These stages included: immediately after fertilization, following the first and fifth divisions, the morula and prism stages, and at two points in pluteal
development, i.e., 24 and 48 h after fertilization.
Thus, the occurrence of ophiuroid and echinoid larvae with vestigial larval feeding structures (e.g., pluteal arms, skeleton, ciliated band) indicates that they have evolved from an ancestral form that had a feeding larva.
The development of four other lecithotrophic Ophionereis species has been described, with two species possessing some pluteal features and the other two completely lacking them (Table 1).
Although the larvae had this continuous ciliated band, a characteristic of ophioplutei, pluteal arms were not formed (Fig.
This sequence of evolutionary changes is similar to the evolution of non-feeding development in echinoids with large eggs, where loss of pluteal arms and acquisition of a simple larval shape also occurs (Olson et al., 1993; Wray, 1996).
The skeletal rods in Ophionereis schayeri larvae are considerably smaller than those of planktotrophic ophioplutei (Mortensen, 1921), and they do not support pluteal arms.
This suggests a potential sequence of morphogenetic change, with those functionally related larval structures (e.g., pluteal arms, skeletal rods) that are formed late in planktotrophic development being the first to be lost in the evolution of lecithotrophic development.