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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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