ABSTRACT
The catch apparatus (CA) is the collagenous ligament holding a spine at the joint of sea urchins. The classic work of Takahashi (1967) showed that the CA increases stiffness in response to acetylcholine (ACh) and seawater containing high concentration of potassium (KASW). This work opened the new field of connective tissue catch, the control of body tone by non-muscular connective tissue. Some physicochemical changes in extracellular materials have been postulated as the mechanism underlying mutability, which was challenged by del Castillo et al. (1995). The CA contains small amount of muscles whose contractile forces cannot account for the increase in stiffness of the CA. Del Castillo hypothesized that the muscular forces were amplified by the friction between fibers in the ligament and ossicles of spine and test to be large enough to account for the stiffness of the CA. We designed the experiment to test the hypothesis that has a falsifiable structure: if the isolated CA without spine and test responds to KASW and ACh, the hypothesis would be falsified. The isolated CA of Diadema setosum was subjected to a creep test and chemical stimulation of KASW and ACh was applied. These chemicals are known to cause contraction of spine muscles and to cause stiffening of the CA with ossicles of spine and test attached. The chemical stimulation reversibly increased the stiffness of the isolated CA, and thus the hypothesis of del Castillo was rejected.
