Three-stage lipid dynamics during development of planktotrophic echinoderm larvae

The eggs of marine species with planktotrophic development must contain, at a minimum, sufficient material for production of a larva that can then sequester additional materials to grow and metamorphose successfully. In echinoderms, lipids perform crucial energy storage and structural functions during larval construction, but their roles during later development and metamorphosis are poorly understood. We investigated lipid-class depletion in early development and accumulation in late development and a lipid nutritional condition index (energetic lipid: sterol ratio) from the egg to the juvenile in the sea star Patiriella regularis and the sea urchin Heliocidaris tuberculata. Three phases were identified: (1) rapid depletion of energetic lipids during embryogenesis and the facultative feeding period (between feeding competence and exhaustion of energetic lipid reserves), (2) larval growth with no improvement in lipid nutritional condition, and (3) rapid lipid accumulation in advanced larvae prior to metamorphosis. Maternally derived energetic lipids were depleted more slowly in fed than unfed larvae but were still exhausted quickly. Patiriella regularis improved their lipid condition index during Phase 3 by accumulating energetic lipid (triacylglycerol [TAG], diacylglycerol ether [DAGE]) reserves that were then partially used to fuel settlement and metamorphosis. In contrast, Heliocidaris tuberculata did not accumulate TAG or DAGE during this phase, suggesting that metamorphosis is fuelled by other reserves, which we hypothesize may be phospholipids.