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Characteristics:

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.

Habitat:

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.

Distribution:

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