Juvenile Mud Crabs Research Articles

Dietary chromium could improve growth, antioxidant capacity, chromium accumulation in tissues and expression of genes involved into glucose and lipid metabolism in juvenile mud crab Scylla paramamosain

Chromium (Cr) is an essential micronutrient for all animals, which is also an important component of glucose tolerance factor. The present study was conducted to evaluate the effect of dietary Cr levels on growth, Cr accumulation in tissues, glucose and lipid metabolism and insulin signal pathway of juvenile mud crab. Six isonitrogenous and isolipidic diets were formulated to contain 1.4 (basal diet), 1.7, 2.0, 2.2, 2.6 and 3.7 mg kg−1 Cr, respectively. The results indicated that crabs fed with 2.2 and 2.6 mg kg−1 Cr diets had the highest percent weight gain (PWG) and specific growth rate (SGR) among all treatments. Cr concentration in hepatopancreas significantly increased with increase of dietary Cr levels. Crabs fed with 3.7 mg kg−1 Cr diet exhibited lower antioxidant enzyme activities and higher malondialdehyde (MDA) in hepatopancreas and hemolymph than those fed the other diets. Crabs fed the diets with 2.2 and 2.6 mg kg−1 Cr showed higher activities of hexokinase (HK), phosphofructokinase (PFK) and pyruvate kinase (PK). The expression levels of genes related to insulin signaling pathway, glycolysis and fat synthesis, glycolipid metabolism (such as phosphatidylinositol 3-kinase (pi3k), hexokinase (hk), and fatty acid synthase (fas)) in hepatopancreas were significantly up-regulated. The expression levels of genes related to gluconeogenesis and lipolysis such as fructose-1,6-bisphosphatase (fbp) and carnitine palmitoyltransferase 1 (cpt1) were significantly down-regulated in crabs fed with optimal Cr diets. In conclusion, the optimal dietary Cr requirement was estimated to be 2.34 mg kg−1 for juvenile mud crab based on two slope broken-line regression analysis of PWG against dietary Cr levels, the results of present study indicated that optimal dietary Cr promoted the growth and Cr accumulation in tissues, and modulated insulin signaling pathway to trigger glycolysis and glycogenesis to maintain glucose homeostasis.

Limb loss and feeding ability in the juvenile mud crab Scylla olivacea: Implications of limb autotomy for aquaculture practice

In Scylla mud crab farming, limb autotomy is commonly used in soft-shell crab production to expedite the moulting process and temporarily achieve high stocking density during crablet transportation. However, little is known about the effect of limb loss on the feeding ability of mud crabs. Therefore, we investigated the effect of various degrees of limb autotomy on the feeding ability of S. olivacea on either live blood cockles or cockle meat, and subsequently the effect of different live cockle size and days after limb removal. The removal of all appendages except for swimming legs compromised S. olivacea’s ability to feed on live cockles but not on cockle meat. When fed with live cockles, the total eaten cockle mass was similar among treatments, but the loss of limbs resulted in lower number of crabs engaged in feeding activity and longer feeding duration. When fed with cockle meat, crabs of all treatments engaged in feeding activity, and only full autotomised crabs showed prolonged feeding duration. Limb loss, but not cockle size nor day after limb removal, affected cockle consumption. Only crabs with one cheliped removed improved their feeding duration after Day 7, whereas crabs that lost both chelipeds did not show change in feeding duration. These results suggest that limb removal affect the feeding ability in mud crabs, and this serves as essential knowledge for future optimisation of the feeding practices of mud crab juveniles right after transportation or during soft-shell crab culture.

Production of juvenile mud crabs, Scylla serrata: Captive breeding, larviculture and nursery production

