Controlled reproduction and larval rearing
Wild or hatchery-reared brood stock can be used for controlled production of seed. If wild brood stock are used, it is considered essential to condition them in holding tanks for two or three weeks. Investigations on the Japanese abalone, H. discus hannai, show the importance of conditioning at an optimum temperature and feeding on the preferred food of fresh seaweed (e.g. Undaria and Laminaria) for successful maturation. According to Kan-No (1975), if the rearing is carried out at 20°C, the species will attain maturity in about 80 days, even in the winter season. Maturity can be maintained for at least three months.
Since the stimuli of elevated water temperature and/or air drying do not provide a reliable means of inducing spawning, the use of ultra-violet irradiated sea water is sometimes adopted to ensure consistent spawning. The gravid animals are exposed to flowing heated sea water, irradiated with UV light. An irradiation of 800 milliwatt hours per litre is reported to be adequate, and spawning occurs in about three hours.
Another method of spawning abalones is the one developed in the USA of exposing brood stock to hydrogen peroxide.This is based on the finding that hormone-like prostaglandins regulate spawning in abalones: hydrogen peroxideactivates the natural enzymic synthesis of prostaglandins in gravid animals and so spawning can be induced. Gravid stock are kept in suitable containers of sea water (temperature 12–18°C) made alkaline to a pH of about 9.1 by the addition of sodium hydroxide. A 6 per cent solution (freshly diluted from a 30 per cent stock solution) of hydrogen peroxide is introduced into the container at the rate of 50ml for each 12l of water. After an exposure of about 2.5 hours, the water is drained and immediately replaced with isothermal fresh sea water.
Spawning can be expected to occur within 2.5–3.5 hours.
If the spawning tank contains both females and males, fertilization of the eggs takes place in the tank itself. A sex ratio of one male to four females is maintained in such tanks. If, however, the males and females are spawned in separate tanks, the gametes can be collected and fertilized separately. The fertilized eggs are washed free from excess sperm and incubated in clean sea water at a temperature of about 14–16°C. The larval trochophores hatch out 18–24 hours after fertilization. In about seven days they reach the veliger stage and if care is taken to maintain water quality and prevent microbial growth in the culture, very high larval survival rates can be expected. Species like H. diversicolor develop faster and may assume a benthiclife within 43–46 hours after hatching.
For larval settlement, special tanks made of fibreglass provided with filtered running sea water are commonly used. The tanks are ‘sea-soned’ with a growth of benthic diatoms, bacteria and microalgae, in particular red algae. Fluorescent lighting is provided to promote the growth of diatoms in the tanks. The stocking density of larvae appears to vary very considerably, depending on the water quality and methods of feeding.
Investigations have shown that the abalone requires a specific biochemical inducer for normal settlement, metamorphosis and rapid subsequent development of juveniles, which has been identified as the amino acid gamma-aminobutyric acid (GABA), contained in the red algae. The addition of a low concentration of this inducer causes rapid synchronous and completely normal settlement, metamorphosis and juvenile growth. Crustose coralline red algae, or specific proteins derived from thesealgae, cause a similar induction of settlement and metamorphosis, but in culture systems predation by microscopic faunae associated with the algae may cause mortalities. The use of GABA is therefore considered more convenient and inexpensive. Another means of inducing larvae to settle is by using the mucus of juveniles and adult animals together with diatoms (Chew, 1986).