Abed Golam Rabbani*
Fisheries and Marine Resource Technology Discipline, Khulna University, Bangladesh
Scylla serrata, Zoea, Megalopa, Larval development, Rotifer, Live feed
Marine and Aquaculture Research Unit of James Cook University, Queensland, Australia
Knowledge Management
Source of larvae: The experiment was carried out at the Marine and Aquaculture Research Facility Unit (MARFU) of James Cook University, Townsville city, Queensland, Australia for 26 days. Spawners of S. serrata with carapace length of at least 140 mm were collected using baited traps from local estuaries. Species was identified following Keenan et al. (1998). Male and female crabs were identified observing abdominal flaps. The crabs were disinfected with 100 ppm formalin overnight and reared in two 5000 l circular tanks until females became ovigerous. Seawater was supplied at a rate of about 15 l min-1 through recirculating system of the research facility. Salinity and temperature in the tanks ranged at 28 - 36 ppt, and 26 – 29 °C respectively. The spawners were fed with squid, mussel, and shrimp or fish meat about 5 to 8% of their body weight once a day in the evening and the uneaten feed was removed in the morning. After extruding eggs into the abdomen, ovigerous females were disinfected in 50-80 ppm formalin for 6 hrs and then transferred to a 300 l indoor tank for egg incubation and hatching. Filtered seawater treated with UV (ultra violet) was supplied through re-circulating system approximately at a rate 1.5 l min-1. Salinity and temperature were maintained at 32 - 36 ppt, and 26 - 29 °C respectively. Egg incubation was required up to 14 days and the berried crabs were not fed during this period in order to reduce the presence of particulate and dissolved organic matter in the tank. Eggs were sampled regularly to check the embryonic development and to predict the hatching time. One or two days prior to hatching, the berried crabs were once again disinfected with 50 ppm formalin for 6 hrs and at the same time the incubation tank was cleaned and used as hatching tank. Hatching normally occurred in the early morning. In the larval rearing tanks seawater was pumped from the storage tank filtered through three cartridges (10, 5 and 1 µm) and UV filters. Salinity of water for the experiments was maintained at 32-35 ppt. Seawater was also treated with 10 ppm Streptomycin.
Feed preparation: Unicellular yellow green microalgae Nannocloropsis occulata was batch cultured in five 1000 l flat bottom oval tanks under natural sunlight. The pathogen free stock culture was collected from the Microalgae Culture Laboratory, James Cook University. Tanks were filled with filtered seawater (32-35 ppt salinity) up to about 80% of their capacity and treated with 10 ppm chlorine for 24 hrs. After 24 hrs of vigorous aeration, the tank was inoculated with 200 l of algae (20% of the tank volume) and fertilized with 20-30 g of soluble plant fertilizer (‘Aquasol’, Hortico Australia Pvt. Ltd). The algal culture turned dark green in 4-7 days and became suitable for rotifer food or used as starter for the next batch of culture. Pathogen-free SS-strain of rotifer Brachionus rotund forms, maximum 150 µm lorica length, was cultured outdoor in three 150 l flat-bottom round tanks using N. occulata as feed. The stock was collected from Marine and Aquaculture Research Facility Unit (MARFU) of James Cook University. Rotifer was maintained at a density of 400 ml-1 primarily by keeping the culture as clean as possible. Filter screens were provided in the culture tanks to accumulate dead algae and detritus. The rotifer culture tanks were cleaned daily by draining 50 to 100 % of the tanks’ volume through a 60 µm sieve and the rotifers were washed with seawater to flush out feces, bacteria and ciliates before they were returned to the tank. The rotifer tank was also cleaned before the rotifers were returned and fed with algae. Every 7 days, the cultures were filtered and transferred to a neighboring tank to allow sequential cleaning of the tanks. When required, rotifers were siphoned from the tank into a 50 l flow-through container fitted with a 63 µm mesh screen to keep them always submerged in water. This method helped from preventing damage to rotifers during harvest. Harvested rotifers were washed with fresh water and seawater the following filtration through a 300 µm mesh screen to remove any debris. Three replicates of 1 ml clean rotifer samples were counted under a dissecting microscope before being fed to the mud crab larvae.
Experimental design and data collection: A single factor experimental design was used in this study to investigate the effects of rotifer densities on larval survival and development. Thirty vigorously swimming and positively phototactic newly hatched zoea were reared up to the megalopa stage in 500 ml glass beakers using three replicates of six rotifer density (0, 5, 10, 20, 40, and 60 ml-1). All culture vessels (glass beakers) were disinfected through chlorination. All the replicates were placed in the water bath, where temperature was maintained between 26 to 30 °C using an immersion heater (Rena Cal 300W). No aeration was provided. A photoperiod of L:D=16:8 was used through out experiment using fluorescent lamps at an intensity of approximately 4000 lax. Larvae were fed according to the protocol provided by Zeng and Li (1999). Every morning, larvae were transferred using a large bore pipette to new culture vessels filled with new seawater and fresh food. At this time survival, mortality, and molts were recorded. Zoeal stages were identified based on characteristics provided by Ong (1964). Experiments were terminated when all larvae either died or metamorphosed to the megalopa stage. Larvae were considered dead when they became opaque, or when no movement of any appendage was observed.
Data analysis: The cumulative survival rate (%) of a particular larval stage was calculated as the number of larvae molted to the next stage divided by the total larval number of a replicate at the beginning of the experiment. Larval development was expressed as the mean intermolt duration (day) and as the mean cumulative development time (day) of a larval stage. Cumulative survival data were arcsin transformed Cumulative development time (day) was log transformed. One way analysis of variance was performed at a 5% level of significance to compare survival and development data among the treatments. When the difference was found significant, then Tukey’s honestly significant test (HSD) was conducted to find out which treatments were different. All statistical analyses were performed using SPSS for Windows, version 10.0 (Microsoft Inc.).
Khulna University Studies, 7(2): 65-70, 2006
Journal