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(M. Sars, 1835)

Umbrella truncate-conical or wide bell-shaped, slightly higher than wide, with jelly thin at sides and thickened in apical region [M.octocostatum-medusa ]; usually (but not always, Arai and Brinckmann-Voss, 1980) with three to seven (sometimes branched) nematocyst tracts on subumbrellar surface in each octant. Velum narrow.
Stomach short, octagonal, with broad base. Mouth with eight simple to slightly folded lips. With eight somewhat broad radial canals; ring canal narrow.
Gonads sinuous-sided, reaching nearly to umbrella margin.
About 64-72 large hollow marginal tentacles with laterally compressed bases, and about as many smaller ones alternate in position [M.octocostatum-margin ]. No marginal cirri or sense organs.
Development: M.octocostatum-development-hab.

Medusa height 11-13 mm, width 10-14 mm.

Stomach, marginal tentacle bases and gonads yellow to yellowish-brown.

Medusae occur in the western North Sea in August-September and December (Evans, 1978), most plentiful in October, maturing till December (McIntosh, in Russell, 1953a).

Depth range
Medusa neritic, not usually occurring more than 30 km from land. Penetrating into brackish fjords, Baltic Sea and other brackish areas (Arai and Brinckmann-Voss, 1980).
— Hydroid noted on the horse mussel, Modiolus modiolus, at 90 m depth (Robson, 1914).

Distribution in the North Sea
Limited; northern North Sea, south as Northumberland and Helgoland (cf. Staurophora mertensii). Skagerrak and Kattegat, and as far as Kiel Bight in the Baltic Sea (Thiel, 1970). Not yet reported from southernmost North Sea.

World distribution
Reported from cool waters in the northern hemisphere and, curiously, from tropical Australasia. In N Atlantic Ocean as north as the Barents Sea and off Iceland and Greenland. In N Pacific Ocean from arctic regions south to Washington State and Japan. In the southern hemisphere, reported from Papua New Guinea (Bouillon, 1984b, 1985b). (Distribution mostly after Russell, 1953a; Naumov, 1969; Arai and Brinckmann-Voss, 1980; see also Fraser, 1972.)

Notes — The description of the Medusa follows Russell, 1953a. That of Arai and Brinckmann-Voss, 1980, based on living NE Pacific material, differed as follows: umbrella deep lens-shaped, up to 20 mm diameter; height ranging from 1/3 width to slightly higher than wide; up to 88 large marginal tentacles, the alternating smaller ones sometimes being almost as large; manubrium, radial canals, gonads and marginal tentacle bases white, orange-yellow or yellowish-brown. Most of the differences perhaps reflect the greater size of their material. They published a photograph of an individual 6.4 mm high, and a diagram of a fertile hydranth with medusa bud from Newfoundland; and like Russell provided detailed discussion of the species.
Gemmill, 1921 recorded swimming as weak, with periods spent on the bottom as cited by Arai and Brinckmann-Voss, 1980: 82, but Russell, 1953a reported the species to be a voracious and presumably active carnivore. Kramp and Damas (1925) described the larger tentacles as bending upwards basally above the bell margin, the alternating smaller ones being directed downwards and trailing behind the swimming medusa.
Very young medusae, up to about 2 mm in diameter, have only four radial canals. They were illustrated and described by (Gemmill, 1921 and Russell, 1953a among others. When about 4 weeks old, at about the 10-tentacle stage, interradial buds appear off the stomach which, over the next two weeks, grow down to meet similar upgrowths from the radial canals hence increasing the number of radial canals to eight by the time the growing medusa has 14-16 tentacles (Gemmill, 1921).

[Description after Cornelius, 1995a]

Melicertum octocostatum