ABSTRACT
The mechanical design of sediment excavating spines of the endemic Mediterranean heart urchin Schizaster canaliferus was examined by cross-sectioning spines at five different points along the axis, and documented using SEM. The moment of inertia was determined by scanning the SEM micrographs. Data points were fitted by an exponential function. The Young’s modulus was determined from bending tests and calculated according to linear-elastic beam theory. Each spine was glued with its base into a metal support and increasingly strained to it’s maximum by adding drops of water of standard weight into a jar, which was glued to the distal end of the spine. Deflection was documented by aid of photography. Fractures occurred randomly along the medio-proximal and distal, but never in the proximal spine region. Spine construction implies that resistance to damage under transverse tip load is constant over the spine length, except in the proximal region. This part of the spine, where the highest bending moment applies, has no lumen and is fortified by an “inner”, labyrinthic stereom. Compared to the long and slender spines of the regular sea urchin Diadema setosum, the Young’s modulus of spines of Schizaster canaliferus was significantly lower. Supported by NSF (FWF) grant # P08919 to HH and GOS.
