Sea, but may be found on occasion cast ashore on the southern North Sea coasts of France or Belgium. There is a single record of P. physalis stranded on the west coast of Belgium near Knokke in April 1912 after a westerly gale (Lameere, 1912). There are hundreds of records of stranded P. physalis on the SW coasts of England, Wales and Ireland. Strandings particularly occur during or after periods of strong SW and W winds, but are not necessarily indicators of the northward spreading of warmer Atlantic waters.
World distribution
Tropical and subtropical panglobal species. Enters the eastern English Channel.