Advancement Mechanics of Growing Teeth in Sand Dollars (Echinodermata, Echinoidea): A Role for Mutable Collagenous Tissue

Olaf Ellers, Malcolm Telford

Abstract

Regulation of growth involves the integration of several body systems including nerves, muscles and connective tissues. We demonstrate how changes in material properties of a connective tissue permit advancement of the continuously growing teeth of sand dollars. During growth, each tooth advances in a tooth slide. During chewing, however, teeth are rigidly attached by collagenous dental ligaments. We found that there was a natural, bimodal variation in tooth looseness where some sand dollars had teeth so loosely attached that they could not crush sand particles without detaching their teeth. We also found that soaking these dental ligaments in divalent cation-free artificial seawater caused more rapid force-relaxation than control artificial seawater. These results suggest that the dental ligaments are a special mutable collagenous tissue (MCT), found in echnoderms, and that sand dollars periodically loosen their teeth via changes in the MCT to allow the teeth to advance. This process could be under nervous control, as material properties of MCT can be altered via nervous control. Thus mutable collagenous tissue in echinoderms is used not only for many skeletal functions, but also for regulation of tooth growth.