Blue jellyfish age can be identified by colour of their bell. They tend to be pale in appearance when young, but mature to have a brightly purple-blue (some yellow) coloured bell. Although it is similar to the
lion's mane jellyfish, the blue jellyfish is not as large, and has a translucent bell.
C. lamarckii has a blue or yellow tone and grows to approximately across the bell, but specimens can grow to . In
Scandinavian seas, this species rarely grows larger than . These jellyfish drift closer to the shore to catch the large abundance of plankton with their tentacles. This jellyfish has many stinging tentacles. The four mouth arms are large with many wrinkles and ripples. The jellyfish live off a diet of
phytoplankton or
zooplankton as well as the eggs and larvae of other aquatic animals such as fish.
Cyanea lamarkii use their tentacles containing
nematocysts to not only catch their prey, but to protect from predators. ==Distribution==