The Function of Surface Sclerites in Gorgonians (Coelenterata, Octocorallia)

Branches of a variety of species of gorgonians representing the common sclerite types were fixed in flexed (bent) positions and examined by scanning electron microscopy. To determine changes in position of sclerites associated with extension and compression, appropriate measures of sclerite density and angle were taken. From these and other data, probable function was hypothesized. Surface sclerites (spicules), called clubs, modify colony flexion by limiting compressibility of the outer cortex when they contact each other. Scaphoids progressively limit extension as their ventral tubercle belts engage similar tubercles on underlying spindles. Radiates limit both extension and compression by catching and locking up after a fixed, free-slide distance. Double-heads severely limit both extension and compression through random orientation of closely packed, spiny protruberences that preclude almost any freedom of movement. Unilaterally spinose spindles act as drawbridge-like, protective covers for polyps. They play no apparent role affecting whole colony mechanics. Possible roles of sclerites as mechanical systems are discussed briefly.