Swimming performance in early development and the "other" consequences of egg size for ciliated planktonic larvae

The evolutionary significance of egg size in marine invertebrates is commonly perceived in energetic terms. Embryonic size should also have direct effects upon the forces that govern swimming, a behavior common to early larval development in the plankton. If swimming is ecologically important, early larvae may need to perform to a certain "standard", or threshold of speed and/or stability. The existence of performance standards in early development could therefore act to constrain the evolution of egg size and the evolution of development. Here we present the key parameters that characterize the upward swimming speed of ciliated spheroidal larvae moving at very low Reynolds numbers. The dependence of maximum supported mass upon larval size, and the independence of neutral-weight swimming speed from size, lead to hypotheses about scaling of swimming speed with size. Experimental studies with thirteen broadcast-spawning planktotrophs demonstrate that free-living embryonic swimmers in all of these species conform to a strong negative scaling of density with size that offsets increases in mass with increasing size. This trend suggests that swimming ability is broadly under selection in early development. In experimental studies and in a hydrodynamic model of larval swimming, the performance of trochophore larvae provides support for our hypothesized scaling relationships, and also for the concept of a standard in swimming speed. Echinoid blastulae, however, show relationships between speed and size that are not predicted by our scaling arguments. Results for echinoids suggest that differences in ciliary tip speed, or possibly in spatial density of cilia over the blastula's surface, result in significant differences in species' performance. Strong phyletic differences in the initial patterning and growth of structures used for swimming thus appear to cause significant differences in the relationship of swimming ability with embryo size.