Crablet of Mangrove Crab, Scylla olivacea Rearing at the Different Salinity Regimes

Mangrove crab growout has been conducted in some area of Indonesia, for instance, in brackishwater pond on the Cenranae mouth river, Bone Regency, South Sulawesi [2], Segara Anakan, Cilacap, Central Java [3] and in Muncar, Banyuwangi, East Java. Soft shell crab production was also developed in some area of Indonesia, such as Barru and Takalar regency, South Sulawesi. Whereas in Pemalang and Brebes regency, Central Java, beside produced soft shell crab also produced mud crab gonadal maturation [4]. Most of them the juvenile and unberried female of mud crab provided from crabbing in the wild.


Introduction
Mangrove crab, Genus Scylla is one of the fisheries commodities with high economic value in Asia Pacific region, there were found four species under genus Scylla, namely S. serrata, S. transquebarica, S. olivacea dan S. paramamosain by Keenan [1].
Mangrove crab growout has been conducted in some area of Indonesia, for instance, in brackishwater pond on the Cenranae mouth river, Bone Regency, South Sulawesi [2], Segara Anakan, Cilacap, Central Java [3] and in Muncar, Banyuwangi, East Java. Soft shell crab production was also developed in some area of Indonesia, such as Barru and Takalar regency, South Sulawesi. Whereas in Pemalang and Brebes regency, Central Java, beside produced soft shell crab also produced mud crab gonadal maturation [4]. Most of them the juvenile and unberried female of mud crab provided from crabbing in the wild.
Since 20 years ago an effort of research was conducted to find out mangrove crab seed production technique. However, complete technique for crablet production is still uncomfortable, cause of low percentage (<5%) of crablet production still and fluctuative. The newest research focused to give attention to the kind of container for megalops rearing. Gunarto [5,6] tested different kind of tanks for megalops rearing namely, (A) circular fiberglass tank volume 4 tons, (B) circular cement tank volume 4 tons, and (C) rectangular cement tank volume 4 ton, they found that the highest crablet-D7 production in A treatment (40.14 ± 0.424%), followed by C treatment (34.65 ± 11.101%) and the lowest in C treatment (22.675 ± 0.954%).
Some constrains encountered in larvae rearing was larvae mortality caused by a bacterial infection such as Vibrio harveyi [7], unsyncronizing of zoea-5 metamorphosis to the megalops, and low vitality of larvae cause mass mortality of megalops [7]. High cannibalisms in megalops also find out in crablet, juvenile and adult stages. Molt crab most often cannibalism by a unmolt crab. Research on seaweed, Gracilaria sp as shelter in crablet rearing was observed that a half of bottom width spreaded with Gracillaria sp as a shelter was found the survival rate at 49.9% during the 30 days rearing, while without shelter, the survival rate at 23% [8][9][10]. From these data can be concluded that even though the shelter was used in crablet rearing, there were still resulted low survival rate of crablet. This research aimed to know the effectiveness of individually crablet rearing in plastic glass floating cages at the different salinity regimes.

Material and Method
Research was conducted in mud crab hatchery of Pond Station Marana, RICA Maros. Crablet of Scylla olivacea provided from hatchery at the range weight 0.04-0.09 g and carapace width 5-10 mm, individually stocked in plastic glass, then they are settled in a floating cage constructed with bamboo fenced, where a piece of styrofoam as the float in surface water in the aquarium. Each floating cage in each aquarium stocked with five crablet in five plastic glasses ( Figure 1 Mangrove crab, S. olivacea morpho and ecologically different with S. serrate as well as S. paramamosain. S. olivacea in the wild obtained in mouth river area and mangrove estuary with dominated by Nypa sp, as was seen in mouth river on Cenranae river, Bone Regency, South Sulawesi. Lewis [11], claimed that S. olivacea are suitable species for stocking enhancement in mangrove area, because of their habitat in hole of mangrove area, while S. paramamosain and S. serrate their habitat more width reach to the sea area.
The mouth river usually with salinity lower than 30 ppt, by that reason S. olivacea commonly found in the lower salinity area. This phenomenon was proven by fast growth rate of crablet S olivacea at the salinity 5 ppt and showed significantly different (P<0.05) with crablet S. olivacea growth rate at the salinity 30 ppt. By this research can be summarized that the good salinity for crablet S olivacea growth (0.07-0.08 g/pieces was at the range 5-20 ppt. Mangrove crab, S. olivacea presumably like tiger shrimp, Penaeus monodon in the salinity adaptation process, where both of them were euryhaline species. The growth rate of tiger shrimp at the salinity 10-20 ppt was higher compared than that of the growth rate at the salinity 30 ppt [12]. The highest crablet survival rate was obtained in the C treatment (100%), then followed by B and D treatment and the lowest was A treatment (73.3%) ( Figure 2). The lowest of the crablet survival rate should not be affected to the highest of crablet growth rate such as in A treatment. It caused not any competition on the feed and space, because crablet reared individually in plastic glass, except if any crablet enter to the neighbour plastic glass and was occuring cannibalisms among crablet and resulted one or both of the crablet were dead. Junaedah [10] was found that crablet survival rate significantly affected by a kind of substrate, where san substrate resulted significantly higher survival rate of crablet (P<0.05) compared than that crablet reared with muddy substrate. Rearing crablet using seaweed, Gracillaria sp as a shelter, Gunarto [8] found that a half of surface bottom with Gracilaria sp as a shelter resulted survival rate 49.99 ± 23.56%, was higher compare than that 100% of surface bottom spread with Gracilaria sp the crablet survival rate 33.33 ± 0%, while without shelter, the crablet survival rate was 24.99 ± 11.78%. Based on this data can be concluded that individually crablet rearing in plastic glass resulted higher survival rate compared than that of crablet rearing using Gracillaria sp as a shelter ( Figure 3).
In A treatment, crablet mortality was caused by molt failure. Greenaway [13], stated that in crab molting was occured calcium ion transportation from and to carapace exdyces, where chitine biosynthesis and degradation was occured. Beside calcium, some hormonal also required in molting process, for instances chitinase, chitobiase, carbonic anhidrase, alcalin phosphatase, Ca ± ATPase, proteinase, trace element dan glicogen [14,15]. In A treatment where salinity only 5 ppt may calcium ion and hormonal required in molting process insufficient to support complete molting process, so that some crablets failure molt and furthermore die. The different condition, when the crablet reared at the salinity 20 ppt, may calcium and hormonal every morning at 8.00-9.00, and the rest of feed was picked out in every morning before fresh feed was given to the crablet. Water exchange at 5% of total volume of water in each aquarium was conducted at every two days. Some biological parameters were monitored be weekly mainly in body weight increment, length and width carapace. Crablet survival rate in each treatment was monitored after one month reared. Water quality monitored in situ, namely water temperature, salinity, pH. 300 mL water sample from each aquarium brought to the laboratory for analyzing ammonium, nitrite and total organic matter (TOM). Complete Random Design was applied to the growth data obtained followed by a Duncan test to know significantly different among the treatments tested.

