Structural Strengthening of Urchin Skeletons by Collagenous Sutural Ligaments

Abstract

Sea urchin skeletons are strengthened by flexible collagenous ligaments that bind together rigid calcite plates at sutures. Whole skeletons without ligaments (removed by bleaching) broke at lower apically applied forces than did intact, fresh skeletons. In addition, in three-point bending tests on excised plate combinations, sutural ligaments strengthened 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 strengthening by ligaments is associated with growth. In fed, growing urchins, sutures overall were looser than in unfed urchins. Looseness was 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 urchins damped vibrations, indicating loss of vibrational energy across looser sutures. Furthermore, bleached skeletons of fed urchins broke at lower apically applied forces than bleached skeletons of unfed urchins, indicating that the sutures of fed urchins had been held together relatively loosely by sutural ligaments. Thus, the apparently rigid dome-like skeleton of urchins sometimes transforms into a flexible, jointed membrane as sutures loosen and become flexible during growth.

The Skeleton of Postmetamorphic Echinoderms in a Changing World

“…Asnaghi et al (2013) reported that the robustness of whole tests was reduced in juvenile Paracentrotus lividus exposed for 30 days to pH 7.7. Unfortunately, this study was carried out on dry tests whose mechanics is deeply altered in comparison with living tests, and the reported results actually illustrate the mechanical properties of the dry ligaments joining the test plates (see Ellers et al, 1998). Holtmann et al (2013) reported breaking forces reduced by 12% for spines of Strongylocentrotus droebachiensis exposed for 6 weeks to pH 7.25.…”

Substratum cavities affect growth-plasticity, allometry, movement and feeding rates in the sea urchinStrongylocentrotus purpuratus

“…As an urchin grows, the sutures between test ossicles loosen (Ellers et al, 1998;Johnson et al, 2001) and "the entire structure is, in a sluggish way, plastic" [Thompson (Thompson, 1917) p. 662]. The flexibility in the test resulting from this kind of growth is capable of producing rapid changes in height:diameter ratios and morphological plasticity (Figs1, 3).…”

Analysis of the bite force and mechanical design of the feeding mechanism of the durophagous horn shark Heterodontus francisci

“…Heterodontus francisci capitalizes upon this when consuming hard prey with composite exoskeletons. Sustained loading after a highvelocity initial impact is effective at fracturing composite structures such as sea urchin exoskeletons (calcite ossicles linked by collagenous ligaments) because composites harden to a saturation point upon initial compression, after which crack nucleation occurs, followed by structural failure (Christoforou et al, 1989;Ellers et al, 1998;Provan and Zhai, 1985;Strong, 1989).…”