The mud crab, Scylla serrata, is one of the most traded and valuable seafood in several Indo-Pacific countries. Most challenges in mud crab hatchery production are rooted not only in the mass production of the megalopa or crab instar 1, but also in the production of juveniles of stockable size in the grow-out production system. In the present study, large-scale production of S. serrata in hatchery was carried out to obtain fundamental data for developing hatchery production of this species. Additionally, the spawning characteristics and reproductive performance of wild broodstocks under captivity were studied to facilitate the management of brood stocks in the hatchery. Adult wild broodstock (n = 202), obtained from the inshore fishery from 2015 to 2019 were eyestalk ablated to promote reproductive maturation. Almost 50% of eyestalk ablated animals successfully spawned within a latency period of ~36 days. The crabs were found to be highly fecund in terms of zoeal production (3.84 × 106 ± 0.65 × 106 zoea 1) and captive spawning was achieved throughout the year, with a peak in November. Juvenile production was carried out in three phases: (1) Zoea 1 to megalopa (3 days old); (2) Pre-nursery phase, megalopa (3 days old), to ~3 g; and (3) nursery phase, ~3 g, to 2 8 g BW. In the first phase, a series of experiments were carried out on pilot scale and at the commercial level to evaluate optimum rearing conditions. In the pilot-scale studies, the highest survival up to the megalopa stage was obtained at lower stocking densities (50–75 larvae/L), and survival declined progressively at higher rearing densities. A series of trials was carried out at commercial scale (5000 L fiberglass tank), and survival from Zoea 1 to megalopa ranged from 0.6% to 7.9%, with a mean of 3.3%. This study also demonstrated that rotifer grown on commercial algal concentrate could be used in larviculture without compromising the survival rate. The second phase of rearing experiment (pre-nursery and nursery) was extended for sixty-eight days. During the pre-nursery phase, the effect of different rearing systems was evaluated: hapa net constructed either in an earthen pond or in open water bodies (lagoon) and direct stocking of megalopa in an earthen pond. During the pre-nursery phase, the highest survival was obtained in the hapa constructed in the earthen pond (60.5 ± 19.9%), whereas a significantly higher growth rate was obtained when megalopa was reared directly in the pond (5.9 ± 2.1 g). During the nursery phase, crabs were reared at 5 crabs/m2 and 10 crabs/m2; crabs reared at the lower stocking density attained significantly higher growth (28.2 ± 2.02 g), without significant differences in crab survival between these rearing densities. These results advance the method for the large-scale production of stockable juveniles of the mud crab, S. serrata

Tank bottom area influences the growth, molting, stress response, and antioxidant capacity of juvenile mud crab Scylla paramamosain

An 8-week trial was performed to determine the effects of different tank bottom areas (TBA) on growth performance, stress response, and antioxidant capacity of juvenile mud crab Scylla paramamosain. Four treatments in triplicates with different k-value (k = A/CW2, where A is the bottom area (mm2) of the tank, and CW is the carapace width (mm) of the juvenile crab), i.e., k-value = 4.42 (K4), 17.67 (K18), 39.76 (K40), and 70.68 (K70), were used in this experiment. The results showed no significant difference in survival rate (83.33–96.67%) was found, but the K40 group had the best growth performance among all the treatments (P < 0.05). With the decrease of TBA, the level of ecdysone and ecdysone receptor (EcR) gene expression decreased, while the molt inhibitory hormone (MIH) gene expression increased. As a result, prolonged molting intervals and reduced weight gain rates were observed in K4 and K18. Higher cortisol, lactic acid, and lower glycogen content were also observed in K4 (P < 0.05), accompanied by higher phosphofructokinase (PFK) and pyruvate kinase (PK) enzyme activities (P < 0.05). On the contrary, lower catalase (CAT) and higher malondialdehyde (MDA) were observed in the K4 and K18 groups. The results demonstrated that TBA significantly impacted the growth performance of S. paramamosain, and the effects could be mediated through hormonal, glycometabolism, and antioxidant pathways. Notably, the optimal TBA was estimated to be 29.23–59.19 (k-value) according to the quadratic polynomial regression analysis of WGR, and the oversized tank could not give extra benefits in terms of growth performance. Therefore, TBA should be deliberated when designing crab culture facilities.

Lipidomic profiling reveals molecular modification of lipids in hepatopancreas of juvenile mud crab (Scylla paramamosain) fed with different dietary DHA/EPA ratios

Untargeted lipidomic analysis was conducted to explore how different dietary docosahexaenoic acid (DHA)/eicosapentaenoic acid (EPA) ratio and, specifically, how an optimal ratio (2.3) compared to a suboptimum ratio (0.6) impacted lipid molecular species and the positional distribution of fatty acids in hepatopancreas of mud crab. The results indicated that major category of lipid affected by dietary DHA/EPA ratio was glycerophospholipids (GPs). The optimum dietary DHA/EPA ratio increased the contents of DHA bound to the sn-2 and sn-3 positions of phosphatidylcholine (PC) and triacylglycerol, EPA bound to the sn-2 position of phosphatidylcholine and 18:2n-6 bound to the sn-2 position of phosphatidylethanolamine (PE). Increased dietary DHA/EPA ratio also led to competition between arachidonic acid (ARA) and 18:2n-6 bound to esterified sites. Appropriate dietary DHA/EPA ratio can not only improve the growth performance and nutritional quality of mud crab, but also provide higher quality products for human consumers.

