TO BE STIFF OR TO BE SOFT - THE DILEMMA OF THE ECHINOID TOOTH LIGAMENT .2. MECHANICAL-PROPERTIES

The teeth of sea urchins are connected to jaws by means of ligaments. Their sliding along the jaw during continuous growth requires a pliant ligament, whereas scraping on rocks for feeding requires a stiff liga ment for firm support. We investigated the mechanical properties of th e tooth ligament of Diadema setosum to clarify how sea urchins solve t his dilemma. In creep tests a load of 30 g caused a shift of the tooth that continued until the tooth was pulled out of the jaw. The creep c urve had three phases: an initial phase of high creep rate, a long pha se of constant creep rate, and a final phase of accelerating creep rat e. The ligaments had a shear viscosity of about 550 MPa . s. Viscosity increased reversibly after stimulation with seawater containing a hig h concentration of potassium ions or acetylcholine. Frozen and rethawe d ligaments did not show an increase of viscosity after stimulation. T he data indicate that sea urchins can change the stiffness of their to oth ligaments through nervous control. We suggest that the tooth ligam ent is a catch connective tissue.