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What is a juvenile sea urchin? A comparative and phylogenetic survey of post-metamorphic juveniles
- Richard B. Emlet
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Echinoid development progresses through embryonic and larval stages to metamorphosis and the adult form. Despite vast differences in embryos and larval forms, including bilaterally symmetric echinopluteus larvae, ovoid non-feeding larvae and brooded embryos, all metamorphose into juvenile sea urchins with pentaradial symmetry. The adult sea urchin body plan is initiated as the juvenile rudiment. The rudiment has been called the phylotypic stage for the class Echinoidea, a designation that implies little variation at this midpoint in development (e.g. Raff et al., 1991; Richardson, 1995; Raff, 1996). However, right at metamorphosis (upon eversion of the juvenile rudiment), variations in test symmetry, shape and number of spines, and number of skeletal plates, podia and pedicellariae are present in juveniles. This variation suggests either that there is no phylotypic stage or that such a stage occurs earlier in rudiment formation. To distinguish between these possibilities, I explored the patterns by which the juvenile rudiment is formed as well as the variation among juveniles approximately 1 day after metamorphosis in 19 echinoid taxa covering a broad taxonomic range including cidaroids, diadematids, irregular echinoids (spatangoids and clypeasteroids), arbaciids, temnopleurids, echinometrids and strongylocentrotids. Most of the material for analysis of juveniles was obtained by the author. Additional information was gathered from classical studies of metamorphosis. Data were collected on the number and shape of dorsal pedicellariae, juvenile and adult spines, primary and secondary podia, and juvenile test shape. When possible multiple individuals within a species were examined, revealing no or only minor trait variation. These data were mapped on a well-resolved phylogeny established from adult characters.
Apical skeletons of sea urchins (Echinodermata: Echinoidea): two methods for inferring mode of larval development
- Richard B. Emlet
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- Paleobiology / Volume 15 / Issue 3 / Summer 1989
- Published online by Cambridge University Press:
- 08 April 2016, pp. 223-254
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Recent data from mollusks suggest that mode of larval development may have important consequences for rates of speciation and extinction of marine organisms. The present study examines two methods that may be used to infer mode of development in the tests of fossil and Recent echinoids: genital pore size and crystallographic patterns of apical plates. Extant species with known modes of development were examined, and the following hypotheses were tested. 1) Species that produce large eggs and have nonfeeding larval development have larger genital pores than species that produce small eggs and have feeding larval development. 2) Orientations of crystallographic axes (c-axes) of genital and ocular plates differ in species with differing modes of development and can be used to infer mode of development. Genital pore size was found to be strongly dependent on body size within a species. For some taxa, pairwise interspecific comparisons of the relationships between genital pore size and body size support the hypothesis of larger genital pores for species with nonfeeding larval development. However, in multiple comparisons of linear regressions, species with nonfeeding larval development always overlapped other species with feeding larval development. An examination of the allometry of genital pore growth showed some species with nonfeeding larval development differed from those with feeding larval development; other species with differing modes of development could not be distinguished on the basis of allometric growth parameters. Orientations of c-axes of genital plates were found to be accurate indicators of mode of development, but orientations of c-axes of ocular plates were not. Among regular echinoids, 71 of 72 species supported the hypothesis that orientation of c-axes of genital plates is indicative of mode of development. Among 19 spatangoid echinoids studied, orientations of c-axes of genital plates generally allowed separation of species with differing modes of development. This method cannot be used to infer modes of development in taxa with reduced numbers of genital plates such as some spatangoids, some cassiduloids, and all clypeasteroids. Taxonomic differences in c-axis orientations require that inferences be made from comparisons between species within families.