Comparative analysis of growth performance, feed utilization, and antioxidant capacity of juvenile mud crab, Scylla paramamosain, reared at two different conditions

Abstract This study was carried out to analyse the growth performance, feed utilization, and antioxidant capacity of juvenile mud crab (Scylla paramamosain) reared in two conditions. Here, a new indoor polypropylene tank culture system and a traditional outdoor pond cage facility were used to perform feeding experiments on juvenile crabs. After eight weeks, the survival rate, weight gain, moulting rate and feed efficiency of crabs cultured in polypropylene tanks were markedly higher than those cultured in pond cages. Moreover, the activities of serum acid phosphatase (ACP) and alkaline phosphatase (ALP), and the antioxidant indices including glutathione peroxidase (GSH-PX), glutathione transferase (GST), superoxide dismutase (SOD) and total antioxidant capacity (T-AOC) in hepatopancreas were higher in crabs cultured in the polypropylene tank system than those in pond cages. Based on these results, compared with the traditional pond cages, the polypropylene tank culture system is more suitable for the short-term culture of mud crabs.

Comparative growth and morphometric assessment between cultures of wild and hatchery-produced mud crabs

This paper reports the comparative growth, nutritional performance, and morphometric variation between wild and hatchery-reared juvenile mud crabs (Scylla olivacea) in earthen ponds. The crabs were fed daily with boiled tilapia paste at a feeding rate of 5–8% body weight for the first two weeks, followed by feeding with chopped eviscerated tilapia until termination of the experiment. Selected phenotypic trains, including carapace width (CW), carapace length (CL) and abdominal width (AW), were measured weekly. The protein content of the muscle (21.13%), gill (13.51%) and egg (43.28%) were significantly higher in the hatchery-sourced compared to wild female crabs (muscle = 19.15%; gill = 10.09%; egg = 38.15%). Likewise, the hatchery sourced crabs exhibited higher lipid content in the muscle (2.45–2.51%) and eggs (7.51%) compared to the wild counterparts (muscle = 1.45–1.47%; egg = 6.15%). These findings suggested a superior nutritional quality of the hatchery-reared compared to the wild-sourced crabs. Although some selected phenotypic traits did not vary among the wild and hatchery-reared crabs (p < 0.05), their survival rates varied significantly depending on the stocking density (p < 0.05). Overall, the findings suggest that the growth characteristics of the hatchery produced and wild-sourced crab were similar, which will help to remove the misconception among the crab farmers about the hatchery seeds and promote diversification of the crab production system for long-term sustainability.

Light intensity impacts on growth, molting and oxidative stress of juvenile mud crab Scylla paramamosain

An 8 weeks regression study was performed to test the effects of increasing light intensities from darkness to 30 W m−2 on growth performance, molting, antioxidant capacity, and gene expression of molting and apoptosis-related genes in Scylla paramamosain. No significant differences were found in survival rates between treatments (ranging from 71.7 to 87.3% at the end of the experiment). However, weight gain and specific growth rate over the study period displayed a curvilinear response to light treatments with peak values in crabs exposed to 10 W m−2. Linear (BLM), quadratic (BQM), and 4-parameter saturation kinetic (4-SKM) models show the optimal light intensities for SGR were 12.98, 18.27, and 11.36 W m−2, respectively. The light intensity did not appear to impact molting. However, crabs reared in darkness showed significantly reduced molt frequency (3.51 ± 0.16) and extended intermolt intervals compared to other treatments. Melatonin levels in the eyestalks were significantly higher in crabs exposed to darkness (502.52 ± 56.24 pg mL−1) than light intensities of 10–30 W m−2 (413.50 ± 32.38 and 384.99 ± 15.56 pg mL−1). Cortisol levels were significantly lower in the 0 and 5 W m−2 groups. Light intensity significantly impacted the activity of antioxidant enzymes, with crabs showing a significant increase in total antioxidant capacity (T-AOC) under 10 W m−2, catalase (CAT), and superoxide dismutase (SOD) under 15 W m−2 and lower malondialdehyde (MDA). Gene expression of the molt-inhibiting hormone (MIH) was downregulated in eyestalks from crabs exposed to 10 W m−2 compared to darkness and 20–30 W m−2. Expression of apoptosis-related genes did not show clear light intensity trends. Taken together, these results suggest light intensity can impact S. paramamosain growth, molting, stress levels, and antioxidant capacity. As such, light regimes used in crab farming should be carefully considered to optimize productivity and welfare.