Result and Discussion
The crablet S. olivacea growth was shown in Figure 1. In the first two weeks until one month rearing showed that crablet in A treatment, salinity 5 ppt tend to grow faster compared than that of the crablet grow in the other treatments. The mean weight of the crablet after one month reared individually inside the plastic glass was 0.80 g/pieces (A), 0.64 g/pieces (B), 0.71 g/pieces (C) and 0.54 g/pieces (D) treatment.
The daily growth rate was 0.024 g/day (A), 0.018 g/day (B), 0.021 g/ day (C) and 0.015 g/day (D). Statistical analysis showed that the daily growth rate of the crablet in A treatment showed significantly different (P<0.05) with D treatment. However, there was not significantly different (P>0.05) with B and C treatments. B and C treatment were also were not significantly different with D treatment (P>0.05) ( Table  1). Earlier studies, Gunarto was observed that crablet weight at 0.31-0.52 g reared at the salinity 25 ppt during one month was resulted grow increment in the range 1.69-1.98 g (Figure 2). Mia and Syah [9] in Bangladesh was observed that S. serrata juvenile with initial weight 1.10-1.95 g and the highest growth increment obtained at the crablet reared at the salinity 25 ppt, with specific growth rate at 4.46%/day and showed significantly different (P<0.05) with the crablet growth increment at the salinity 10 and 5 ppt. However, there were not significantly different (P>0.05) with salinity 20 and 15 ppt. Junaedah [10] declared that crab seed S. paramamosain reared in the soil substrat resulted significant gowth rate (P<0.01) compared than that of crab seed reared in the san substrat. Different letter at the same column are shown significantly different at a level 5% (p<0,05).

Water quality
Some water quality parameter monitored were presented in Table  2. Water temperature as the environmental factor controlling to the metabolism acceleration process, oxygen consumption, growth and survival of organisms cultured [16]. Water temperature in the morning at 8.00-9. to the each aquarium. The relationship between oxygen consumption and salinity regimes for crab rearing was studied by Karim and Syahrul [17] and they found that the lowest oxygen consumption was 0.23 mg/g body weight/hour at the rearing salinity 25 ppt and the high oxygen consumption at 0.37 mg/g body weight/hour at salinity 5 ppt. Dissolved oxygen in all treatments were >4 mg/L, and that condition was not a limiting factor to the crablet growth.
Alkalinity range was 105-160 ppm (A), 105-160 ppm (B), 139-205 ppm (C) and 92-214 ppm (D). The suitable alkalinity for crablet rearing can be interpreted on the relationship between alkalinity and the crablet survival rate in each treatment. It was performed in C treatment at the range 139-205 ppm, where crablet survival rate attains 100% and not any crablet failure molt during one month rearing.

Conclusion
The salinity 5 ppt was performed highest growth weight, while salinity 20 ppt was performed highest survival rate. The salinity ranged 5-20 ppt could be better for crablet S. olivacea rearing to increase growth weight and survival rate.  Table 2: Some water quality parameters in crablet rearing at the different salinity regimes.