Examination of the large triacts of the calcareous sponge Leuconia nivea grant by scanning electron microscopy

Abstract

Examination of the ends of broken spicule rays by SEM revealed only traces of structure in the spicule calcite. Occasionally a transverse concentric banding could faintly be discerned, the width of the bands being about 4μm. On part of one surface a finer concentric lamination was visible, with bands about 0.65μm wide separated by clefts about 0.05μm wide. Small inclusions were embedded in the clefts. After etching with 10% D-tartaric acid a pattern of transverse concentric bands bordered by projecting ridges became evident on polished sections and broken surfaces of the rays. The bands tended to be uniform in width in a given section but the widths varied from section to section, e.g. 0.29μm on one, 0.53μm on another. The etching of the calcite varied, so that some ridges were particularly conspicuous, giving rise to a series of broad secondary bands superimposed upon the primary band pattern. The spacing of the secondary bands depended upon the number (2 to 6 or more) of primary bands incorporated. There was no evidence of a polycrystalline structure, and no vacuoles could be detected in the calcite. Beneath an infra-red microscope (wavelength 0.7–1.2μm) the spicules appeared completely transparent and homogeneous. The cause of the growth ring pattern has still to be determined.