Effects of delayed metamorphosis and delayed post-settlement feeding on post-larval survival and growth of the abalone Haliotis diversicolor.

Abstract To elucidate the effects of delayed metamorphosis and delayed post-settlement feeding on larval competence and post-larval survival and growth in the small abalone Haliotis diversicolor , we examined three sets of experiments. First, a pebble coated with crustose coralline algae (CCA) was used to induce competent H. diversicolor larvae to metamorphose at 2, 5, 8, 12, or 17 days (Group A) or 2, 6, 10, or 12 days (Group B) after fertilization. To observe spontaneous attachment and metamorphosis, some larvae from Group B were maintained without the addition of a CCA-coated pebble. Rapid metamorphosis was observed in older larvae of both CCA-induced groups; 2 days after the addition of the CCA-coated pebble, 50–75% of the 2-day-old larvae and more than 85% of the 5–12-day-old larvae had metamorphosed. The percentage of spontaneously metamorphosed individuals increased 14 days after fertilization, reaching 50.6 ± 16.7% (mean ± S.E.) at 18 days. Second, metamorphosed post-larvae that had experienced larval periods of 5, 8, 11, or 15 days (Group A) or 4, 7, or 10 days (Group B) were reared at 26 °C with or without the benthic diatom Nitzchia ovalis as the sole food source. The survival and growth rates of the fed post-larvae in both groups decreased with the length of the larval swimming period; of the post-larvae from larvae of more than 7–11 days old, fewer than 70% still survived 16 days after metamorphosis. Mean growth rates during the 6 days after metamorphosis were lowest for the post-larvae from the larvae with the longest period of swimming. Third, newly settled post-larvae from 2-day-old larvae were reared at 26 °C with delays of 0, 3, 6, 9, or 12 days before first feeding ( N. ovalis ). Feeding delays of longer than 6 days dramatically depressed post-larval survival rates; approximately 1 week after the onset of feeding, more than 65% and fewer than 20% of the larvae subjected to feeding delays of 0–3 days or 6–12 days, respectively, still survived. These results indicate that H. diversicolor is more susceptible to the detrimental effects of delayed metamorphosis and delayed post-settlement feeding than are the larger abalone species Haliotis discus hannai and Haliotis iris .