Use of plate growth measurement for the estimation of skeletal growth of two sand dollars, Astriclypeus manni (Verrill 1867) and Clypeaster japonicus (D^|^ouml;derlein 1885), in Jeju, Korea

Abstract

The skeletal growth patterns of two irregular sea urchins, Astriclypeus manni (Verrill 1867) and Clypeaster japonicus (Doderlein 1885), widely distributed in the shallow coastal Jeju Island, were investigated in this study. After charring by flame and subsequent clearing with xylene, a series of light and dark bands appeared on the plates consisting oral and aboral tests of both species. A strong correlation was observed between the number of dark bands and the size of A. manni as a test diameter (TD, r2=0.921); as the number of dark band increased, TD increased logarithmically. From digitized image of the plate, the radius and area of each dark band was measured using an image analyzing software. The size of sand dollar was also positively correlated with radius of the plate (r2=0.886 for A. manni and r2=0.422 for C. japonicus) or area of the plate (PA, r2=0.904 for A. manni and r2=0.704 for C. japonicus). As the correlation coefficient indicated, PA was found to be a better estimation for the skeletal growth of the sand dollars. The proportion of each dark band area in a plate increased dramatically from the first band to the second and third band then it dropped gradually as the number of dark band increased. After the 7 or 8th bands, the area or radius could no longer be measured since the distance between the two adjacent bands become too close. It was believed that the alteration of dark and light bands appeared on the plate represents the annual or seasonal growth ring, as was reported from other sea urchins and the skeletal growth of the two sand dollars are possibly age-dependent.