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(Forbes, 1848)

Description
Fully grown umbrella nearly hemispherical, with fairly thick jelly. Velar aperture ca 3/4 of bell diameter [P.quadrata-medusa ].
Stomach short, quadrate, with small base. Mouth with four short lips with slightly folded margins. The four radial canals and ring canal narrow.
Gonads short, elongate-oval, with median groove, situated on distal 1/3 of radial canals but not reaching margin.
Marginal tentacles 16-32, with globular bases. Without marginal or lateral cirri. Eight adradial closed marginal vesicles [P.quadrata-marg vesic ]each with 2-8, usually 3-4, concretions, each supported on small bulbous swelling.
Young specimens: P.quadrata-young-habitus; P.quadrata-young2-habitus.

Size
Diameter of bell recorded as up to 13 mm.

Colour
Stomach and marginal tentacle bases reddish yellow to reddish brown, gonads yellowish; often with four black interradial spots on base of stomach.

Ecology
Medusae occur in all months of the year, but are most numerous 'between April and September' (Russell, 1953a) when they may be abundant. Their long presence in the plankton might indicate two generations per year (Russell).

Depth range
Medusa in coastal plankton.
— Hydroid in shallow water, occasionally intertidally (Roscoff, Teissier, 1965).

Distribution around the British Isles
Occurs all around, possibly less commonly from Hampshire to Northumberland than elsewhere. Scottish records of the medusa were summarised by Fraser, 1972: W coast, Minch to Shetland; E coast, Shetland to Firth of Forth; not Faeroes nor Faeroe-Shetland Channel. Abundant in The Clyde 1940 but not found in or shortly before 1956.

Distribution in the North Sea
All coastal waters throughout the North Sea.

World distribution
This species, or it and others closely similar, is widespread in coastal waters in Atlantic and Indo-Pacific Oceans. Often reported along coasts of N France, Belgium, Holland, Denmark and Sweden (Kramp, 1935a; Vervoort, 1946a; Leloup, 1952; W. J. Rees and Rowe, 1969); and occasionally in Mediterranean and Adriatic Seas. Known also from the Sea of Japan (Naumov, 1969), New Zealand (Russell, 1953a), Chile (Russell, 1970a), Morocco (Patriti, 1970) and the Gulf of Guinea (Kramp, 1961).

[After Cornelius, 1995a]

Phialella quadrata