Jellyfish It is quite common to see masse swarms of jellyfish around the coast of Britain, because warm sea currents and strong winds push the jellyfish to shore. They usually disappear when the weather changes again. However, people should avoid touching them, turning them over or playing with them, because their sting is dangerous for anyone alergic to them. The moon jellyfish s the UK's most common species and they grow up to 40cm in size.