Hydractinia echinata Fleming, 1828

The colonies of H. echinata have feeding polyps (gastrozooids), reproductive polyps (gonozooids), and defensive polyps (dactylozooids), although the dactylozooids may be absent depending on the substrate and conditions the colony is growing on. The ‘root’ system of the colony (also called hydrorhiza) forms a crust-like layer from which the polyps, spines, and prickles emerge.

The gastrozooids are usually 5–10 mm high and have 15–20 tentacles. The gonozooids are much smaller compared to the gastrozooids. The gonozooids lack tentacles and end instead in buttons of stinging cells, which is a unique characteristic for this species. The colonies often have spines, and these are characteristically large (up to 2 mm) and have several irregularly spikey ridges. The gonophores are fixed sporosacs, they develop in a single whorl in the upper region of the gonozooid.

The colonies of Hydractinia echinata grow preferably on snail shells inhabited by the hermit crab Pagurus bernhardus, although they can also grow on other substrates such as other hermit crabs, crab carapaces, and wood. Hydractinia echinata is a shallow water species with a preferred depth range of 0–100 m.

Hydractinia echinata is one of the most commonly reported hydractiniids in Norway, although many of these records need to be validated. It occurs in the Northeastern Atlantic from the Arctic Ocean south to NW Africa, including the North Sea and parts of the Baltic Sea, but probably not in the Mediterranean Sea, where its records are likely due to misidentification.