Ossicle change in Holothuria scabra with a discussion of ossicle evolution within the Holothuriidae (Echinodermata)

Ossicles of Holothuria (Metriatyla) scabra Jaeger 1833 vary mainly in early juveniles between 0.9 and 15mm long. While ossicles were not observed in auricularia and doliolaria larvae, which instead possessed elastic balls, ossicles were present in late pentactulae. Specimens 0.9–1.5mm long have tables with tall spire (4–5 cross beams), no buttons, and large irregular perforated plates. Specimens 5–6mm long have tables with moderate spire (2–4 cross beams) and a few smooth buttons. Specimens 9–16mm long have tables with low spire (1–2 cross beams) and knobbed buttons. From 30mm, ossicle are similar to those of adults, with more buttons and fewer tables. Several features of the ossicles of early juveniles, including their size, shape and prevalence, are unique to the species. Comparison with holothurian juveniles of other species indicates that presence of tables with tall spire and absence of buttons are plesiomorph characters in the evolution of the Holothuriidae.