Clava multicornis (Forskål, 1775)

Hydroids

The colonies of C. multicornis are composed of several polyps that are all similar to each other. These polyps are very large in comparison with all the other species in the family, being able to reach 30 mm when fully extended. Each polyp has 20–40 tentacles scattered in the upper part of the body, not arranged in whorls. In fertile colonies all or almost all polyps will be bearing gonophores. The gonophores are small fixed sporosacs and they are distributed in a band around the gonophore, just below the tentacles.

Clava multicornis is a species easy to identify thanks to its habitat, preferred substrate, and the distribution of the tentacles on the polyps. It cannot be confused with any other species of hydractiniid in Norway.   

This species can locally be the most abundant and conspicuous hydrozoan in localities with good tidal flow. The colonies grow preferably on brown algae, especially Ascophyllum nodosum and several species of Fucus, but they have also been reported from other substrates including rocks and barnacles. Their main depth range is between the mean tide level and the low water mark of ordinary spring tides, and therefore the polyps are adapted to exposure to air and are able to withstand desiccation. Under good conditions the colonies are perennial. While C. multicornis is mostly a marine species, there are records of colonies living in estuaries with reduced salinities.

This is one of the most commonly reported hydractiniids in Norway, where it is distributed from the Arctic Ocean to the Skagerrak and inside the fjords. It is mainly a North Atlantic species that occurs in the Northeastern Atlantic from the Arctic Ocean south to northern Portugal, including also the North Sea, Iceland, and the western Baltic Sea.