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Brandt, 1838

Description
Bell large, when full grown about 1/8 high as wide, and when above ca 30 mm diameter lower than hemisphere; jelly thick. Velum narrow, 1/6 of bell radius or less.
Stomach, mouth and the four radial canals combined to form large gastric cross, reaching from centre nearly to umbrella margin. Mouth also cross-shaped, in form of four long slit-like openings extending nearly along whole of radial canals and with conspicuous much-folded marginal lips; opening along whole length directly into overlying radial canals which have numerous short lateral diverticula which increase prominence of the cross; short distal portions of radial canals, and ring canal, narrow.
Gonads on diverticula of radial canals, extending nearly to bell margin and virtually to centre.
Marginal tentacles numerous, up to several thousands in large specimens. in length about 1/6 of full-grown bell diameter but proportionately longer in smaller specimens, often coiled; with elongate conical basal bulbs and well-developed pointed gastrodermal spurs. No marginal cirri. Adaxial ocelli on each tentacle base. Many cordyli, approximately alternating with tentacles, with narrow stems and lacking nematocysts.
Young specimen: S.mertensi-young .

Size
Diameter of bell usually stated as 'up to 200-300 mm' though specimens 45 mm in diameter and less already have the peculiar gastric system and may be sexually mature. For proportions see above.

Colour
Colouration (after Russell, 1953a; Naumov, 1969; Arai and Brinckmann-Voss, 1980): umbrella with glass-like transparency, sometimes tinged bluish; gonads, radial canals and ring canal white to brown; marginal tentacles white to rosy; mouth-lips rosy to yellowish; ocelli dark violet or dark brown. The gastric cross has been reported pink in at least some preserved specimens.

Ecology
No information on reproductive season.

Depth range
Medusa usually 0-150 m depth (Naumov, 1969).
Hydroid probably coastal (Naumov, 1951, Naumov, 1969).

Distribution in the North Sea
Northern and southern North Sea, Skagerrak, N and W Jutland (where indigenous, Kramp, 1927); vagrants from the north arrive with an inflow of water from the Faeroes region
The cold winter sea-temperatures characteristic of deeper parts of the Clyde, and possibly of some W Scottish sea lochs and of Norfolk coastal waters, might enable sparse and isolated hydroid populations to survive there and to produce the occasional young medusae recorded (Edwards, 1968). Hamond and Williams, 1977 and R. Hamond (personal communication) report that in the W North Sea there is, indeed, a resident population of the hydroid off the Norfolk coast, which gives rise to the young medusae found in the spring plankton, although fully-grown medusae have yet to be reported so far south.

World distribution
Considered bipolar; in cool water. In the northern hemisphere, recorded as arctic-boreal circumpolar, extending south in E Atlantic to the North Sea, and to Cape Cod in the W Atlantic; reported from the Falklands and South Orkneys in the South Atlantic Ocean. Reported from N Japan and S Alaska in the Pacific Ocean.

[After Cornelius, 1995a; Naumov, 1960; Naumov, 1969; Edwards, 1968; Russell, 1970a; Fraser, 1972]

Staurophora mertensii