GROWTH DYNAMICS OF THE RADIAL SHIELDS OF THE EURYALID SNAKE STAR ASTROBRACHION CONSTRICTUM (ECHINODERMATA, OPHIUROIDEA)

摘要

Growth bands are evident in sections of the radial shields of Astrobrachion constrictum and on direct examination of dissected material. These bands result from the sequential addition of scales to the radial shields, a phenomenon not typical of ophiuroid radial shields. The rate of addition of these scales and the pattern of calcite deposition within them was studied using the skeletal growth marker calcein, and the validity of using the radial shield scales as age markers was examined. Individual brittle stars, injected intra-coelomically with calcein solution in the field at 6-month intervals, were collected and transverse fluorescent growth marks were found on radial shield scales. Measurement of the amount of stereom laid down between and after injections indicated the amount of growth in successive 6-month periods. Growth rate, as determined by measurement of the length of the radial shields, was faster during winter/spring, but the number of new radial shield scales increased during summer/autumn. Deposition of calcite within the scales also varied seasonally, greater amounts of large-pored stereom being deposited in winter. Growth bands were not laid down annually; as many as 11 bands appeared in a single year. Larger brittle stars did not appear to deposit new radial shield scales; 23 mm may be the maximum disc diameter for A. constrictum. [References: 50]