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(Romanes, 1876)

Description
Umbrella hemispherical [Mitrocomella polydiademata ] or slightly higher when small (sometimes already fertile), becoming proportionately shallower when larger, with diameter about twice height; jelly fairly thick. Velum prominent, ca 1/4 bell radius.
Stomach short, four-sided, with small base attached to subumbrella along arms of perradial cross; mouth with four short lips with slightly folded margins. Radial canals four, these and ring canal narrow.
Gonads linear but slightly sinuous with 4-5 shallow zigzags; along 1/2 - 4/5 of radial canal with median division which is somewhat indistinct in female [M.polydiademata-female; M.polydiademata-male ].
Marginal tentacles recorded as 36-48 in fully grown European medusae, but up to 64 in NW Atlantic Ocean; with long, tapering basal bulbs [M.polydiademata]. No ocelli. Marginal cirri usually 5-9 between adjacent tentacles, sometimes more; coiling spirally. Marginal vesicles open, typically 16 (Edwards, 1973c, reported 10-19, the number sometimes differing between quadrants), each with 15-20 concretions, arranged in two rows or irregularly distributed.

Size
Diameter of umbrella when ripe 9-30 mm. Though Edwards found maximum diameter in Clyde ca 16 mm medusae up to 30 mm recorded Isle of Man (E. T. Browne, in Russell, 1953a: 260).

Colour
Stomach and marginal tentacle bases 'purplish or rosy, gonads yellowish-brown, purplish or rosy'; or stomach, gonads, marginal tentacle bases and tentacles pink or 'pale yellowish brown (often suffused with pink), with the yellowish brown sometimes more intense, almost ochreous'; Romanes (1876, 1877) recorded the colour as rosy.

Ecology
In British Isles, medusae reported mainly spring and early summer; earliest medusae recorded in the Clyde (February) March-April, mature ones April-June (July) (Edwards, 1973b). Other British records summarised by Russell (1953a) are from a similar period.

Depth range
Coastal.

Distribution around the British Isles
Much more numerous than Mitrocomella brownei. Recorded off Northumberland and Isle of Man northwards, being widespread in shelf waters east and west of Scotland, N of Orkney and Shetland, around Faeroes and a few stragglers west to Rockall and Lousy Bank (Fraser, 1972).

Distribution in the North Sea
All North Sea, also Skagerrak and Kattegat.

World distribution
Temperate to northern boreal, ranging from the British Isles and North Sea, including Belgium, the Norwegian and N Russian coasts as far east as the Kara Sea; hydroid stage also provisionally identified off southern Iberia (Ramil and Vervoort, 1992a). In NW Atlantic Ocean reported from W Greenland and Gulf of Maine. In N Pacific, from Washington State to Arctic Ocean (summary in Arai and Brinckmann-Voss, 1980).

Remark
The cuspidellid polyp only known from culture.

[Description after Cornelius, 1995a]

Mitrocomella polydiademata