Bridges between radial wedges (septs) in two diadematid spine types

Diadematid spines differ in the details of their structural architecture, and this paper examines differences in the architecture of Astropyga radiata and Diadema antillarum spines revealed by synchrotron micro Computed Tomography (microCT). The focus is on the radial wedges (septs) comprising the majority of the calcite volume of the spines and, in particular, on the bridges joining adjacent wedges. Bridges in D. antillarum are much larger and sparser than those in A. radiata. The A. radiata bridges tend to be regularly spaced with rows of three to five bridges running perpendicular to the spine axis; smaller cross-sections, found closer to the spine axis, tend to be circular and larger, more abaxial bridges have oval cross-sections. In D. antillarum, the bridges have oval cross-sections and join the inner portions of the wedges; further, bridges between groups of six or more adjacent wedges spiral helically along the spine axis.