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(Linnaeus, 1767)

Description
(Mainly after Russell, 1953a). Umbrella diameter up to 20 mm, hemispherical to slightly flatter with fairly thin jelly, velum ca 1/6 bell radius [C.hemisphaerica-female-habitus ].
Stomach short, quadrate, with small base; no peduncle. Mouth with four short lips. Radial canals straight, usually four but individuals having. up to twelve are not uncommon in some populations; ring canal narrow.
Gonads elongate-oval, on distal 1/3 of radial canals, without median furrow. Male: C.hemisphaerica-males; females: C.hemisphaerica-females.
Marginal tentacles typically 32 but fewer in young medusae, smooth, hollow, with spherical bases; marginal bulbs few, partially developed; no cirri. Marginal vesicles closed, each with one concretion, rarely two; 1-3+ between adjacent tentacles.
Young: C.hemisphaerica-young-habitus.

Size
Bell diameter typically up to ca 20 mm but 1arger specimens sometimes occur, C. gracilis being rather smaller. Fully extended tentacles 2-3 times diameter of bell (Kramp, 1919). Russell (1938a) gave details of annual variation in dimensions but some of his data probably referred to C. gracilis.

Colour
Colouration varied, marginal tentacle bases and stomach yellowish, yellow brown, reddish brown, greenish or purple; gonads yellowish.

Ecology
Often confused with C. gracilis, so much published data unreliable. The following is certain: medusae released spring and summer off Plymouth (Russell, 1953a; perhaps March-July, cf. Russell, 1938a), and 'late summer and early autumn' when the water temperature reaches ca 15°C in W Sweden (Östman, 1979b).

Depth range
Medusa in coastal plankton.
— Hydroid on a wide variety of substrates, possibly more frequent on algae than C. gracilis. Recorded several times rafting on fish and on their crustacean ectoparasites. Commonly occurring from shallow subtidal to several tens of metres, but recorded also intertidally and down to ca 150 m; some much deeper unchecked records. Records from Baltic indicate tolerance of brackish water. [After Cornelius, 1982a].

Distribution in the North Sea
Both hydroid and medusa are among the commonest leptolids throughout; occurs also in the Skagerrak, Kattegat and Baltic.

World distribution
Near-cosmopolitan in coastal waters but difficulties of identification in this genus make nearly all records suspect. Still, it seems that either this species or its close siblings have an enormous geographical range. In Baltic Sea, once recorded as far east as Kiel Bight (Thiel, 1970, as Campanalaria johnstoni).

[After Cornelius, 1995b]

Clytia hemisphaerica