Dietary soybean oil aggravates the adverse effects of low salinity on intestinal health in juvenile mud crab Scylla paramamosain

Salinity is one of the important factors affecting the physiological state of crustaceans in marine environments. Lipid plays major roles in energy supply and is main sources of essential fatty acids for membrane integrity, which is critical in adaptations to changes in salinity. Here we evaluated the effects of salinity (medium, 23 ppt and low, 4 ppt) and dietary lipid source (fish oil, FO and soybean oil, SO) on intestinal health of the marine crustacean mud crab Scylla paramamosain. The results indicated that low salinity and dietary SO (LSO group) significantly affected intestinal histomorphology, with a significant decrease of intestinal fold height and width as well as down-regulation of intestinal mRNA levels of tight junction genes compared to crab reared at medium salinity and fed FO diets (MFO group). Crabs reared at low salinity and fed SO showed an increased inflammatory response in intestine, which stimulated a physiological detoxification response together with apoptosis compared to crab in the MFO group. Low salinity and SO diets also could be responsible for multiply the pathogenic bacteria of Photobacterium and inhibit the beneficial bacteria of Firmicutes and Rhodobacteraceae in intestine, and act on a crucial impact on the development of intestinal microbial barrier disorders. The results of microbial function predictive analysis also support these inferences. The findings of the present study demonstrated that soybean oil as the main dietary lipid source could exacerbate the adverse effects of low salinity on intestinal health of mud crab, and provided evidence suggesting that dietary lipid source and fatty acid composition may play vital roles in intestinal health and the process of adaptation to environmental salinity in marine crustaceans.

Dietary DHA/EPA ratio affects growth, tissue fatty acid profiles and expression of genes involved in lipid metabolism in mud crab Scylla paramamosain supplied with appropriate n-3 LC-PUFA at two lipid levels

An 8-week feeding trial was conducted to determine the optimal dietary docosahexaenoic acid/eisosapentaenoic acid (DHA/EPA) ratio of mud crab (Scylla paramamosain) supplied with optimal n-3 LC-PUFA at two dietary lipid levels. Eight isonitrogenous diets were formulated to contain 7% and 12% crude lipid, each with DHA/EPA ratios of 0.6, 1.2, 2.3 and 3.2, respectively. Each diet was randomly assigned to triplicate groups of 30 juvenile mud crabs (initial weight 20.9 ± 0.6 g) that were stocked in single crab cells. In crabs fed 7% lipid, the diet with a DHA/EPA ratio of 2.3 showed significantly higher weight gain than crabs fed the other ratios while in crabs fed 12% lipid, lower weight gain and specific growth rate were observed in crabs fed the diet with a DHA/EPA ratio of 0.6 than crabs fed the other ratios. Lipid content in hepatopancreas significantly increased as dietary DHA/EPA ratio increased from 1.2 to 2.3 in crabs fed 7% lipid, while no differences were observed among crabs fed the diets with DHA/EPA ratios higher than 0.6 when fed 12% lipid. Total fatty acid and DHA contents and DHA/EPA ratio showed increasing, and EPA decreasing, trends in muscle and hepatopancreas with increased dietary DHA/EPA ratio, at both dietary lipid levels. The hemolymph triacylglycerol and total cholesterol contents were higher in crabs fed dietary DHA/PA ratios of 1.2 and 2.3 than those fed ratios of 0.6 and 3.2 at 7% dietary lipid, and lowest low and high-density lipoprotein cholesterol contents were observed in crabs fed DHA/EPA dietary ratios of 0.6 and 3.2 at 7% and 12% lipid, respectively. The expression levels of fas, aco3 and fatp4 were significantly up-regulated, and cptI, hsl and ldlr were down-regulated, with increased dietary DHA/EPA ratio in crabs fed 7% lipid. In crabs fed 12% lipid, the expression levels of g6pd, 6pgd, srebp-1, aco1 and fatp4 were down-regulated, and fabp-1 was up-regulated, with increased dietary DHA/EPA ratio. The expression levels of elovl4 and Δ6 fad initially increased and then decreased as dietary DHA/EPA ratio increased from 0.6 to 3.2 in crabs fed both 7% and 12% lipid. Based on analysis of weight gain versus dietary DHA/EPA ratio, the optimal dietary DHA/EPA ratios of mud crab S. paramamosa were estimated to be 2.2 and 1.2 when supplied with optimal n-3 LC-PUFA at 7% and 12% lipid, respectively.

