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(M. Sars, 1835)

Description
(Mainly after Russell, 1953a; also Naumov, 1969). Umbrella when young somewhat flatter than a hemisphere with moderately thick jelly, becoming strongly flattened at sexual maturity [T.multicirrata-medusa ]. Velum narrow, 1/20 - 1/30 bell radius.
Stomach start with small base, attached to subumbrella along arms of perradial cross; broad peduncle below. Mouth with four fairly long and broad lips with much-folded margins. Radial canals 4, these and ring canals narrow.
Gonads linear, somewhat sinuous, situated along middle 1/2 - 2/3 of length of radial canals; with median division in both sexes.
Marginal tentacles typically 200-300 but sometimes more, with broad swollen bases without ocelli [T.multicirrata-margin A]. No marginal cirri. Marginal vesicles eight, open, each with about 12-13 concretions and with one black ocellus at its base [T.multicirrata-ocell/marg; T.multicirrata-margin B].
Young specimen: T.multicirrata-young-habitus

Size
Diameter of umbrella usually up to 20 mm, sometimes up to 30 mm; sexually mature at 10 mm, more usually 14 mm diameter (Russell, 1953a).

Colour
Stomach, gonads and tentacle bases dull yellow, tentacle bases with black pigment granules which may also be present in dorsal walls of ovaries; manubrium and tentacle base light brown, gonads almost black.

Ecology
In British Isles, medusa found mainly March onward mature about July (Russell, 1953a); at Roscoff reported during September (Teissier, 1965).

Depth range
Medusa recorded in shallow coastal plankton (Naumov, 1969).
— Hydroid found so seldom that its habitat in Europe is unknown, although Rees, 1941b reported a colony on a Buccinum shell from off Millport, W Scotland, at 30 m; said to occur in 'well-washed', shallow places (Naumov, 1969). In British Columbia, reported common on stems of other hydroids and algae; and to feed on a range of prey and also, possibly, on decayed material and dissolved metabolites (summary in Arai and Brinckmann-Voss, 1980).

Distribution in the North Sea around the British Isles
Might occur throughout the North Sea, also in Skagerrak and Kattegat.

World distribution
Cool-water circumpolar. In North Atlantic Ocean from N Brittany (Teissier, 1965) and the New England states north to Barents Sea and W Greenland; (Russell, 1953a; Kramp, 1961). Apparently generally distributed in N Pacific Ocean (Kramp, 1961, 1968; Arai and Brinckmann-Voss, 1980; Yamada and Hirano, 1983). Known from southern coasts of Britain and Ireland but there are more records from further north; widespread in Scottish region (Fraser, 1972). A dense occurrence at Falmouth in 1895 was thought by Russell, 1953a to reflect unusual oceanographic conditions rather than the normal distribution. Later work on water masses in that area supports this.

[Description after Cornelius, 1995a]

Tiaropsis multicirrata