Data from: A newly-identified left-right asymmetry in larval sea urchins

Directional asymmetry in body form is a wide-spread 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 directional asymmetry: a left-side invagination in mid-stage larvae leads to the formation of the 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 the there is a corresponding directional asymmetry 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 likely 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.