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Early larval feeding is most important in larval diet especially newly hatched larvae which had a limited yolk or no yolk reserve. It is important to provide a good nutritional and suitable diet for early larval stages as support larval survival and development of mud crab (Scylla spp.) larvae from zoea to crablet stage.
With different feeding regime, only larvae fed on rotifer at Z1 and Z2 survived to the crablet stage while other treatments failure develop to the crablet stage. Even though, high survival rate from Z1 to Z4 was obtained in combination feeding of rotifer and Artemia umbrella. Although, the survival rate of larvae was poor by only feed on rotifer at the Z1 to Z2 stage, but the zoea larvae were able metamorphose to the crablet stage. Several studies shows that rotifers are most preferred feed in early mud crab larval (Z1-Z2) because there are easy to consume due to small size (Chang & Wu, 1985) and digestible compared to Artemia nauplii (Watanabe et al., 1978; Dhert, 1996). Suprayudi et al. (2002) got a similar result on the mud crab Scylla serrata, Z1 to Z5 larvae had highest survival when fed with a combination of rotifer (40 ind/ml) and Artemia nauplii (5 ind/ml), but none of the larvae survived to the megalopa stage, while the larvae fed with rotifer at Z1 and Z2 and Artemia nauplii at Z3 stage onwards survived to the crablet stage, even though the survival was poor at beginning of zoeal stage (Z1-Z5). For scylla paramamosain, higher survival was obtained when rotifer given at Z1 and change to Artemia nauplii as the larvae molt to Z2 stage. Similar feeding regime was observed in other crustacean species such as Chinese mitten crab, Eriocheir sinensis larvae (Sui, 2008). This result is not comparable to other report for mud crab (Scylla sp.) larvae that was successfully reared either with Artemia alone or combination of rotifer and Artemia. Suprayudi (2002) and Baylon and Failman (2001) reported that in mud crab larvae Scylla traquberica and Scylla serrata, Z1 to megalopa had highest survived when fed with a combination of rotifer and Artemia. According to Hai (2001), umbrella stage Artemia umbrella could be used in replacement of rotifer feeding during Z1 and Z2, but the survival was higher when the larvae fed with combination of rotifer and Artemia umbrella compared to single diet on Artemia only, thus increase the encounter rate between prey and larvae as increased the prey density in larval culture. In Vietnam, most of hatcheries were preferred to use solely on Artemia throughout larval culture due to convenient form of live feed compared to rotifer, which were slightly difficult to culture and required more space and labor. In this aspect, the balanced nutritional content of rotifer and Artemia should be considering in larvae diet to complement each other, thus enhance larval morphogenesis to megalopa stage. Rotifer and Artemia would be good combination diet in larval rearing if an appropriate nutritional value and feeding regime could be provided on the zoea larval.
Although, umbrella stage Artemia can be consumed by zoeal larvae but did not provide sufficient nutritional to sustain survival rate and development of mud crab larvae from Z1 to the megalopa stage (Li et al, 1999; Kobayashi et al., 2000; Suprayudi, 2002). Generally, Umbrella stage Artemia are small size, non-moving and tend to float in the water column (due to the attached chorion) compared to Artemia nauplii, making them availability to be captured by early zoea larval (Z1-Z2). However, the disadvantages using Artemia are inadequate nutritional content and inconsistent nutritional profile, which varies highly between geographical sources (such as Great Salt Lakes, San Francisco, China or Vietnam) thus, resulted in inconsistent survival rate among mud crab larvae culture (Leger et al., 1987).
The results also demonstrated that the mud crab larvae could not metamorphoses to the megalopa stage by receiving Artemia umbrella at the Z1 and Z2 stage, significantly high LSI and high body length were obtained in this treatment. According to Hamasaki, Scylla serrata larvae fed with Artemia nauplii from early larval stage accelerated metamorphosis and had a greater chela length at the Z5 stage are tend to fail to molt successful to megalopa stage. This similar to what observer other crustacean species, Ohs et al. (1998) reported a significantly higher LSI in the M. rosenbergii larvae fed with Artemia only. Although, the development of mud crab larvae was rapid with Artemia feeding but mass mortality during metamorphose to the megalopa stage was observed. According to Suprayudi (2004), mass mortality during metamorphose to the megalopa stage could be due to imbalance of unsaturated fatty acid (n-3 HUFA) in diet of mud crab larvae such as eicosapentaenoic acid (22:6n-3-DHA) and decosahexanoic acid (20:5n3-EPA). In addition, most of end Z5 larvae died had undergone advanced morphological features similar to megalopa such as a larger cheliped (Suprayudi, 2004; Hamasaki, 2002 & Li et al., 1999). Similarly, phenomenon of molt death syndrome (MDS) has been observed in many crustacean species such as Brachyuran crab (Arai et al., 2004), swimming crab and shrimp.
