Effects of settlement cues on behaviour and substrate attachment of hatchery reared winged pearl oyster (Pteria penguin) larvae

Successful hatchery production of commercial bivalve species requires optimal recruitment of larvae to substrates. This study used two experiments to investigate the effects on recruitment of Pteria penguin pediveliger larvae of (1) treating the culture medium with alternate concentrations of three chemical compounds both in the presence/absence of a bio-film and (2) exposure to five substrate types both in the presence/absence of a chemical cue. Small 10 mL aquaria were stocked with 1 larvae mL⁻¹ and given a 25x25 mm piece of horizontally-oriented flat substrate. The proportion of larvae within each treatment that displayed various behaviours (swimming/sitting/crawling), had settled using byssal attachment or had undergone mortality was recorded at 24, 48 and 72 h. After 48 h, settlement of P. penguin was 65% greater in aquaria containing a substrate covered by a naturally formed bio-film than in control aquaria. The neuroactive compounds 5-hydroxytryptamine (serotonin) and γ-aminobutyric acid (GABA) and the chemical compound potassium chloride (KCl) all acted to decrease larval swimming behaviour when compared to controls with untreated seawater. Exposure to KCl (20 mM) also resulted in more larvae crawling over the substrate surface. After 72 h, settlement of larvae in aquaria treated with serotonin (10⁻³ M) or KCl (20 mM) was significantly greater than in control aquaria by 75% and 84%, respectively, while exposure to GABA had no effect. The positive effect of a bio-film on settlement did not change significantly according to chemical treatment of the culture medium, suggesting that the techniques can be used in conjunction. Settlement in response to 20 mM KCl was enhanced by the presence of a red nylon mesh substrate with 5 mm between filaments. Roughly three times more larvae settled onto this substrate during the first 24 h when compared to the same material with a smaller mesh size, black fibreglass screen and smooth plastic substrates. Comparisons with settlement behaviour of other bivalves and the commercial implications of this research are discussed