|Researched by||Ruby Nash||Refereed by||This information is not refereed|
|Other common names||Barrel jellyfish||Synonyms||Medusa octopus Gemlin, 1791|
Rhizostoma octopus is one of the largest jellyfish to inhabit UK waters. It has a smooth and somewhat flattened shaped umbrella (or bell) that can reach 90 cm in diameter. It is generally white or bluish in colour but may be red, brown, or grey. The blue colouration of the ring muscle of the dorsal umbrella can help differentiate it from closely related species. Beneath the umbrella is a manubrium that bears four pairs of oral arms. There are 16 three-winged scapulets at the upper part of these arms. The oral arms are longer than the bell diameter and end with three-winged bulbs that are wider at their end. There are 112 rounded lobes (lappets) around the margin of the umbrella, and the inner veil bears six pairs of velar lappets per octant. There are no marginal tentacles.
Recorded around all coasts of the British Isles but reported to aggregate in Carmarthen Bay, Tremadoc Bay and Rosslare.
Recorded along the Atlantic coasts of Europe from the south coasts of Sweden, the coasts of Denmark to the north coasts of Germany, Netherlands and Belgium, and along the north and west coasts of France.
Rhizostoma octopus is a pelagic species. It is known to aggregate in bay areas but it has also been observed in open water.
Rhizostoma octopus can be mistaken for Rhizostoma pulmo. Rhizostoma octopus has 112 marginal lappets; the proximal section of the oral arms is shorter than the ending bulb section and the bulb appendages are widest at their ends. Whereas, Rhizostoma pulmo has 96 marginal lappets; the proximal section of its oral arms is longer than the ending bulb section and the bulb appendages are widest at their forepart.
The lifecycle of Rhizostoma octopus includes strobilation (a form of asexual reproduction) whereby immature medusae (juveniles) are produced. This strobilation can be activated by a 5°C increase or decrease in temperature and it is thought to occurs twice annually, in autumn and in spring. The sex of Rhizostoma octopus can be determined by the colouration of the gonads. Female gonads appear brown whereas, male gonads appear blue. This species is eaten by sea turtles and is commonly known to host Merlangius merlangus (whiting) between the oral arms.
- none -
Jarms, G., Morandini, A.C. & Haeckel, E., 2019. World Atlas of Jellyfish: Scyphozoa Except Stauromedusae. München: Dölling und Galitz Verlag.
O'Connor, B. & McGrath, D., 1978. The occurrence of the scyphozoan Rhizostoma octopus (L.) around the Irish coast in 1976. Irish Naturalists' Journal, 19, 261-263.
Russell, F.S., 1970. The medusae of the British Isles. Vol II - Pelagic Scyphozoa, with a supplement to the first volume on hydromedusae. Cambridge University Press
This review can be cited as:
Last Updated: 14/01/2021