Variable response from abalone larvae (Haliotis iris, H. virginea) to a range of settlement cues

Abstract

We exposed competent larvae of two abalone species (Haliotis iris and H. virginea) to a range of potential settlement cues, and observed a hierarchy of responses. The end point and the timing of the settlement process varied, depending on the cue provided . Response was qualitatively consistent between experiments, but showed some quantitative variation. Rocks coated with crustose coralline algae (CCA) induced 80–100% of larvae to attach and metamorphose, and 50–100% to grow peristomal shell, within 2 days. A distilled water extract of CCA induced almost 100% attachment within 2 days, but metamorphosis occurred gradually between 2 and 5 days. Shell growth was minimal, apparently inhibited by chemical interference from the extract. γ-Aminobutyric acid (GABA, I μM) induced 90–100% attachment and 20–60% metamorphosis in H. iris within 2 days. For H. virginea, I μM GABA induced attachment (65–100%) but typically only 0–5% metamorphosis within 2 days. KCl (10 mM) added to seawater induced attachment (50–70%) but less than 10% metamorphosis. Different diatoms induced responses ranging from rapid or gradual induction of metamorphosis, to minimal response . Some combinations of cues produced synergistic effects. E.g ., GABA (I μM) plus diatom biofilm induced more metamorphosis (49%) than biofilm (28%) or GABA (1%) alone. Addition of CCA crude extract to the same biofilm hastened metamorphosis. Observations of attachment without subsequent metamorphosis suggest that there may be separate cues for attachment and metamorphosis. This study highlights the complexity of the settlement response in abalone.