Delayed metamorphosis by larval abalone in the field

Delayed metamorphosis has been thought to be an important factor affecting recruitment in marine benthic invertebrates with lecithotrophic larval stages including abalone. The effects of delayed metamorphosis on larval competence, and post-larval survival and growth of Ezo abalone Haliotis discus hannai were examined, and age determination method for larval and newly metamorphosed post-larval H. discus hannai was developed by the laboratory experiments. In addition, we determined the age of field caught larvae and post-larvae, and actual larval period of H. discus hannai in the natural habitat was estimated. Larval Haliotis discus hannai required about 4 days before becoming competent to metamorphose, and metamorphosis was successfully achieved up to 19 days after fertilization. The survival and growth rates of post-larvae that metamorphosed after a 19-day larval period, however, were significantly lower than those within less than 15-day larval period. Competent veliger larvae had a radula and regularly added rows of radular teeth with age, in the absence of metamorphosis. The number of radular teeth rows increased linearly with age, but rows of radular teeth were added more slowly at lower temperatures. The effect of temperature on the development rate of the radula was quantified by the regression and the temperature coefficient, Q10. From these results, an age determination method of veliger larvae and newly metamorphosed post-larvae (within a day of metamorphosis) was established; using the number of radular teeth rows. The age of veliger larvae and newly metamorphosed post-larvae captured in the field was determined by the age determination method. Nine percent of veliger larvae were estimated to be 4-6 days old. The remaining 91% of larvae were classified as younger than 4 days old. All newly metamorphosed post-larvae were estimated to be 4-6 days old.