The ultrastructure of spines in sea urchins of the family Strongylocentrotidae

The ultrastructure of primary spines (microscopic relief of the surface of radial wedges on the spines and cross-sections of the spines) was studied by scanning electron microscopy in seven sea urchin species of the family Strongylocentrotidae. The spines were taken from the ambitus area of equally sizes individuals with a test diameter of 50 ± 5 mm. According to the number of wedges on their spines, the studied species can be divided into three groups: Strongylocentrotus intermedius, S. pallidus (18–25 wedges), S. droebachiensis, S. polyacanthus, Allocentrotus fragilis (24–32), and Mesocentrotus franciscantus, S. nudus (45–70). The species visibly differ in the microrelief of the wedges, which can be longitudinally streaked, with protuberances, with cross-dentate or non-dentate wedges, or with cross-series of denticles; in some species, the relief is absent. In S. intermedius, spines with smooth surface of the wedges, longitudinally streaked, with sparse protuberances, and with numerous cross-series of denticles only distally, only proximally, or over the entire length of the spine have been found. Wedge surface is convex or flattened in cross-sections; wedge shape in cross-section is rectangular (S. droebachiensis, S. intermedius, S. polyacanthus), triangular (S. pallidus), trapezoid (S. fragilis), or ansiform (M. franciscanus, M. nudus). Species of the genus Mesocentrotus are readily distinguished from the other species by the stereome of their spines: wider than the height of the wedges and more homogeneous, without regular concentric circles. Data on the ultrastructure of primary spines confirm the generic status of Mesocentrotus Tatarenko et Poltaraus, 1993 and do not support the recognition of Allocentrotus Mortensen, 1942 as a distinct genus.