Larval locomotion of the lancelet.

The ontogeny of locomotion in the Florida lancelet (Branchiostoma floridae) is described for the early developmental stages through to metamorphosis. Recently hatched larvae swam at speeds up to 1 mm s-1 using their epidermal cilia; this speed decreased to approximately 0.2 mm s-1 by 60 h after fertilization. Changes in cilia-powered fluid flow could be related to changes in the distribution and density of the epidermal cilia during development. Cilia-powered hovering was the dominant behaviour until metamorphosis. The amount of energy expended by ciliating larvae ranged from 10(-9) to 10(-11) W depending upon the age of the larvae and the model used for estimating the power output. The majority of the energy expended was in the ciliary sublayer next to the body. The first muscular movements were seen in larvae 16 h old. These simple flexions increased in complexity during the first 72 h until a complete undulatory (approximately sinusoidal) wave was propagated down the body in the adult manner. The frequency of undulatory beating increased to approximately 10 Hz during the first 48 h, and the larval head showed a large degree of yaw. Lancelet larvae were also capable of high-speed undulations 5­10 times faster than regular swimming motions. In contrast to ciliating larvae, the energy expended during undulation was at least an order of magnitude greater (10(-8) to 10(-6) W) and radiated beyond the ciliary sublayer.