Behavior of pigment cells closely correlates the manner of gastrulation in sea urchin embryos

To know whether behavior of pigment cells correlates the process of gastrulation or not, gastrulating embryos of several species of regular echinoids (Anthocidaris crassispina, Mespilia globulus and Toxopneustes pileolus) and irregular echinoids (Clypeaster japonicus and Astriclypeus manni) were examined. In M. globulus and A. crassispina, the archenteron elongated stepwise like in well-known sea urchins. In the embryos of both species, fluorescent pigment cells left the archenteron tip and migrated into the blastocoel during gastrulation. In T. pileolus, C. japonicus and A. manni, on the other hand, the archenteron elongated at a constant rate throughout gastrulation. In these species, no pigment cell was observed at the archenteron tip during invagination processes; pigment cells began to migrate in the ectoderm from the vegetal pole side toward the apical plate without entering the blastocoel. These results clearly indicate that the behavior of pigment cells closely correlated the manner of gastrulation. Further, it was examined whether the archenteron cells are rearranged during invagination, by comparing the number of cells observed on cross sections of the archenteron at the early and late gastrula stages. The rearrangement was not conspicuous in A. crassispina and M. globulus, in which archenteron elongated stepwise. In contrast, the archenteron cells were remarkably rearranged in C. japonicus, alothough the archenteron elongated continuously. Thus, neither the behavior of pigment cells nor the manner of gastrulation matches the current taxonomic classification of echinoids.