Regulatory effects of mussel (Aulacomya maoriana Iredale 1915) larval settlement by neuroactive compounds, amino acids and bacterial biofilms Andrea C. Alfaro ⁎, Tim Young, Annapoorna Maitrayee Ganesan School of Applied Sciences, Faculty of Health and Environmental Sciences, Auckland University of Technology, Private Bag 92006, Auckland 1142, New Zealand abstract article info Article history: Received 21 March 2011 Received in revised form 29 August 2011 Accepted 29 August 2011 Available online 16 September 2011 Keywords: Larval settlement Ribbed mussels Aulacomya maoriana Chemical cues Bacterial biofilms Larval settlement responses of the ribbed mussel, Aulacomya maoriana Iredale 1915, were investigated after exposure to various chemicals and mono-species bacteria. Identification of settlement inductive compounds assists in the elucidation of intermediary biochemical mechanisms involved in the neuronal control of settlement behaviour downstream from primary cue reception. Neuroactive compounds and amino acids (potassium ions, GABA, acetylcholine, L-Phenylalanine, L-Tyrosine, dopamine, epinephrine, L-Tryptophan, and 5-HTP) and planktonic bacteria, biofilms and biofilm exudates of Macrococcus sp. AMGM1, Bacillus sp. AMGB1, and Pseudoalteromonas sp. AMGP1 were tested for their abilities to induce larval settlement. Toxicity effects of each treatment also were simul- taneously identified by recording larval mortalities. Results indicate that all chemicals used induced larvae to settle, with acetylcholine being the most effective (~24% at 10 -6 M compared to b 2% in control assays). Toxicities of treatment compounds were low at optimal settlement inducing concentrations, except for L-Tryptophan (~32%) and GABA (~59%). Our data suggest that catecholamines (and their precursors) play an important role in the biochemical mechanisms of settlement for A. maoriana. While serotonin precursors did induce low levels of larval settlement at some concentrations, high toxicity responses to 5-HTP at 10 -5 M, combined with com- plete settlement inhibition indicate that the mechanism of action may be more complex than can be elucidated in this study. Larval settlement responses to bacterial treatments were low for planktonic and biofilm phases across all three strains, and settlement inhibition was observed when larvae were exposed to biofilm exudates of all bacterial strains. Comparisons of A. maoriana responses to other endemic and worldwide distributed mussel species are provided as a means to highlight potential evolutionary differences in chemoreception mechanisms. © 2011 Elsevier B.V. All rights reserved. 1. Introduction A great majority of marine invertebrates has planktonic larvae, which after a period of minutes to months (Hadfield and Paul, 2001) settle onto benthic substrates. A wide range of environmental and biological stimuli or cues mediate this settlement process (Hadfield, 2011; Harder et al., 2002; Pawlik, 1992; Steinberg et al., 2002; Wikstrom and Pavia, 2004). Chemoreception involves the binding of chemicals to receptors in the neural tissues of larvae, which activate neuronal net- works (Hay, 2009). Factors which regulate larval settlement behaviour have been investigated extensively for many marine taxa (reviewed by Hadfield and Paul, 2001; Steinberg and De Nys, 2002; Murthy et al., 2009), but the complex chemoreception process has yet to be elucidated. For example, within the Class Bivalvia, many cues have been found to induce larval settlement in oysters (Tamburri et al., 2008; Yu et al., 2008; Yu et al., 2010a), scallops (Leyton and Riquelme, 2008; Mesías-Gansbiller et al., 2008), clams (García-Lavandeira et al., 2005; Neo et al., 2009; Sumin et al., 2006), and mussels (Alfaro et al., 2006; Bao et al., 2007; Dobretsov and Qian, 2003; Ganesan et al., 2010; Young, 2009). Settlement responses to different cues appear to be genus-, species- and even intraspecies-specific(Ritson-Williams et al., 2010; Rodríguez et al., 1993; Williams et al., 2008). These differ- ences suggest evolutionary variations in cue-binding receptors, endog- enous biochemical processes, and the metabolites produced during the settlement process. Chemical compounds that mediate larval settlement often are produced by bacteria or the biofilms they form on just about every surface in the marine environment (Hadfield and Paul, 2001). The chemical cues generated by bacteria may be surface-bound (bound to bacterial cells or exopolymeric substances) or water-soluble (produced by free-swimming planktonic bacteria or released by their biofilms) (Hadfield, 2011). The surface-bound cues induce larval settlement only when larvae come into contact with the bacteria (Hadfield, 2011). On the other hand, water-soluble cues (e.g., low and high mo- lecular weight polyshaccharides [Dobretsov and Qian, 2004; Harder et al., 2004], low molecular weight peptides [Tamburri et al., 1992; Zimmer-Faust and Tamburri, 1994], and even neurotransmitters [Mountfort and Pybus, 1992]) may regulate larval settlement without the need for them to contact the substrate (Browne and Zimmer, 2001; Tamburri et al., 1996). For example, studies on the green-lipped Aquaculture 322-323 (2011) 158–168 ⁎ Corresponding author. Tel.: + 64 9 921 9999x8197; fax: + 64 9 921 9743. E-mail address: andrea.alfaro@aut.ac.nz (A.C. Alfaro). 0044-8486/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2011.08.038 Contents lists available at SciVerse ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online