Different feed regime from the Z1 to Z2 stage affected survival and development of mud crab larvae especially during metamorphosis to the megalopa stage. The present study generally concluded that rotifer is still the preferred diet for early larval feeding (Z1-Z2) as support development to crablet stage, followed with Artemia nauplii from the Z3 stage onward.
Optimum prey density of rotifer and Artemia in larval rearing from zoea to crablets stage
Appropriate prey density is one of the most important feeding methods for mud crab larval in promoting a good larval survival and growth (Wang et al., 1995). The Z1 and Z2 larvae were fed with rotifer and change to Artemia nauplii from the Z3 stage onward. Ultimately, the purpose of this study is to maximize larval survival rate from zoea to crablet stage by optimization of prey density of rotifer and Artemia nauplii.
Rotifer density for feeding early larval stages, Z1-Z1stages
Although, survival rate of larvae increased with increasing rotifer density from 20 ind/ml to 40 ind/ml but there was no significant difference in larval survival between rotifer density of 40, 60 and 80 ind/ml. The results show that high survival to crablet was recorded at rotifer density of 40, 60 and 80 ind/ml compared with feeding rotifer at density of 20 ind/ml. This is similarly reported by Yunus (1992), which provided rotifer at density of 60 ind/ml were enhanced larval survival rate (50 %) up to Z2 stage. According to Gui-zhong (2005), zoea survival was higher when fed with rotifer at density of 30 ind/ml and 40 ind/ml. Other studies also shows that higher rotifer densities (30-80 ind/ml) are required to optimize survival rate and growth of Scylla serrata (Suprayudi et al., 2002) and Scylla paramamosain larvae (Djunaidah et al., 1998; Zeng and Li, 1999 & Suprayudi et al. 2002). Generally, Z1 and Z2 larvae are relatively passive feeder and do not persuade the prey (Baylon et.al, 2002). Higher rotifer density in rearing tank give a chance of zoea to encounter rotifer more frequent (Zeng et. al, 1999) and stimulate zoea to flipping their tail more rapidly resulted in a higher feeding incidence at zoea mouth, thus improving larval survival and growth (Heasman and Fielder, 1983). Meanwhile, survival rate to crablet was poor at rotifer density of 20 ind/ml. The results indicate that rotifer should be supplied at an optimum density of 40 ind/ml during Z1 and Z2 larvae. During larval culture, water quality of rearing water is easily deteriorated by waste from rotifer and it contributes high ammonia level in rearing water and it is crucial for larval rearing to maintain a good water quality (Zeng et al, 1999). Increase of rotifer density in larval culture resulted in increasing of waste into rearing water, which is toxic for zoea larvae. In practice, regular exchange water (25-50%) should be applied to reduce ammonia level as well to maintain a good larval rearing condition. However, most of the hatcheries are preferred use rotifer at low densities (10-25 ind/ml) to feed mud crab larval in maintaining a good water quality management (Quinitio, 2001 & Hai et. al, 1999).
Maximum survival rate recorded from Z1 to megalopa stage was 28%ÂÂ±7 but suffered high mortality (3.3%ÂÂ±0.3) during megalopa stage due to cannibalism. Cannibalism still occur even provide with plastic net shelter. Probably the plastic net is still not sufficient to provide shelter for megalopa stage. Newly hatched megalopa are generally aggressive after molting and tend to attack Z5 stage, which were weaker and slower swimmer relative to megalopa. Other factor contribute in cannibalism is population density, which is stocking density should be reduced to 5 ind/l (Zainodin, 1999) or directly transferred to the pond with large area once the larvae molt to the megalopa stage (Quinito, 2001).
