Plate growth and age determination of sea urchin, Echinostrephus aciculatus, in Maoaw Bay of northeastern Taiwan

Echinostrephus aciculatus, the most dominant species in Maoaw Bay of the Northeastern Taiwan, was surveyed in this study. From April 2005 to March 2006, samples were taken every month to conduct the following experiment. First, the researcher measured the shell diameter and the plate of interambulacral zone to analyze the relation between. Second, the researcher compared the shell diameter and the dry weight of the coronal plate. Third, the researcher analyzed and compared the stable carbon and oxygen isotope ratios in horizontal growth axle of the periproct, the ambitus and the peristome of the interambulacral zone of two sea urchins. Then, the researcher analyzed the stable carbon and oxygen isotope ratios in the horizontal and vertical growth axle of the peristomes’ plates of two samples. Last, the researcher derived the age of sea urchins by analyzing the inverse relation between isotope ratios and temperatures.
Results showed that positive correlation existed among the shell diameter, the plate of the interambulacral zone and the height of sea urchins. As to the shell diameter, samples collected during March and April reached the maximum, with a decreasing trend to the minimum in November. As to the dry weight, the maximum fell on samples collected during March and April while the minimum in July, which showed that Echinostrephus aciculatus had completed reproduction as well as larvae settlement and metamorphosis by July and then joined to a new colony. By analyzing the stable carbon and oxygen isotope ratios of the periproct, the ambitus and the peristome of the interambulacral zone of a sample collected in April, 2005, the researcher derived different values of plates of different areas, and inferred the difference in growing time. Moreover, a trend of mirror symmetry was observed when the researcher analyzed the stable carbon and oxygen isotope ratios in the horizontal and vertical growth axle of the peristomes’ plates of two samples (No. 05041604 and No. 05041659). This finding, combined with the inverse relation between isotope ratios and temperatures, drew the inference of two growth zones in horizontal and vertical axle with both inward and outward bone regeneration. The rates of inward and outward regeneration were similar in horizontal axle while in vertical axle the rate of inward regeneration was higher than that of outward regeneration. In this way, it was inferred that the plates had the circular cambium, from which bone regenerated inward and outward. The researcher then inferred the sample No. 05041604 to be 4.9 months old from the inverse relation between isotope ratios and temperatures. Besides, the carbon isotope ratio didn’t show any significant positive or negative correlation with the temperature.