STRUCTURAL STRENGTHENING OF URCHIN SKELETONS BY COLLAGENOUS SUTURAL LIGAMENTS

Sea urchin skeletons are strengthened by flexible collagenous ligament s that bind together rigid calcite plates at sutures. Whole skeletons without ligaments (removed by bleaching) broke at lower apically appli ed forces than did intact, fresh skeletons. In addition, in three-poin t bending tests on excised plate combinations, sutural ligaments stren gthened sutures but not plates. The degree of sutural strengthening by ligaments depended on sutural position; in tensile tests, ambital and adapical sutures were strengthened more than adoral sutures. Adapical sutures, which grow fastest, were also the loosest, suggesting that s trengthening by ligaments is associated with growth. In fed, growing u rchins, sutures overall were looser than in unfed urchins. Looseness w as demonstrated visually and by vibration analysis: bleached skeletons of unfed urchins rang at characteristic frequencies, indicating that sound traveled across tightly fitting sutures; skeletons of fed urchin s damped vibrations, indicating loss of vibrational energy across loos er sutures. Furthermore, bleached skeletons of fed urchins broke at lo wer apically applied forces than bleached skeletons of unfed urchins, indicating that the sutures of fed urchins had been held together rela tively loosely by sutural ligaments. Thus, the apparently rigid dome-l ike skeleton of urchins sometimes transforms into a flexible, jointed membrane as sutures loosen and become flexible during growth.