The development of Colobocentrotus mertensii from embryos to larvae and early juveniles was observed to give the first detailed description of larval and juvenile formation and skeletal structures in echinometrid sea urchins. The first larval spicules appeared at the mesenchyme blastula stage, whereas, in many echinoids, spicules were formed after gastrulation. From late eight-armed larva to juvenile, body color of C. mertensii was deep red, which has never been described for any echinoid before.

The adult form of C. mertensii is characteristic in that the spines at the aboral side are short, truncated and pavement-like. The first sign of peculiar adult features could be seen in the juvenile spines and adult spines, which are broader than those of closely related Anthocidaris crassispina. The primary podia emerged on the left side of larval body were more stout and thicker in C. mertensii than in A. crassispina. The present study shows that developmental process of larval structure of C. mertensii is in general similar to the A. crassispina and the differences is first seen in juvenile structure including the distribution of pigment spots and morphology of adult spine.