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Characteristics, distribution and ecology
Taxonomische classification
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(Linnaeus, 1758)

Description
Umbrella flat, flexible, transparent; with eight simple marginal lobes arising from exumbrella slightly above umbrella margin, shallowly — almost imperceptibly — cleft between.
Marginal tentacles small, filiform, up to a few hundred to more than 1000 in large specimens, arising nearly at bell edge. Subumbrellar musculature inconspicuous. From stomach lead 16 unbranched adradial canals, the eight perradial and eight interradial canals typically fork roughly dichotomously into branches, which connect approximately straight to ring canal.
Gonads invaginated, with external subgenital pits, appearing as four horseshoe-shaped ribbons in the gastric cavity, conspicuous due to their colour and transparency of bell.
The four unbranched oral arms as long as umbrella radius, tapered, V-shaped in section, with thick firm mesogloea and much-crenulated lips with many small tentacle-like processes along their margins [A.aurita-subumbrella ]. Oral arms in life typically held horizontally when young (when shorter than bell radius), dangling down in older specimens (when longer than radius).

Size
Diameter of umbrella usually up to 250-400 mm.

Colour
Transparent, some specimens colourless throughout, sometimes gonads coloured red through magenta to blue, in some specimens the extreme umbrella margin or the entire umbrella may be coloured the same as the gonads.

Ephyra
Developmental stages are shown in figure A.aurita-development. Ephyra are illustrated in A.aurita-ephyra, and juveniles are illustrated in A.aurita-juvenile.

Ecology
In British waters the planktonic ephyra stage may be expected any time from January to early April; mature medusae are usually found from the end of April to late August, some adults surviving into the winter in deeper waters. Ephyrae tend to appear later in more northern or colder waters. Generally an inshore species, it may establish local races up estuaries and probably in harbours; it may also be very sporadic in its appearance, its absence and sudden appearance probably due to wind effects.

Depth range
Usually found at the surface, it may go deep under adverse weather conditions.

Distribution in the North Sea
The most common Scyphomedusa throughout the North Sea; may be locally absent in some years.

World distribution
Cosmopolitan from northern boreal to tropical waters.

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

Aurelia aurita