COMBINED EFFECTS OF LIGHT CONDITION (CONSTANT ILLUMINATION OR DARKNESS) AND DIATOM DENSITY ON POSTLARVAL SURVIVAL AND GROWTH OF THE ABALONE HALIOTIS RUFESCENS.

Abalone (Haliotis spp.) postlarvae are cultured in systems that provide natural or artificial light to promote the growth of benthic diatoms that are grazed by postlarvae. Larger abalones (>2 cm) grow better in dark conditions and the possibility that this is true for postlarvae is explored in this contribution. Two independent experiments with Haliotis rufescens postlarvae fed the diatom Navicula incerta were conducted in 10-mL vessels with daily water changes. Two factors were tested following split-plot experimental designs: six diatom densities (from 500-10.000 cells/mm²) and two light conditions (constant light at 19-33 μE/m²/s and darkness). Experimental units in darkness were kept inside black plastic bags but subjected to -30 min of ambient light every day for maintenance. Food (N. incerta) was supplied as required to maintain diatom densities. The first experiment started with 14-day-old postlarvae and was conducted for 20 days: the second trial started with 2-day old postlarvae and was performed for 32 days. In general. postlarval growth increased as diatom density increased but stabilized at high densities (ca. >2.000 cell/mm²) and was significantly higher in darkness in both trials. In Experiment 1, average growth rate in darkness was 2.4 times higher than in light conditions (34.7 and 14.4 μm/d, respectively). In Experiment 2, average growth in darkness was 3.0 times higher than under constant illumination (14.4 and 4.8 μm/d, respectively). These results are discussed in terms of postlarval behavior and possible changes in the nutritional quality of diatom films. The potential implications for abalone culture are also addressed. [ABSTRACT FROM AUTHOR]