Establishment of Broodstock Conditioning Culture of Razor Clam Solen regularis Dunker, 1862

Natural stock of razor clam Solen regularis in western part of Sarawak had showed sign of overexploitation. However, effort to culture this species is facing a problem due to its natural annual reproductive cycle and the presence of endoparasite (trematode and nematode) within the broodstock gonad. Therefore, broodstock conditioning technique is essential in the provision of S. regularis for its larval culture. Thus, this study aimed to design a best broodstock conditioning culture system that can give better survival. Furthermore, impacts of broodstock conditioning toward Gonadal Condition Index (GCI), oocyte diameter and parasite prevalence on S. regularis were also compared between spawning and resting periods. Clams with shell length of more than 4 cm (4.04 – 5.79 cm) were obtained from intertidal area of Asajaya Laut, Asajaya Division for laboratory experiment. Two species of microalgae (Isochrysis spp. and Chaetoceros spp.) with mix ratio 1:1 were given daily to the broodstocks at approximately 1% of S. regularis mean dry meat. Experiments were performed in two phases. During Phase 1, the survival rate and growth rate of S. regularis broodstocks were tested with different treatments in order to establish a control treatment in terms of stocking density, sediment compactness, duration of culture and sediment type. In Phase 2, the broodstock conditioning experiments tested on different sediment thickness, seawater volume and seawater temperature for 25 days in two gonadal development stages (resting period and spawning period). During this Phase 2, the survival rate, growth rate, qualitative observation, gonadal improvement of broodstock and parasite prevalence were evaluated. Overall, results showed that the mean survival rate of razor clams ranged from 19.71 ± 6.60% to 71.30 ± 28.91%. Among treatments, five treatments were significantly (t-test, p < 0.05) and treatment 10 L seawater volume and 30 °C seawater temperature consistently have low survival rate. However, weight loss of razor clams was observed in all treatments due to insufficient amount of feed supplied during the conditioning culture. In term of shell growth, both gain and loss in shell length were observed throughout the study period with no specific trend which may be due to the damage of shell periostracum. Based on the results of mean survival rate, the broodstock conditioning culture system for ten clams can be set up in a tank with closed system containing 20 L seawater with daily 50% water exchange, 5 cm thickness of natural sediment and reared at 24 °C (ambient temperature). This system is considered as low cost and easy to handle. There was no significant difference pattern in mean survival rate between spawning and resting period, thus this indicates that the treatment design gave more influence on the survival than the broodstock physiology condition. After 25 days of conditioning culture, all broodstocks in both gonadal development stages showed increase in the mean GCI values compared to both control treatments (wild before conditioning treatment and wild after conditioning treatment). This broodstock conditioning technique also had successfully removed the endoparasite within the razor clam gonad content. In conclusion, this study had shed some important information on broodstock conditioning technique of S. regularis in small laboratory scale and this may lead to the development of a successful razor clam spat production in future.