Introduction
The seeding of abalone larvae has been considered as a viable alternative for stock enhancement programs due to high production costs of raising juvenile abalone for seeding (Preece et al., 1997). However, poor larval settlement and metamorphosis in the hatchery is one of the significant constraints with seed production on abalone farms, resulting in low post-larval survival (Gapasin and Polohan, 2004; Li et al., 2006). Improving settlement and metamorphosis of abalone larvae is critical to ensure that abalone larvae settle within the seeding site for ocean ranching or to increase production in hatcheries. This study investigated the effect of biological (planktonic Nitzschia sp.) and chemical (potassium chloride) cues in inducing settlement and metamorphosis of Haliotis midae larvae. Specifically, the effects of dilution of such cues upon release of the larvae into the ocean is immitated by removing the settlement cues after a certain time period.
Materials and Methods
Three potassium chloride concentrations (10, 15, and 20 mM) and a control (with no KCl) were used to determine the optimal concentration for potassium ions to induce larval settlement for Haliotis midae. The optimal concentration of KCl was used to determine the effect of longer exposure period (12- and 24-hours) on the larval settlement and metamorphosis with the treatments also include the combination of KCl optimal concentration and biological cues (Nitzschia). Larval settlement and metamorphosisi was also determined after a brief exposure to chemical and biological cues and a combination of both using settlement sheets that were coated with diatoms.
Results
For larvae exposed to different KCl concentrations (10–20 mM), settlement was highest at 10 mM in the first 20 hours. Larval settlement exposed to KCl-12 hours and KCl and Nitzschia combined-24 hours was higher in the first 20 hours than larvae exposed to KCl-24 hours and the controls (Figure 1). However, in both experiments, larval settlement in the controls increased over time and surpassed other treatments after 20 hours. Finally, the settlement was very low on uncoated sheets, compared to diatom-coated sheets, regardless of exposure to different combinations of KCl and Nitzschia.
Discussion
Due to the dramatic decrease in the mean settlement post-exposure, these results should be interpreted with caution when drawing biological conclusions for field studies. We, therefore hypothesize that exposure of Haliotis midae larvae to 10 mM KCl and Nitzschia will not enhance settlement in the ocean, as the inducers are primarily effective when KCl concentration levels remain at 10 Mm. The level of K+ in the ocean are typically below 10 mM (Todd et al.,1991) and it does not improve settlement without adding extra KCl in the seawater. However, long-term exposure to both KCl and Nitzschia over 24 hours could be used in hatcheries to improve the settlement of Haliotis midae larvae.
Reference
Gapasin, R.S.J., Polohan, B.B., 2004. Induction of larval settlement and metamorphosis in the donkey-ear abalone, Haliotis asinina Linnaeus, by chemical cues. Hydrobiologia 519, 9–17.
Li, H., Wei, L., Zhang, G., Cai, Z., Cai, G., Chang, Y., Xing, K., 2006. Enhancement of larval settlement and metamorphosis through biological and chemical cues in the abalone Haliotis diversicolor supertexta. Aquaculture 258, 416–423.
Preece, P.A., Shepherd, S.A., Clarke, S.M., Keesing, J.K., 1997. Abalone stock enhancement by larval seeding: Effect of larval density on settlement and survival. Molluscan Res 18, 265–273.
Todd, C.D., Bentley, M.G., Jonathan, N.H., 1991. Larval metamorphosis of the opisthobranch mollusc adalaria proxima (Gastropoda: Nudibranchia): The effects of choline and elevated potassium ion concentration. Journal of the Marine Biological Association of the United Kingdom 71, 53–72.