Artemia density for feeding later larval stages, Z3-Z5 stages
From study shows that the larvae fed with a high Artemia density of 2.5 ind/ml at the Z3 and increased to 5 ind/ml at the Z4 and Z5 stages, resulted in high body length of the Z5 stage but had a negative effect on survival rate and died during metamorphose to megalopa stage. Although, excessive Artemia density did not have any effect on larval survival and metamorphosis up to the Z4 stage, but greatly effect on the morphological of zoea feature at the end Z5, such as having a large cheliped and high body length of Z5 larvae. According to Suprayudi (2004), excessive level of Artemia resulted in imbalance nutritional value obtained by zoea larval and leads to the mass mortality of larval in rearing of Scylla serrata. Meanwhile, the larvae survived better when fed with a low Artemia density of 0.5 ind/ml at the Z3 stage and increased to 1 ind/ml at the Z4 and Z5 stage. This phenomenon had been also observed in Shrimp Peneaus marginatus larvae culture, where the larval survival was decreased when high Artemia density introduced during larval rearing (Gopalakrishnan, 1769). This is probably due to inappropriate prey density in larval rearing, which is caused an imbalance nutritional value on the zoea larvae. Furthermore, the excessive prey density was found to have an effect on morphology of the red frog crab Ranina ranina larvae (Minagawa & Murano, 1993). On the other hand, Hamasaki (2002) also reported that a high molting failure at the Z5 and megalopa stage was observed when high densities Nannochloropsis sp. was introduced into the larval rearing tank.
Suggested that Artemia should be given at density up to 1.5 ind/ml as the larvae develop to Z5 stage to obtain a good survival rate and to reduce molt death syndrome (MDS). Supplying Artemia at density of 0.5 ind/ml at Z3, increased to 1 ind/ml at Z4 and further increased to 2 ind/ml at Z5 would enhance larval survival during the Z4 and Z5 stage by increased the chance of zoea to capture Artemia nauplii (Zeng & Li, 1999; Brick 1974; Heasman & Fielder 1983 & Quinitio et al. 2001). On the basis of the present study, high mortalities at the Z5 stage occur when Artemia introduced at density of 2 ind/ml or higher.
Several studies had been done on the determine prey consumption of larval mud crab Scylla sp. in optimizing of prey density (Nghia, 2004; Baylon et al., 2004 & Juliana, 2007). According to Nghia (2004), suggested that the Artemia nauplii should be supplied to the Z3, Z4 and Z5 at density of 1.5, 2.5 and 3.7 ind/ml, respectively. In mass production of Scylla serrata recommended that the late Z2 stage should fed with Artemia at density of 0.5 to 3 ind/ml until Z5 stage and increased to 5 to 7 ind/ml at the late Z5 to the megalopa stage (Qunitio & Parado-Estapa, 2003). In other study reported that Artemia are given up to 10 ind/ml at daily rate for Scylla serrata (Brick, 1974) and Scylla paramamosain larvae (Nghia & ngut, 2011).
In commercial hatchery, high quality of crab seed (growth and survival rate of larvae) and low operation costs (utilities, feed and labor cost) should be consider in formulating feeding regime of mud crab larvae (Moretti, 1999). Currently, rotifer is preferred to use at early larval stage due to economical aspect, low prices than Artemia cyst. However, it required labor skill to culture. Meanwhile, Artemia are available for use and required less space but the market price of Artemia cyst often fluctuating and limit their use in hatcheries. In other hand, mud crab larvae reported successful to culture using specific Artemia strain such INVE, Vietnam and china. Generally, a good quality Artemia is definitely high priced and some of cyst type are not commercially available. Additionally, the nutritional value and production of Artemia cyst are often fluctuation with season. This could be the factor that impedes commercial production of mud crab seed.
In this aspect, suggested that rotifers should be supplied from the Z1 to Z2 stage at an optimal density of 40 ind/ml and change to Artemia nauplii from the Z3 stage onwards. During the zoea 1 to zoea 2 stage (Z1-Z2), rotifers were fed at a density of 40 to 80 ind/ ml. As the larval moulted into a new zoeal stage (Z3-Z5), the diet was changed from rotifers to Artemia nauplii at a density of 0.5 to 1.5 ind/ml. As the zoea 5 larvae metamorphosed into megalopa stage (M-C1), Artemia nauplii density was increased to 5