Effects of delayed metamorphosis on larval competence, and postlarval survival and growth of abalone Haliotis discus hannai

The effects of delayed metamorphosis on larval competence, and the postlarval survival and growth of Haliotis discus hannai (H. discus hannai) were examined. Competent larvae were induced to metamorphose at 5, 10, 15, and 19 days after fertilization by the addition of 1 muM gamma-aminobutyric acid (GABA). Larvae in another group were maintained until individuals metamorphosed spontaneously. Metamorphosed individuals (postlarvae) were reared in the laboratory at 20 C fed on a benthic diatom Cylindrotheca closterium or without any food in equivalent conditions. Metamorphosis, survival, and growth (determined from shell length) rates were measured. The percentage of individuals that had metamorphosed 2 days after the addition of GABA increased with the length of larval swimming period; larvae that were 5 and 19 days old when induced to metamorphose showed 19% (+/- 5.6; S.E.) and 96% (+/- 3.6) metamorphosis, respectively. The percentage of postlarvae that metamorphosed spontaneously increased after 17 days from fertilization and reached 96% (+/- 3.0) at 24 days postfertilization. Survival rates of fed postlarvae depended on larval swimming period. For larvae that swam for less than or equal to 15 days, more than 80% of postlarvae survived throughout the 20-day experimental period. In contrast, the survival rate was only 57% (+/- 2.9) for 19-day-old larvae. Postlarval growth rates did not differ significantly between larval swimming periods less than or equal to 15 days. However, the growth rate of postlarvae from 19-day-old larvae was significantly lower than that of less than or equal to 15-day-old larvae. In the starved treatments, survival rate was lower, and the final shell length of the dead animals was less, as larval period became longer. (C) 2002 Elsevier Science B.V. All rights reserved.