Swimming behavior and velocities of barnacle cyprids in a downwelling flume

It has been proposed that barnacle cyprids can maintain position in shorewardpropagating fronts by swimming upward against a downwelling flow, potentially mediating onshore transport of larvae toward intertidal habitat. This study developed a novel flume to characterize swimming behavior of barnacle cyprids in a laboratory downwelling flow. Seawater was pumped through a cylindrical observation chamber fitted with diffusers to produce a homogeneous downwelling velocity field. The flume generated plug flow with mean downwelling velocities (indicated by negative sign) of 0 to –47.3 mm s–1. Behavior experiments were done with wild Semibalanus balanoides cyprids. Vertical swimming rates and behaviors were estimated from video observations, and a mixture model was used to estimate velocity distributions for distinct behavioral modes. Larvae exhibited multiple behaviors but typically swam upward in response to downwelling, with a maximum estimated vertical velocity of 72.3 mm s–1. When faced with downwelling flows, cyprids alternated between upward swimming and downward swimming to maintain their vertical position in the chamber. As downwelling velocities increased, cyprids that remained in the field of view of the cameras exhibited faster mean upward swimming velocities. It is unclear how long individual S. balanoides cyprids can maintain depth, but cyprids were able to maintain depth throughout the 2 min observation period. This study supports earlier hypotheses based on field observations by demonstrating that S. balanoides cyprids swim well enough to counter downwelling velocities characteristic of convergence zones. Swimming against downwelling flow could be an adaptive behavior that enables shoreward transport in the absence of any larval ability to swim toward shore or even to sense its direction.