Divers and boats in the Falmouth area (England) are encountering more and more barrel jellyfish (Rhizostoma octopus, Rhizostoma pulmo).
These large pelagic species can reach up to 90cm across, the sort of
size that has been encountered. They feed mostly on plankton and are
thought to spend most of the time at depth, coming up when the plankton
blooms. For marine biological enthusiasts, if you manage to see one and
get close, look for the various creatures that live within the