Evaluation of the different forces brought into play during tube foot activities in sea stars.

Sea star tube feet consist of an enlarged and flattened distal extremity (the disc), which makes contact with the substratum, and a proximal contractile cylinder (the stem), which acts as a tether. In this study, the different forces brought into play during tube foot functioning were investigated in two related species. The tube feet of Asterias rubens and Marthasterias glacialis attach to glass with a similar mean tenacity (0.24 and 0.43 MPa, respectively), corresponding to an estimated maximal attachment force of 0.15 and 0.35 N. The contraction force of their retractor muscle averages 0.017 N. The variation of the retractor muscle contraction with its extension ratio follows a typical bell-shaped length-tension curve in which a maximal contraction of approximately 0.04 N is obtained for an extension ratio of approximately 2.3 in both sea star species. The tensile strength of the tube foot stem was investigated considering the two tissues that could assume a load-bearing function, i.e. the retractor muscle and the connective tissue. The latter is a mutable collagenous tissue presenting a fivefold difference in tensile strength between its soft and stiff state. In our experiments, stiffening was induced by disrupting cell membranes or by modifying the ionic composition of the bathing solution. Finally, the force needed to break the tube foot retractor muscle was found to account for 18-25% of the tube foot total breaking force, showing that, although the connective tissue is the tissue layer that supports most of the load exerted on the stem, the contribution of the retractor muscle cannot be neglected in sea stars. All these forces appear well-balanced for proper functioning of the tube feet during the activities of the sea star. They are discussed in the context of two essential activities: the opening of bivalve shells and the maintenance of position in exposed habitats.