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Hydroid stage: The colony is build from slender, erect stems with a maximum of 5 hydranths on each. Side branches shoots out just below the hydranths, turns in a right angle to continue to grow in parallel with the original stem. A mature hydranth may have 18-24 tentacles. The total height of a colony is typically 5 mm, so it is easily overlooked by divers.

Medusa stage: The umbrella is slightly flattened and may reach a diameter of 25-35 mm. It has four radial canals. The stomach reaches slightly below the umbrella, which has a large number of short, marginal tentacles. The stomach, gonads, and the base of the marginal tentacles are all colored bright sepia.


The medusa is costal plankton, found near the surface during daytime. It is present during the summer (May-September around the British Isles, propably most frequent late in the period in northern areas. The hydroid has been recorded at shallow, subtidal depths, on rocks and crab shells.


There are widespread registrations from the North-East Atlantic Ocean, as well as from the Pacific coast of Canada, California and even Japan.