Helicospiral growth in the whip black coral Cirrhipathes sp. (Antipatharia, Antipathidae).

The coiling pattern of the whip black coral Cirrhipathes sp. (Antipatharia, Antipathidae) was studied in four sites of the Bunaken Marine Park (North Sulawesi, Indonesia) characterized by different hydrodynamic conditions. The colonies show a helicospiral growth, with the polyps irregularly arranged on the external convex side of the turns of the stem. Only the colonies living in a very slow current environment reach the greatest lengths (up to 5 m) and show the highest number of rotations (up to 10, 3600°). The turns shape changes along the stem of the colonies, from the base to the apex, as evidenced by the ratio between the pitch and the diameter of each coil. In particular, the turns close to the basal plate have a smaller diameter and a larger pitch, while in the most distal turns, the diameter increases and the pitch decreases concurrently. We hypothesize that the shape variation of the whorls is triggered by the relationship between the growing stem and the intensity of the prevailing current. In this way, the colony can initially grow fast, moving away from the bottom, and then extend into the current, maximizing the plankton capture.