Growth Zones in the Echinoid Skeleton

Abstract

Growth zones in echinoid skeletal ossicles are mainly the result of differences in structural characteristics. In the test plates, opaque zones, which appear light in reflected light, dark in transmitted light, and are X-ray dense, have relatively larger trabecules and smaller intertrabecular channels. Translucent zones, which appear dark in reflected light, light in transmitted light, and are less X-ray dense, have relatively smaller trabecules and larger intertrabecular channels. Organic material in the plates, especially when pigmented or charred, enhances the appearance of the growth zones. Opaque zones result from relatively fast plate growth while translucent zones result from relatively slow plate growth; food deprivation leads to the formation of translucent zones. The growth zones appear to be formed seasonally, at least in some cases, probably in relation to seasonal changes in growth rates, and perhaps in relation to seasonal reproductive activity