The bilaterally asymmetrical larval form of Stomopneustes variolaris (Lamarck)

A newly identified left-right asymmetry in larval sea urchins

Directional asymmetry (DA) in body form is a widespread phenomenon in animals and plants alike, and a functional understanding of such asymmetries can offer insights into the ways in which ecology and development interface to drive evolution. Echinoids (sea urchins, sand dollars and their kin) with planktotrophic development have a bilaterally symmetrical feeding pluteus larva that undergoes a dramatic metamorphosis into a pentameral juvenile that enters the benthos at settlement. The earliest stage of this transformation involves a DA: a left-side invagination in mid-stage larvae leads to the formation of the oral field of the juvenile via a directionally asymmetric structure called the echinus rudiment. Here, we show for the first time in two echinoid species that there is a corresponding DA in the overall shape of the larva: late-stage plutei have consistently shorter arms specifically on the rudiment (left) side. We then demonstrate a mechanistic connection between the rudiment and arm length asymmetries by examining rare, anomalous purple urchin larvae that have rudiments on both the left and the right side. Our data suggest that this asymmetry is probably a broadly shared feature characterizing ontogeny in the class Echinoidea. We propose several functional hypotheses-including developmental constraints and water column stability-to account for this newly identified asymmetry.

Morphological evolution of newly metamorphosed sea urchins--a phylogenetic and functional analysis

Newly metamorphosed juvenile sea urchins are highly variable across taxa. This contribution documents and illustrates structural, functional, and phylogenetic variation among newly metamorphosed juvenile sea urchins for 31 species from 12 ordinal or familial lineages. The classic juvenile with five primary podia, 20 interambulacral spines, and variable numbers of juvenile spines is found commonly among new metamorphs across lineages, but there are many examples, which depart from this pattern and most likely reflect adaptation to settlement habitats. At metamorphosis juveniles can have 5-25 functional podia. They can have 0-65 spines, 0 or 5 sphaeridia (balance organs). They may have zero or up to eight pedicellariae. While competent larvae that delay metamorphosis may continue to develop juvenile structures, variation across species is much greater than within species and there are strong phylogenetic and functional differences among juveniles. Heterochronic changes in expression of these structures can account for differences among taxa. Based on this sample, juvenile characters such as spines, podia, and larval pedicellariae are expressed in ways that suggest they are developmental modules whose expression can be readily changed relative to one another and to the time of metamorphosis.