Characterisation of the spherulocyte subpopulations in Eucidaris tribuloides (Cidaroida: Echinoidea)

Echinoderms are reported to have up to eight coelomic cell types, but this number can be lower in some classes. In Echinoidea, only three basic types are recognised: phagocytes, vibratile cells and spherulocytes. Most studies on these coelomocytes are based on either live cell suspensions or transmission electron microscopy, while cytochemical studies are scarce. In this work we offer a detailed characterisation of Eucidaris tribuloides (Cidaroida: Echinoidea) spherulocytes using an integrative approach (live cells, cytochemistry and transmission electron microscopy). Additionally, evidence on the maturation of these cells is presented. Three distinct spherulocyte types were found in the coelomic fluid: the already-known red and colourless spherulocytes, and a poorly known subpopulation named herein as granular spherulocytes. Three distinct maturation stages could be detected, which are characterised by a remarkable reduction in nuclear diameter while the number and size of cytoplasmic spherules increases.