Mathew C. Ferner, Jason Hodin, Gabriel Ng, and Brian Gaylord
In coastal ecosystems, attributes of fluid motion can prompt animal larvae to rise or sink in the water column and to select microhabitats within which they attach and commit to a benthic existence. In Echinoid (sea urchin and sand dollar) larvae living along wave-exposed shorelines, intense turbulence characteristic of surf zones can cause individuals to undergo an abrupt life-history shift characterized by precocious entry into competence: the stage at which larvae will settle and complete metamorphosis in response to local cues. However, the mechanistic details of this turbulence-triggered onset of competence remain poorly defined. Here, we evaluate in a series of laboratory experiments the time course of this turbulence effect, both the rapidity with which it initiates, and how long it perdures. We find that larvae become competent with turbulence exposures as brief as 30 sec, with longer exposures inducing a greater proportion of larvae to become competent. Intriguingly, larvae can remember such exposures for a protracted period (at least 24 hrs), a pattern reminiscent of long-term potentiation. Turbulence also induces short-term behavioral responses that last less than 30 mins, including cessation of swimming, that facilitate sinking and thus contact of echinoid larvae with the substrate. Together these results yield a novel perspective on how larvae find their way to suitable adult habitat at the critical settlement transition, and also open new experimental opportunities to elucidate the mechanisms by which planktonic animals respond to fluid motion.