A review of settlement cues for larval abalone (Haliotis spp.)

Settlement of abalone larvae involves larval attachment (a reversible behavior) followed by metamorphosis (which involves irreversible physical changes), Coralline algae induce attachment and metamorphosis in all abalone species tested, and there is some evidence of settlement preferences for certain coralline species. The settlement-inducing chemicals from corallines have not been identified, In one case (Haliotis rufescens) a small peptide is implicated, while in another (Haliotis discus hannai) halomethanes are thought to be critical. Corallines are generally regarded as unsuitable for use in hatcheries, but their potential use has not been fully evaluated. Many abalone hatcheries rely on biofilms to induce larval settlement. The activity of biofilms increases with their age. Ungrazed films are generally dominated by fast-growing benthic diatoms, and settlement on these films is variable and often low. Few diatom strains are consistently good for settlement, and strains that are excessively mobile, or form 3-dimensional colonies, can prevent successful settlement. The chemistry of settlement induction by biofilms, and the role that bacteria play, are poorly understood. Bacteria can induce settlement, but metamorphosis occurs gradually over several days, if at all. Pregrazing by conspecific abalone improves the settlement-inducing activity of a biofilm. The mucus from the foot of grazers probably contains chemicals (not identified) that trigger attachment or metamorphosis, particularly when combined with another cue (such as dibromomethane or a biofilm). Various pure chemicals induce attachment and/or metamorphosis of abalone larvae. These may bind to larval receptors (e.g., gamma-aminobutyric acid = GABA) or act "downstream" of, or "parallel to", the receptors (e.g., compounds that depolarize membranes or alter levels of cyclic AMP or calcium). None of these chemicals is considered to be a natural settlement cue, and only GABA has been used in abalone hatcheries. The timing and end point of the abalone settlement response vary among cues, and among abalone species. Cues can combine synergistically to enhance settlement (e.g., GABA with lysine, biofilm or coralline extract, dibromomethane with mucus). Signal reception pathways are known only for the case of Haliotis rufescens and GABA-mimetic inducers. The response is controlled through a "morphogenetic pathway", which is subject to up- and down-regulation by molecules acting on a separate "regulatory pathway". Larval attachment and metamorphosis are often uncoupled in abalone. Cues for attachment are much more widespread than cues for metamorphosis.