Dietary lipid and n-3 long-chain PUFA levels impact growth performance and lipid metabolism of juvenile mud crab, Scylla paramamosain.

An 8-week feeding trial was conducted to evaluate the effects of dietary n-3 LC-PUFA levels on growth performance, tissue fatty acid profiles and relative expression of genes involved in the lipid metabolism of mud crab (Scylla paramamosain). Ten isonitrogenous diets were formulated to contain five n-3 LC-PUFA levels at 7 and 12 % dietary lipid levels. The highest weight gain and specific growth rate were observed in crabs fed the diets with 19·8 and 13·2 mg/g n-3 LC-PUFA at 7 and 12 % lipid, respectively. Moisture and lipid contents in hepatopancreas and muscle were significantly influenced by dietary n-3 LC-PUFA at the two lipid levels. The DHA, EPA, n-3 LC-PUFA contents and n-3:n-6 PUFA ratio in hepatopancreas and muscle significantly increased as dietary n-3 LC-PUFA levels increased at both lipid levels. The expression levels of -6 fatty acyl desaturase and acyl-CoA oxidase in hepatopancreas increased significantly, and expression levels of fatty acid synthase, carnitine palmitoyltransferase I and hormone-sensitive TAG lipase were down-regulated, with increased dietary n-3 LC-PUFA regardless of lipid level. Based on weight gain, n-3 LC-PUFA requirements of S. paramamosain were estimated to be 20·1 and 12·7 mg/g of diet at 7 and 12 % dietary lipid, respectively. Overall, dietary lipid level influenced lipid metabolism, and purified, high-lipid diets rich in palmitic acid reduced the n-3 LC-PUFA requirement of juvenile mud crab.

Influence of dietary zinc on growth, zinc bioaccumulation and expression of genes involved in antioxidant and innate immune in juvenile mud crabs (Scylla paramamosain).

The aim of the present study was to investigate the effects of dietary Zn level on growth performance, Zn bioaccumulation, antioxidant capacity and innate immunity in juvenile mud crabs (Scylla paramamosain). Six semi-purified diets were formulated to contain dietary Zn levels of 44·5, 56·9, 68·5, 97·3, 155·6 or 254·7 mg/kg. Dietary Zn level significantly influenced percentage weight gain (PWG), with the highest observed in crabs fed the diet containing 97·3 mg/kg Zn. Tissue Zn concentrations significantly increased as dietary Zn levels increased from 44·5 to 254·7 mg/kg. Retention of Zn in hepatopancreas increased with dietary Zn levels up to 68·5 mg/kg and then significantly decreased. Moreover, inadequate dietary Zn (44·5 and 56·9 mg/kg) reduced antioxidation markers including total superoxide dismutase (SOD) and Cu/Zn SOD activities and total antioxidant level. Crabs fed the diet with 44·5 mg/kg Zn also showed significantly lower expression of genes involved in antioxidant status, such as Cu/Zn SOD, glutathione peroxidase, catalase and thioredoxin than those fed diets containing 68·5 and 97·3 mg/kg Zn. The highest activities of phenoloxidase and alkaline phosphatase were recorded in crabs fed the diets containing 68·5 and 97·3 mg/kg Zn. Expression levels of prophenoloxidase and toll-like receptor 2 were higher in crabs fed the 97·3 mg/kg Zn diet compared with crabs fed the other diets. Based on PWG alone, the optimal dietary Zn level was estimated to be 82·9 mg/kg, with 68·5 to 97·3 mg/kg recommended for maintaining optimal Zn bioaccumulation, oxidation resistance and innate immune response of juvenile mud crabs.

