© 2000 by The Society for Integrative and Comparative Biology
On the Number of Rays in Starfish1
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Multiradiate starfish evolved independently in fourteen living families. Twenty living families are strictly 5-rayed. The FIVE-PLUS hypothesis is that supernumerary rays develop separately from the five primary rays. The ontogeny of the primary rays is proposed to be highly integrated ("en bloc" hypothesis), closely timed (synchronic hypothesis) and a developmental constraint ("tamper-proof" hypothesis). The "en bloc" hypothesis postulates that the five primary rays develop as a unit. The deep structure of this unit is believed to be a 2-1-2, BA-A-BA, organization. The synchronic hypothesis postulates that there is only a brief time at metamorphosis during which the "en bloc" pathway operates. There is a pause before the development of supernumerary rays. The "tamper-proof" hypothesis postulates that the "en bloc" pathway has no heritable variation and cannot be co-opted for the production of supernumerary rays. There is diversity of timing and pattern in the development of supernumerary rays. Postgeneration of rays in the rudiment and intercalary regeneration of rays in the imago are independent ray-producing pathways that may have been co-opted variously and recurrently in the multiple origins of multiradiate starfish.