Ciliary Hovering in Larval Lancelets (=Amphioxus)

Larvae of lancelets (=amphioxus) are of special interest because they figure prominently in debates about vertebrate origins (1), can sometimes grow into a giant "amphioxides" form (2, 3), have a puzzling right-left asymmetry (4), and constitute a major zooplankton resource in parts of the Atlantic (5). By using improved methods (6, 7) to culture and observe healthy pre-metamorphic larvae in relatively deep containers, we demonstrated a prominent hovering behavior. The larvae spend most of their time suspended in midwater by metachronal beating of epidermal cilia. The body is usually tilted at an angle such that the anterior end and ventral side are oriented towards the water surface. This posture is maintained in the dark and in the light, although there is directional photosensitivity. Hovering may help account for the giant "amphioxides" and may be related to the curious asymmetry of the larval body.