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Péron and Lesueur, 1809

Description
Umbrella saucer-shaped with uniformly thick jelly thinning suddenly at bases of marginal lappets; exumbrella with central surface smooth, with prominent nematocyst clusters. Margin with eight deep adradial tentacular clefts, and four perradial and four interradial rhopalar clefts less deep; marginal lappets sloping asymmetrically outwards from the tentacular clefts towards the rhopalar clefts.
Marginal tentacles hollow, arranged in up to four incomplete serial rows, arising from subumbrella surface at a distance from umbrella margin; tentacles in adradial groups of 40-60 alternating with the eight marginal sense organs [C.lamarckii-habitus ].
Stomach without interradial gastric septa; with many gastric filaments arranged in four interradial groups on subumbrellar wall where central stomach cavity passes into gastrovascular sinus. Gastrovascular sinus divided into sixteen pouches by an equal number of radial septa extending from near the proximal border of coronal muscle to distal margin of umbrella, merging into fused areas of marginal lappets; pouches branched, terminal ramifications without anastomoses.
Subumbrella with radial and circular muscles [C.lamarckii-circular muscle ]. Coronal and radial muscle folds without, or occasionally with only few, pit-like intrusions from gastrovascular sinus; 16-20 coronal folds between radial septa.
The four oral arms arise from a ring of thickened subumbrellar mesogloea surrounding the base of the manubrium; each arm slightly shorter than the umbrella radius and thins out to form membranous lips on either side of a median furrow. Adjacent oral arms connected by a membranous, much-folded curtain about half the length of the arms.
The four interradial, very much-folded gonads hang freely down from the subumbrellar surface.

Size
Umbrella 60-150 mm, up to 300 mm in diameter.

Colour
Pale yellow to bright blue or deepest violet [C.lamarckii-habitus ], never the deep reddish brown colour of Cyanea capillata. The intensity of the blue might be due to rising temperature; all the darker colouration is in the gastrovascular endoderm and clearly visible through the transparent umbrella.

Ephyra
C.lamarckii-ephyra

Ecology and depth range
Tends to appear earlier than C. capillata, ephyrae may appear as early as in November and from British waters a late ephyra was taken in January. Very young medusae have been recorded off the Dutch coast in February; peak abundance of mature specimens in the southern North Sea from mid-May to the end of June, with mature specimens usually not found later than August to mid-September.

Distribution in the North Sea
Common throughout the North Sea, including the Skagerrak.

World distribution
More southern species than C. capillata, reported as far north as the Faeroes and Iceland and off the Norwegian coast just north of Bergen; absent from the Baltic.
Apart from being reported from the southern coasts of England and Ireland, and the Bay of Biscay, little is known about its southern limits.

[After Russell, 1970a; Russell, 1978a]

Cyanea lamarckii