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Jellyfish are unlike other cnidarians because the medusa stage is the most conspicuous. They are the largest zooplankton and frequently reach over 1 m in diameter. The lion´s mane jellyfish (Cyanea capillata) is the largest jellyfish in the world and in fact in the Guinness book of records as the longest invertebrate in the world with tentacles of up to 37 m (120 feet) long. This species is quite common in Eyjafjörður.
Six to seven species of jellyfish have been found around Iceland and most of these have been found in Eyjafjörður. They are most common during late summer and autumn when the benthic polyp stages develop into the larger pelagic medusa stages.
No really dangerous jellyfish species can be found around Iceland. Some, such as the lion´s mane jellyfish can cause rashes or even burns on the skin. In Eyjafjörður the moon jellyfish (Aurelia aurita) is more common and this species is smaller and harmless. In the tropics species scan be found that are potentially fatal to humans.
The medusa stage is the mature stage of the jellyfish. The medusa reproduce sexually and the resulting larvae finds a suitable bottom habitat to wait as a small polyp during winter. The polyp grows during the next summer and releases parts of it as medusa late next summer.
As jellyfish are voracious predators they are probably harmful to many commercial species around Iceland by eating their juveniles. Potentially they also compete with fish species for food.
Jellyfish have also caused damage to fish farms because the caged fish cannot escape the jellyfish bloom in the autumn. The relationship between commercial fishes and jellyfish can hovers e complex because, deadly as they are, juvenile cod do sometimes seem to want to be close to jellyfishes as there they are in less danger of being eaten by their mother.
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