The importance of flow and settlement cues to larvae of the abalone, Haliotis rufescens Swainson.

In flow tank experiments, I tested the relative importance of active and passive processes to larvae settling on manufactured casts that were hydrodynamically rough at a small scale (mm to <1 cm). I predefined two distinct regions of small-scale flow that I used to manipulate larval settlement behaviour of the red abalone Haliotis rufescens Swainson. The larvae show a stringent settlement response associated with coralline red algae. Haliotis rufescens larvae settled preferentially to an inducer regardless of the flow conditions, as expected. However, the ability of H. rufescens larvae to show this stringent behaviour was altered by changing the small-scale flow. When the free-stream velocity was low, the larvae responded to a settlement cue regardless of the small-scale hydrodynamics. When free-stream velocity was higher, the larvae acted increasingly as passive particles in their deposition, but settled only in response to an inducer. The results were consistent in two flow tanks, across 2 years and between different batches of larvae.