Rhizostoma pulmo (previously Rhizostoma octopus)
The Barrel Jellyfish is an impressive creature with a huge dome jelly up to 90cm wide and cauliflower like fused tentacles dangling up to 1.9m below. The stings are harmless to humans. A blue line runs round the edge of the dome with tiny dots evenly spaced. These sensory statocysts help them work out their orientation in the water.
This year has seen a swarm of Barrel jellyfish around Cornwall. Here, COAST Chair Howard Wood shot video footage near our shores. Experts believe the growing numbers of Jellyfish in British seas could be a result of pollution, over-fishing or climate change. Jellyfish are the staple diet of the critically endangered leatherback turtle and the small jellyfish polyps are eaten by fish.
The scientific name Rhizostoma means 'root pores'. The cauliflower like fused tentacles hang down below the solid dome. Hundreds of small mouths (the pores) are surrounded by tiny stinging tentacles that collect plankton and take it to a highly branched digestive system.
It is also known as the dustbin-lid jellyfish or the frilly-mouthed jellyfish.
The barrel jellyfish plays host to amphipods (Hyperia galba), crustaceans up to 1cm long, that hang onto the jelly with spine-like feet, often inside the stomach or reproductive cavities. These can change colour to match the jellyfish tissue to avoid being eaten by fish. If an over infestation occurs they can kill the jellyfish.
In the spring tiny anemone-like jellyfish 'polyps' attached to the sea bed expand in size and then bud off thousands of tiny larvae.
Most years these larvae will perish but in years where the conditions are good, with plenty of planktonic food and when predators do not eat them all, large numbers of them will survive creating jellyfish swarms. A lack of predators caused by over fishing can lead to a sea where nothing but jellyfish remain.
Latin name: Rhizostoma pulmo, (Rhizostoma octopus)
Habitat: Pelagic zone (being neither close to the bottom nor near the shore)
Distribution: It is found on Southern and Western shores of the British Isles. Northeast Atlantic, and in the Adriatic and Mediterranean Sea. It is also known from the Southern Atlantic off the western South African coast.
Size: Between 40cm - 90cm in diameter, tentacles up to 1.9m long
Sources: en.wikipedia.org, www.marine-conservation.org.uk/ukjellyfish.html www.dailymail.co.uk/news/article-2674978/BARREL-JELLYFISH-SPOTTED-IN-ESTUARY.html
The Marine Conservation Society is inviting people to take part in a jellyfish survey. The full-colour MCS jellyfish photo-ID guide can be downloaded from www.mcsuk.org, where jellyfish encounters can also be reported in detail on-line. Participants should look carefully at jellyfish before reporting them, but should not touch them as some species have a powerful sting.