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(Macri, 1778)

Description
Umbrella hemispherical, translucent; exumbrella surface finely granular, jelly thick, central portion stiff, thinner and flexible in outer third. With 8-12 velar marginal lappets per octant; marginal tentacles absent. Eight rhopalia; rhopalial lappets smaller than inter- rhopalar, pointed.
Subumbrellar musculature in eight distinct peripheral muscle fields.
Stomach occupying central third of bell, roughly square with concave sides; from it 16 substantial canals connect to bell edge; younger specimens have narrow ring canal which follows closely outline of each marginal lappet; in many older specimens ring canal apparently absent in places and perhaps in some is completely lacking; an intermediate ring canal about 1/3 of radius in from margin, broad; centripetal to this is a coarse, irregular anastomosing network of canals, connecting only with intermediate ring canal and not with radial canals.
Peripheral to intermediate ring-canal a similar but finer meshwork, branchings become increasingly more fine towards perimeter.
Manubrium short, massive and translucent; concealed by 16 scapulets upon it. Each scapulet small, inverted Y-shaped in section, bearing numerous mouthlets. The eight oral arms are inverted Y-shaped in section, supporting two long, massive, outwardly-directed blades also bearing numerous mouthlets.
Oral arms without lateral clubs and filaments, each arm with a large, translucent terminal club. Four gonads, each a much convoluted lobe fundamentally forming most of a circle but not obvious due to convolutions. In older animals surface of gonad bearing grooves extending to its edge.

Size
Umbrella diameter typically to ca. 40 cm, exceptionally to 90 cm.

Colour
Surface of umbrella typically pale grey to colourless but may also be pale milk-white, opalescent, and rose-red. The gonads are largely white and may show diffusely through in (near-)mature specimens. Marginal lappets narrowly edged with ultramarine blue, violet, or brown line of dense colour, constant in width and following margin, rhopalar lappets less so or colourless. Scapulets with pink, bluish, violet, yellowish or reddish ‘frills’; canals in the terminal clubs of the oral arms may be bright blue, yellowish-brown, pink, deep purple-brown, or colourless. Maturing gonads tinged brown to reddish-brown or blue.

Ephyra
Ephyra are illustrated in figure R.pulmo-ephyra and the developmental stages are depicted in figure R.pulmo-development.

Ecology
Strobilation and the production of the ephyra stage seem restricted to the summer months; peak abundance of mature medusae in late summer and autumn with large numbers cast ashore in autumn and winter storms.
Specimens living in deeper offshore waters will probably survive the winter and can be encountered as late as June of the following year.

Depth range
Medusae are usually recorded at or near the water surface, but probably being more abundant in the (coastal) water column as the result of the strong currents of ebb and flow and resting on the bottom during slack-water periods.

Distribution in the North Sea
Indigenous in the southern North Sea with young medusae recorded off the Belgian and Dutch coasts and in the German Bight, occasionally found in the northern North Sea; also in the Baltic Sea.

World distribution
North and South Atlantic Oceans, Mediterranean, Black Sea, Red Sea.

[After Kramp, 1961; Russell, 1970a; Russell, 1978a; Bouillon, 1999]

Rhizostoma pulmo