Shear vibes: Delineating movement and cilia stroke patterns in the planula of the hydrozoan Clytia gregaria

Abstract

Many factors contribute to a larva’s ability to navigate its three-dimensional domain, including abiotic influences such as currents and flow as well as organismal traits including larval morphology and behaviour. Using the hydromedusa Clytia gregaria (L. Agassiz, 1862) (Cnidaria: Hydrozoa), we investigated the behaviour of planulae in different hydrodynamic conditions and anaesthetised and de-ciliated larvae to observe mechanisms behind movement. In this study, we provide evidence that cnidarian planula are capable of both actively righting themselves and passively moving upwards via positive buoyancy. Planulae maintain their orientation even in high shear, suggesting their ability to sense shear and control their movements in response. We also provide a first detailed description of ciliary strokes and movement patterns for a hydrozoan planula, highlighting the potential control of active and inactive cilia. Our results suggest planulae may move in a yet unknown combination of ciliary movement and muscular activity. We provide important insights into an often overlooked yet crucial cnidarian life history stage as well as a comparative reference point for further studies of larvae in this highly diverse phylum.