Six species in the area, of which the medusae can not be separated sufficiently:
¥ Obelia bidentata Clarke, 1875
¥ Obelia dichotoma (Linnaeus, 1758)
¥ Obelia geniculata (Linnaeus, 1758)
¥ Obelia longissima (Pallas, 1777)
¥ Obelia lucifera Forbes, 1848
¥ Obelia nigra Browne, 1900
Description
(All species). [Obelia-medusa] Umbrella flat, with velum minute or lacking (authorities differ). Stomach short, with quadrangular base, without peduncle; mouth with four short simple lips. Radial canals four, straight; they and ring canal narrow, the latter hard to discern.
Gonads spherical to ovoid, sac-like, hanging from middle to end-regions of radial canals.
Numerous short solid marginal tentacles, somewhat stiff, not extensile; with axial core of single row of gastrodermal cells; each tentacle with small basal swelling, with short gastrodermal root into mesogloea of umbrella margin. No cirri. Eight closed adradial marginal vesicles, each situated on underside of basal bulb of marginal tentacle and containing a single concretion.
Note — the medusa stages of Obelia spp. are common in coastal and offshore plankton throughout the world. They occur all round the British Isles. At present, however, no certain characters have been published by which they can be distinguished morphologically. A review of available information and an assessment of the nominal species has appeared elsewhere (Cornelius, 1990a; see also Cornelius, 1975a; Cornelius, 1982a).
Size
Diameter of full-grown umbrella 2.5-6.0 mm.
Colour
Bases of marginal tentacles pale straw, yellowish, brown, dark brown, black, or colourless, the colour possibly dependent on recent food source; gonads yellowish brown or brown.
Ecology and depth range
Medusa stages are common in coastal and offshore plankton.
Distribution in the North Sea
They occur all round the British Isles.
World distribution
The genus has a worldwide distribution except the high-Arctic and Antarctic seas.
Remark
Medusa of the Obelia complex are not distinguishable by simple means and are usually indicated as Obelia spp. There may be characters in the detailed structure of the nematocysts.
[After Cornelius, 1995b]