Colonies are stout and fleshy and up to 40 cm long. The seapen is composed of a basal peduncle embedded in the substratum, from which arise leaf-like branches. Fused polyps form large triangular 'leaves' in alternate opposing lateral rows. The back of the peduncle is covered in inhalent polyps (siphonozooids). The flesh is translucent, yellowish or pale pink with white polyps. The colony may also be a deep reddish-pink owing to the presence of red skeletal plates in the tissue. The central axis of the colony is often bent-over at the tip like a shepherds crook.Pennatula phosphorea luminesces in a blue/green colour when stimulated. The luminous waves proceed in either direction, with a measurable velocity, to the extremities of the animal. Light is emitted by the polyps, which are believed to be connected by a nerve net. There is a gradual increase in light intensity with increasing number of disturbances as a result of facilitation (Nicol, 1958). Pennatula phosphorea contracts when disturbed, and swells by up-taking water when unperturbed (Nicol, 1958). It is also capable of withdrawing into a tube below the mud surface (Mackie, 1998). Extracts from this species have been found to show narcotic and anorectic properties, acting as feeding deterrents for the octocoral's main predator the Dover sole, Solea solea (Mackie, 1998). They have stinging organelles like most sea anemones and all cnidarians, which are contained in polyps along the branches off the long peduncle.
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