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.



A 3 cm wide hydra medusa swimming along an algae covered cliff.

The hydrozoas medusae are often small and transparent. Their mouth is situated

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.


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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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson