Biological zero point in hybrid Pacific abalone

Journal of Shellfisheries Research, Dec 15, 2004 by Hongen Zhao, Jinshi Zhang, Lihong Huang, Limin Sun

The biologic zero point (BZP) of hybrid Pacific abalone Haliotis discus hannai (China) and Haliotis discus discus (Japan) at temperatures of 16[degrees]C, 20[degrees]C, and 22[degrees]C was calculated from the developmental hatching rate, formation of larval retractor muscle, 90 degree torsion and formation of epipodial tentacles. BZP for the early development of hybrid Pacific abalone is 6.22[degrees]C, which is an important result for the hybrid breeding of Pacific abalone.

KEY WORDS: hybrid, Pacific abalone, biological zero point

INTRODUCTION

Pacific abalone, Haliotis discus hannai is a key aquaculture organism, cultivated in China since the 1980s. Commercial seed production commenced in 1987 and, from this time to 1992, abalone aquaculture grew greatly. Most research was focused on development of hatchery seed production techniques and grow-out modes. A series of key techniques involving spawning, larval rearing, and juvenile and seed nursing were established, as well as grow-out systems. Disease and abnormal mortality has recently, however, severely disrupted the industry and promoted the hybridization of Haliotis discus hannai (China) and Huliotis discus discus (Japan). Hybrid [F.sub.1] shows a significant heterosis in survival and growth and is playing an important role in Chinese abalone culture.

A successful culture and high yield are assured by altering rearing conditions at three important stages during larval development: hatching, development of larval shell, and settlement (Hahn 1989). Information on BZP and effective accumulative temperature (EAT) is necessary for the culture of broodstock and hatchery management. BZP is one of the values used to calculate EAT. BZP varies among abalone species, depending on water temperature in each location. The BZP has so far been investigated in seven abalone (Sawateerap et al. 2001) and this study investigates the BZP of hybrid [F.sub.1] of Pacific abalone.

MATERIALS AND METHODS

Conditioning of Broodstocks

Mature broodstock of Haliotis discus hannai and Haliotis discus discus, with a shell length of 8 cm and a weight of 140-180 g, were used. Males and females were separately reared in different nets and placed in 0.7 [m.sup.3] plastic tanks. Filtered seawater was used and changed daily. Rearing temperature was maintained at 20[degrees]C with constant aeration. Broodstock were fed with fresh kelp. The dietary grazing rate was calculated daily and gonad maturation was determined by size and color according to Ebert and Houk. (1984).

Induced Spawning

Spawning was induced in April 2000. Individuals with mature gonads were chosen for spawning. Female abalone was dried in a shade room (24[degrees]C) for 1.5 h, males for 40-60 min. Broodstock were separated, one by one, into 20-L tanks with filtered water irradiated by ultra-violet at 700 mw.L/h. At the same time, abalone for spawning was induced with flow-through seawater at 1-2 L/min and from 24[degrees]C to 22[degrees]C. Spawning occurred after about an hour.

Artificial Fertilization

Eggs and sperms were collected separately. Eggs were placed in the experimental container at a density of 6 eggs/mL and then maintained at different temperatures (16[degrees]C, 20[degrees]C, 22[degrees]C). Each experiment was performed in triplicate.

Rearing Management

The zygote was washed once every hour with sand-filtered seawater. After hatching, healthy larvae were selected for continued rearing and water was changed every 8 h.

Observation

Larval development was observed microscopically every hour until the epipodial tentacle stage. Development rate of hatch-out, formation of larval retractor muscle, 90 degree torsion and formation of epipodial tentacle were recorded.

Data Analysis

The relationship of rate (t) of larval development and water temperature was 1/t = AT + B provides the basis for BZP calculation.

RESULTS

Experiments were repeated three times and data is showed in Table 1.

The BZP was calculated at water temperatures of 16[degrees]C, 20[degrees]C and 22[degrees]C. The average BZP was 6.22[degrees]C (Table 2).

DISCUSSION

BZP provides the day/hour when larvae are best ready for washing, selection, and settling and is necessary, therefore, for the effective handling of fertilized eggs and larvae in abalone seed production. It also provides a means of obtaining synchronously developed material for this species and presumably others, for developmental, physiologic, ecologic and biochemical investigations.

BZP varies among species, depending on specific geographical location (Table 3). Moreover, the same species in different geographical locations, also has different BZP, hence H. discus hannai of Japan is 7.6[degrees]C and H. discus hannai of China is 4.2[degrees]C (Zhao 1999).

In this experiment, abalone larvae were reared at different temperatures. From the equation of the relationship between water temperature and time for larval development, we know that the larval growth rate shows large discrepancies at different temperatures. Lower water temperature leads to slower development rate and higher mortality. Water temperature is an important factor in many stages of larval development (Seki & Kan-no 1977). In this study, reared larva were reared at a water temperatures of 13[degrees]C and 14[degrees]C, but the larval mortality was so high that the expected result was not reached. A water temperature of 22[degrees]C is optimum for larval development and, therefore, to reduce mortality and improve larval quality, reared larvae should be transferred as soon as possible to high temperature.