Like hydroids, jellyfish develope from an attached polyp-generation to a free swimming medusae-generation. The medusae is the jelly-like stage as seen by many visitors to the beach during the summer. The polyps are rarely noticed. They are seldom larger than 1 centimetre and are usually attached under cliffs, rocks and algae.
    The moon jellyfish reproduces sexually during the summer. Occasionally moon jellyfish are seen in pairs - as shown in the picture on page 1. It is not known if the males sexually reproduce at this stage, or if the female catches free swimming sperm from the water.
    When the eggs hatch the larvae stay with the female for a few weeks. Because of this, yellow clusters of planula-larvae are seen on the females mouth arms during the late summer and early autmn. Every female can carry up to 1 million 0,1-0,6 mm larvae.
    When the larvae are mature enough they swim to the bottom where they develope into small attached polyps, that during the autumn give rise to a new generation of a few millimetre large jellyfish medusae. These young are called ephyrae and hibernate on or close to the bottom during the winter. In the spring they rise to the surface. Growth is rapidly attained to such an extent that during the first month the medusae can became adults with a diametre of 15-25 cm.
    Moon jellyfish polyps can be a few years old, while the medusae hardly became more than a year old in scandinavian waters.

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Medusae and polyps

Many jellyfish

Catch food with the whole of their body

Jelly eaters

Moon jellyfish     More facts     Other names
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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson