The tube feet of sea urchins and sea stars contain functionally different mutable collagenous tissues.

Echinoderms possess mutable collagenous tissues (MCTs), which are capable of undergoing rapid changes in their passive mechanical properties mediated by secretions from a specific cell type, the juxtaligamental cell. In this study, the possible presence of MCTs in the tube feet of the echinoid Paracentrotus lividus and the asteroid Marthasterias glacialis was investigated by measuring their extensibility, tensile strength, stiffness and toughness after different treatments known to influence the physiological state of MCTs. Calcium removal reversibly induced a significant plasticization of the tube feet of both species. When exposed to cell-disrupting solutions, the tube foot stem of sea urchins and sea stars showed a significant increase in strength, stiffness and toughness in the absence of calcium. This response, combined with the ultrastructural observation of juxtaligamental-like cells in the connective tissue, confirms that an MCT is present in both echinoid and asteroid tube feet. It was observed, however, that the tube foot stems of P. lividus and M. glacialis are affected differently by exposure to cell-disrupting solutions in the presence of calcium, indicating that their MCTs could be functionally different. In their soft state, MCTs could assist the muscles in tube foot protraction, bending and retraction; in their stiff state, they could play a role in the energy-sparing maintenance of position; for example, during strong attachment to the substratum to resist hydrodynamically generated loads.