Ossicle development of the crinoid Florometra serratissima through larval stages

Crinoids are the oldest living class of echinoderm and sister group to the remaining eleutherozoan clade and so are key to discussions on the evolution and development of the echinoderm skeleton. Here we present the intraspecific variation of ossicle development of the feather star Florometra serratissima (A.H. Clark, 1907) during its three larval stages: doliolaria, cystidean, and early pentacrinoid. To induce settlement, larvae were cultured on a sea table in glass bowls containing coralline algae. The soft tissues of 60 larvae were dissolved to isolate and to observe the ossicles with compound microscopy and scanning electron microscopy. From the late doliolaria stage to 56-day-old pentacrinoids, a total of four types of ossicle developed: oral plates, basal plates, columnar stalk ossicles, and an attachment disk. Occasionally, an additional plate was found under the basal plates, which may represent a vestigial infrabasal plate. The shape of the attachment disk was plastic to accommodate the substrate...