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In Swedish waters very few notice the
siphonphores as the species that is common
in Swedish waters is completely transparent, small and does not tolerate
low salinities in surface waters. The portuguese
man-o-war is a very large siphonphore
that floats on the surface and can be very common in the warmer parts
of the Atlantic.
In fresh or brackish
water it is possible to find hydras.
They are small and solitary. Hydras reproduce by buding off during the
summer, and sexually during the autumn. The fertilized eggs have a hard
shell and lay dormant over the winter until they hatch in the spring.
Medusa-stage
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A 3 cm wide hydra medusa swimming along an algae covered
cliff.
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The hydrozoas medusae
are often small and transparent. Their mouth is situated
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on a drawn out tube and along the inner side of the "umbrella"
is a peripheral fold. The medusa has tentacles with stinging cells
and between the tentacles are the small sight and balance organs.
The medusae of the hydrozoa swim in the same way as
the jellyfish with the help
of the muscles and jelly in the "umbrella". The muscles draw it
together, while the jelly streches it out again so that they can
swim with a rythmic pumping motion.
There are two forms of hydra medusae: anthomedusae
och leptomedusae.
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Reproduction
The hydrozoas that have medusae reproduce
sexually. The medusae are both
male and female. The polyps in this case have no sexual orientation, but
reproduce by budding off. The polyps bud new polyps or small medusae.
With certain specie, the medusa stage has disappeared and it is the polyp
that ejects the eggs and sperms.
The group hydrozoas consists of about 90 Swedish specie
and about 2 700 specie world-wide.
Read more about the different
hydrozoas!
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Page 6
of 13
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