Vibratile cells in the class Echinoidea: Comparative morphology, physiological function, and their occurrence in other echinoderms

Vibratile cells are flagellated free-swimming coelomocytes typical of echinoids and sometimes reported in other echinoderms. Despite their intriguing characteristics, vibratile cells are poorly understood, and several aspects of their morphology, occurrence, and physiological role have yet to be determined. Thus, in the present study, we: (1) provide a morphological characterization of the vibratile cells in Eucidaris tribuloides; (2) compare the vibratile cells in E. tribuloides with those in other regular and irregular echinoids; (3) investigate the distribution of vibratile cells in the phylum Echinodermata; (4) provide a critical discussion on the putative physiological roles of vibratile cells; and (5) present a clear definition of a vibratile cell, providing for more accurate identification of this coelomocyte. Vibratile cells from individuals of E. tribuloides were analyzed through live and stained preparations, scanning and transmission electron microscopy, and X-ray spectroscopy and compared to those of six regular and irregular Brazilian and Italian echinoids. Based on a literature survey, we investigated the occurrence of vibratile cells in other echinoderms and discussed their physiological function. Our results showed that the vibratile cells of all analyzed echinoids are similar in morphology, morphometry, and cytochemistry. However, the extreme variation in these characteristics reported for the vibratile cells of Crinoidea, Asteroidea, Ophiuroidea, and Holothuroidea indicated that in these groups other structures or cells were likely misidentified as vibratile cells. Consequently, we recommend the name flagellated cells for the flagellated corpuscles from non-echinoid echinoderms. Lastly, we critically review the hypotesys about the physiological function of these coelomocytes, and provide evidence that the vibratile cells are involved in immune responses, possibly participating in clotting. Thus, our study indicates that vibratile cells have conserved characteristics shared by regular and irregular echinoids, and are quite different from the flagellated cells found in other echinoderms.