The Functional Morphology of Starfish Tube Feet: The Role of a Crossed-Fiber Helical Array in Movement

The morphology and mechanics of the tube feet, ampullae, and lateral and radial canals of the water vascular systems of Luidia clathrata and Astropecten articulatus (Echinodermata, Asteroidea) were analyzed. Histological methods, based on embedding in both paraffin and glycol methacrylate, were used to document the arrangement of muscle and connective tissue. The tube foot wall includes longitudinal muscles and connective tissue fibers, the latter arranged in a crossed-fiber helical array, with a fiber angle of about 67{deg} in elongated tube feet. No evidence was found for the circular rings of connective tissue reported in earlier studies; the appearance of rings is probably an artifact of folding. The ampullae are bilobed and include circumferentially arranged muscle fibers and connective tissue fibers aligned 90{deg} to the muscle. The lateral canals are short and equipped with oneway flap valves similar to those described for other echinoderms. The radial canal is thin-walled, nonmuscular, and enclosed in the connective tissue and ossicles of the ambulacrum. Frame-by-frame video analysis of both intact animals and animals with "windows" cut in the arm wall was used to document the movements of the tube feet and ampullae. No evidence was found for the previously suggested role of the radial canal in protracting the tube feet. The ampullae protract the tube feet and antagonize the tube foot musculature. The fiber angle of the connective tissue allows protraction and prevents dilation of the tube feet, and limits elongation of the ampullae.