Life history and early ontogeny determine vertical swimming behaviors in the larvae of Caribbean corals

Abstract

Marine larval dispersal is strongly influenced by ocean currents, but larvae themselves possess traits and behaviors that can curtail or augment dispersal, and affect their settlement. Energetics and metabolism, buoyancy, and swimming can all influence the ultimate fate of marine larvae through biophysical interactions with the environment. These biophysical interactions can be difficult to observe in situ, and ecologists often resort to biophysical models to predict dispersal pathways and estimate connectivity. These models aim to assist in marine reserve design and conservation; however, larval propagules have often been modeled as passive particles, which ignores the influence of the often complex early life-history of larvae on their own dispersal. Here we provide a systematic survey of vertical larval swimming behaviors in six species of Caribbean coral larvae, throughout their early ontogeny. We deployed novel larval observation systems ex situ to obtain continuous video footage of larval vertical velocity and behavior, tracked larvae using computer vision, and calculated time-series of vertical velocity distributions. Results add depth and resolution to the dramatic contrast in swimming behaviors between brooded and broadcast spawned coral larvae. In the absence of settlement cues, brooders often settled quickly post-planulation and exhibited mainly downward swimming behaviors throughout the first two weeks of life. Though they exhibited within-cohort variation, in the absence of settlement cues broadcast spawners typically continued swimming upward even after attaining competency. Behaviors in brooded larvae suggest that there is strong pressure to settle upon planulation. However, observations of detachment and continued downward swimming suggest an active search for suitable habitat. Oppositely, broadcast larvae were observed to prolong dispersal in the absence of settlement cues. Velocity distributions obtained in this study can be used parameterize propagule behaviors in biophysical models to examine the effects of behaviors on connectivity and larval retention in high-resolution coastal hydrodynamic models.