Lipid dynamics in the embryos of Patiriella species (Asteroidea) with divergent modes of development

Evolution of lecithotrophic development in sea stars involved a modificatio n in maternal provisioning from the production of yolk-dominated to lipid-d ominated eggs. The dynamics of lipid reserves in the embryos of four Patiri ella species differing in their lipid provisions were examined. Patiriella regularis had small yolk protein-dominated eggs (150 mu m in diameter) and an ancestral mode of development through planktotrophic larvae. Patiriella calcar, Patiriella exigua and Patiriella pseudoexigua had large eggs (390-4 40 mu m in diameter) and lecithotrophic planktonic, benthic and intragonada l larvae, respectively. Patiriella exigua deposited negatively buoyant eggs containing substantial yolk protein and lipid reserves onto the substratum . In contrast, the planktonic eggs of P. calcar and the intragonadal eggs o f P. pseudoexigua were dominated by lipid and were neutrally and positively buoyant, respectively. By the blastula stage there was little trace of lip id in P. regularis embryos. Blastulae of the lecithotrophic developers, by contrast, had conspicuous lipid droplets distributed through their cells. I n parallel with the change from cuboidal to columnar epithelium during the blastula to gastrula transition, lipid reserves became redistributed into t he basal cytoplasm. The extent of lipid transport reflected the amount of l ipid reserves. In P. pseudoexigua embryos with the greatest lipid load, bas al shunting was followed by secretion of lipid into the blastocoele where i t was stored for the perimetamorphic period. Evolution of lecithotrophy in Patiriella appears to reflect selection to provide metamorphic stages with nutrients normally accrued by feeding larvae with the consequence that earl y development is burdened by voluminous, potentially inert nutritive stores . Lipid redistribution coincident with a major developmental stage transiti on may be required to facilitate unimpeded morphogenesis. This phenomenon m ay be characteristic of lecithotrophic development in echinoderms and appea rs pre-adaptive for extrusion of lipid in species like P. pseudoexigua with particularly extensive lipid reserves.