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(Linnaeus, 1758)

Description
Umbrella saucer-shaped with uniformly thick jelly thinning suddenly at bases of marginal lappets; exumbrella with central surface smooth, without 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 complete serial rows, arising from subumbrella surface at a distance from umbrella margin; tentacles in adradial groups of 70-150 or more.
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.capillata-circular muscle ]. Coronal and radial muscle folds with pit-like intrusions from gastrovascular sinus; 13-15 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 as long as 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 usually 300-500 mm, up to 2,000 mm in diameter.

Colour
Usually yellowish brown with dark red manubrium (manubrium darkens with age), sometimes paler or almost colourless [C.capillata-habitus ]. Viewed from above the darker pigmentation of stomach and gastrovascular pouches shows up strongly with a clear pattern due to the weakly pigmented radial and ‘false’ septa.

Ephyra
The ephyra is immediately recognisable as a Cyanea by the long horn-like diverticula at the distal corners of the radial canals [C.capillata-ephyra ]. Developmental stages are illustrated in figure C.capillata-development.

Ecology and depth range
In British waters the ephyra stage may appear as early as February; small medusae are usually found in April-May; more mature specimens are not found until June with a peak abundance in July to September, a few large specimens may survive the winter in deep water. In the southern North Sea peak abundance of mature specimens from June to September, in the northern North Sea from August to September. In Danish waters ephyrae may appear throughout the summer leading to a second peak abundance in autumn, adults of which are rarely seen as they probably seek the bottom to avoid the winter storms. Adults from this second wave have been reported off the Swedish coast in January or from February to April. Very young medusae usually cling to the bottom by their oral arm folds; mature medusae are a common sight on the surface at all hours of the day, probably only driven down by rough weather. Light sensitivity has also been reported with medusae swimming to the surface as evening approaches after spending the daylight hours at a depth of a few meters or deeper.

Distribution in the North Sea
Common species in the northern and southern North Sea, usually outnumbered by Cyanea lamarckii.

World distribution
Northern boreal species with circumpolar distribution, possibly cosmopolitan.

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

Cyanea capillata