EMLET, R.B.: Maternal provisioning, buoyancy and redesign of echinoderm larvae

Living echinoderms have independently evolved (> 20 times) large, organically rich eggs and nonfeeding larvae from small eggs and feeding larvae. Reduction or loss of feeding structures and decreased planktonic intervals are commonly documented patterns in the evolution of nonfeeding larval development. The functional consequences of changes in shape, size and ciliation are rarely considered. The simplified body plans of nonfeeding larvae converge on low-drag shapes. Larger larvae typically have uniform ciliation instead of ciliated bands. The correlation of large size and uniform ciliation can be explained on biomechanical grounds: more cilia than those of the ciliated bands are required to match drag and buoyant forces during locomotion. Floating eggs have evolved in half of all instances where nonfeeding larval development has arisen. Departures from neutral buoyancy influence swimming and orientation. I have examined swimming and buoyancy of ontogenetic stages of four lecithotrophic species. Embryos, initially positively buoyant, usually swim upward, but they can swim down against their buoyant force. Later stages become negatively buoyant and tend to swim downward. Swimming speeds range up to 2 mm/s, are 3 to 5 times greater than those of planktotrophic larval echinoderms, and are correlated with larval specific gravity. The positively buoyant stages are oriented with anterior ends up and posterior ends down. When moving downward, embryos and larvae swim blastopore first, moving water anteriorly. This pattern is opposite that of planktotrophic embryos. Bouyancy in the larvae may in part be controlled by skeletalization. Within the echinoderms, maternal provisioning has created larval forms redesigned to meet functional requirements of swimming and non-neutral buoyancy.