Meeting Abstract
Echinoderm eggs vary in volume over several orders of magnitude, but most that develop into planktotrophic larvae cluster in the nanoliter range (0.3–4 nl). Egg size is an important aspect of maternal investment, and the correlation with developmental mode implies an influence of egg size on developmental mechanisms and larval performance. From the starting material provided by the egg, planktotrophic larvae must build certain essential structures to survive to metamorphosis. These include the ciliary band, which is the primary larval feeding and swimming organ; an alimentary tract; and, for echinoids and ophiuroids, a larval skeleton. We therefore wondered whether scaling constraints preclude echinoderms from making even smaller eggs, and hence more propagules. We simulated egg size reduction in the sand dollar Dendraster excentricus, which has an egg size (1 nl) that is nearly modal for obligately planktotrophic echinoids, and observed the effect on larval development. To create half- and quarter-sized “eggs,” we separated blastomeres at the two- and four-cell stages. It is well known that echinoid larvae can “regulate”, creating complete larvae from each of four blastomeres, but we asked: can these partial larvae effectively move water and capture particles? Can they successfully reach metamorphosis? Do essential structures like the ciliary band scale locally, or with overall embryo size? All three sizes – full, half, and quarter – were able to move water and capture particles successfully, and we successfully raised all three sizes of larvae to metamorphosis, although quarters suffered a substantial burden of defects and asymmetry. We found that the proportion of cells in the ciliary band and the number of nerve cells along the band appears to scale with embryo size. This suggests that although developmental regulation might guarantee partial larvae a complete anatomy, further reduction might erode essential body organs to the point of disfunction, thereby conferring a constraint on egg size.
