A newly identified left–right asymmetry in larval sea urchins

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.

Hormonal Regulation of Programmed Cell Death in Sea Urchin Metamorphosis

“…Still, in recent years, new data on cell death regulation and function has emerged from a diversity of taxa and this data suggests that PCD mechanisms are used in a range of morphological, physiological, and developmental contexts. For example, animals with an indirect life-history, characterized by a drastic morphological and physiological transition (Tata, 1993;Balon, 1999;Bishop et al, 2006;Heyland and Moroz, 2006;Ishizuya-Oka et al, 2010;Heyland et al, 2018;Tettamanti and Casartelli, 2019) have been repeatedly shown to employ PCD for the removal of cells and the transformation of the larval body plan into a juvenile (Heyland and Moroz, 2006;Hodin et al, 2016;Lutek et al, 2018). Here we integrate available mechanistic information on PCD from whole organism studies with our understanding of the regulatory mechanisms of metamorphosis with an emphasis on endocrine regulation.…”

Thyroid hormone-induced cell death in sea urchin metamorphic development

“…Echinoid larvae typically reduce their arm growth as they approach metamorphic competence and settlement ( Hart and Strathmann, 1994 ; Heyland and Hodin, 2004 , 2014 ; Heyland et al, 2006a , 2004 ; Hodin et al, 2016 ; Reitzel and Heyland, 2007 ; Soars et al, 2009 ; Strathmann et al, 1992 ). If the model outlined above is correct, this could be accomplished by reducing cell proliferation, increasing apoptosis or both.…”

Combined effect of microplastics and global warming factors on early growth and development of the sea urchin (Paracentrotus lividus)

“…The evolutionary pressures shape the larvae morphology to fit within opposing functional constraints such as feeding ability and stability in the water column. Feeding structures generally require large surface area for particle capture, whereas stability relies upon minimal surface area (Hodin et al, 2016). Thus, deformities and altered growth can compromise both crucial activities and therefore the chance of survival of P. lividus larvae.…”