Abstract
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.
Highlights
Many species of benthic invertebrates have a planktonic larval phase, which may allow these taxa to exploit alternative resources across life-history stages, increase their dispersal ability and maintain connectivity among populations [1,2,3]
We tested whether echinopluteus larval arms were directionally asymmetrical during mid- to latelarval development, and whether any detected asymmetry changed as a function of developmental age and stage
Our data from two disparate echinoids, separated by approximately 250 million years of evolution, suggests that this consistent, previously undescribed asymmetry in multiple arms, and in overall larval shape, may be a common feature of late-stage echinopluteus ontogeny
Summary
Many species of benthic invertebrates have a planktonic larval phase, which may allow these taxa to exploit alternative resources across life-history stages, increase their dispersal ability and maintain connectivity among populations [1,2,3]. The echinoderms—including sea urchins, sea stars and sea cucumbers—exhibit a wide variety of such planktonic larval forms, both feeding and non-feeding [4,5,6,7,8]. These forms are the result of evolutionary pressures that appear to shape larval morphology within the confines of opposing functional. Constraints, in particular on feeding ability versus stability in the water column [9,10]. 2 feeding structures generally require large surface area for particle capture, whereas stability, especially in turbulent waters, relies upon minimal surface area [8]
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