Autotomy and arm number increase in Oxycomanthus japonicus (Echinodermata, Crinoidea)

We have explored the process by which crinoids increase arm number as they grow. Two hypotheses have been proposed: (1) arm autotomy with subsequent bifurcation and regeneration of a pair of arms, and (2) growth of a pinnule into an additional arm. We have traced the development of Oxycomanthus japonicus for about a year after fertilization and provide the first confirmation that the number of arms increases by autotomy, bifurcation, and subsequent regeneration of a pair of arms. The next such addition tends to occur at some distance from the previous pair. Thus, increase of arm number takes place in such a manner that the density of the arms remains relatively constant, and an efficient filtration fan for feeding is maintained. Although arm autotomy in crinoids has been considered to occur only as a response to physical or chemical disturbance, the present results suggest that autotomy also occurs as a specific, intrinsically programmed event during normal development.