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Marine Biology (2004) 145: 8799 DOI 10.1007/s00227-004-1293-y
RESEARCH ARTICLE
P. Selvakumaraswamy M. Byrne
Metamorphosis and developmental evolution in Ophionereis (Echinodermata: Ophiuroidea)
Received: 6 July 2001 / Accepted: 19 December 2003 / Published online: 17 February 2004 Springer-Verlag 2004
Abstract The brittle star Ophionereis schayeri has abbreviated non-feeding development through a reduced ophiopluteus and a vitellaria larva. The metamorphic changes involved with development from a bilateral larva to a radial juvenile were examined in detail. The reduced ophiopluteus has a continuous ciliated band that breaks up into discontinuous ciliary ridges at the vitellaria stage. As the vitellaria develops, the juvenile rudiment forms in the mid-ventral region. The rudiment then undergoes a morphogenetic movement to the left of the larval anterior/posterior axis. This results in a dramatic transformation from bilateral to radial symmetry and is accompanied by development of juvenile structures and settlement, 67 days after fertilisation. Ophiuroid development through one larval stage, the ophiopluteus, is termed Type I; whereas development through two larval stages, the ophiopluteus and vitellaria, is termed Type II. We examined the evolutionary changes in the expression of Type II development in Ophionereis by comparing the ontogeny of six species with a range of larval forms. O. fasciata has the ancestral-like planktotrophic ophiopluteus. Vestigial pluteal structures in the larvae of O. schayeri, provide a link between ophiopluteal and vitellarial forms during evolution of non-feeding development. The diversity of larval forms in Ophionereis indicates that evolution of non-feeding development through a vitellaria (Type II) may have involved an increase in egg size, reduction of pluteal structures and shortening of the time to metamorphosis. Assessment of the phylogenetic signicance of Types I and II development awaits additional com-
parative data on the metamorphic stages of other ophiuroid genera.
Introduction The evolution of development in the Ophiuroidea is thought to have diverged along two developmental pathways, termed Type I and Type II, from an ancestral planktotrophic life history (Mortensen 1921, 1931; Mladenov 1985; Byrne and Selvakumaraswamy 2002). Types I and II development dier in the larval stage that metamorphoses and carries the developing juvenile (Mortensen 1921, 1931). The study of metamorphosis in ophiuroids thus appears to be key to understanding the evolution of development in this class of echinoderms.
The transformation from bilateral to radial symmetry...