Toxicological mechanism of excessive copper supplementation: Effects on coloration, copper bioaccumulation and oxidation resistance in mud crab Scylla paramamosain

Copper is a widespread pollutant in marine environments, and marine animals can ingest large amounts of copper through the food chain. Here, an 8-week feeding trial was designed to investigate the effects of different dietary copper levels on coloration, copper bioaccumulation, stress response and oxidation resistance of juvenile mud crab Scylla paramamosain. The results indicated that crabs fed the diet with 162 mg/kg copper exhibited a dark-blue carapace and hemolymph. The accumulation of copper in tissues was positively correlated with the level of copper in feed. High/excess dietary copper (162 mg/kg) up-regulated the expression of stress response related genes, and reduced the expression/activities of anti-oxidation genes/enzymes. The activity of phenoloxidase decreased significantly when dietary copper level was 86−162 mg/kg, and the expression of hemocyanin was up-regulated in crab fed the diets with 28−162 mg/kg copper. Overall, the results of the present study indicated that high dietary copper led to parachrea in carapace and hemolymph of mud crab, and caused copper deposition abnormality in carapace and hepatopancreas. The data suggested that the toxic effects of dietary copper were concentration-dependent such that, excess dietary copper (162 mg/kg) had adverse impacts on oxidation resistance.

Role of crustacean female sex hormone (CFSH) in sex differentiation in early juvenile mud crabs, Scylla paramamosain

Recent studies have shown that crustacean female sex hormone (CFSH) is involved in the development of reproductive phenotype. In the present study, observation of sexually dimorphic traits revealed that gender could be distinguished from the third stage juveniles onwards in the mud crab, Scylla paramamosain. Sp-cfsh expression levels were analyzed in early juveniles. The results showed that, Sp-cfsh expression levels differed among individuals at post-molt of the first stage and second stage, and significantly different between the two sexes at post-molt of the third stage, which suggested that Sp-cfsh might participate in the sex differentiation in early juveniles. The expression of Sp-cfsh was examined during the molting cycle at the third stage juveniles, and the results showed that it was highest at the pre-molt stage. Based on the results, the expression of Sp-cfsh at pre-molt stage was further analyzed between the sexes from the third stage to the fifth stage, and it was found that the expression of Sp-cfsh was similar between two sexes at the third stage and the fourth stage; whereas at the fifth stage, when the gonopores occurred, the expression of Sp-cfsh significantly increased in females but decreased in males; suggesting that the expression of Sp-cfsh could influence the formation of gonopores. Finally, the role of Sp-cfsh in the reproductive phenotypes was confirmed through RNA interference knockdown. The combined results suggest that CFSH is involved in the regulation of sex differentiation of early juvenile in S. paramamosain.

Dietary protein requirement of juvenile mud crab Scylla paramamosain

This experiment was carried out to investigate the optimal dietary protein levels for juvenile Scylla paramamosain. Six isoenergtic and isolipidic semi-purified diets were formulated with graded protein levels, and the final protein concentrations were 32.69%, 38.54%, 42.92%, 49.89%, 52.09% and 60.07%, respectively. Each diet was offered to triplicate groups of 28 mud crabs (initial weight 0.04 ± 0.001 g, carapace length 3.3 ± 0.05 mm) for 8 weeks. The results showed that the mud crabs fed diets with 32.69% and 60.07% protein had significantly lower survival than those in other treatments (p < .05). The juvenile mud crab consuming 42.92%, 49.89% and 52.09% dietary protein obtained significantly higher weight gain (WG) and specific growth rate (SGR) than other groups (p < .05). Crude protein content in whole body significantly increased with increasing dietary protein level (p < .05). Crabs fed diet with 49.89% had the lowest hepatic aspartate aminotransferase (AST) activity (p < .05). A relatively higher of hepatic alkaline phosphatase (AKP) activity was observed in groups with 42.92% and 49.89% protein diet (p < .05). The hepatic superoxide dismutase (SOD) activity significantly increased with dietary protein level increasing to 49.89% and leveled off thereafter (p < .05). The EcR mRNA expression level in the eyestalk slightly decreased with increasing protein level to 42.92%, and it then obviously up-regulated with dietary protein content further increasing to 52.09% (p > .05). According to the polynomial regression analysis, 47.06% dietary protein was required for juvenile S. paramamosain to obtain maximum growth rate.

Different biochemical composition and nutritional value attribute to salinity and rearing period in male and female mud crab Scylla paramamosain

Crabs show major differences in biochemical composition and nutritional value responding to various culture environments, growth stages and even sexes. In the present study, juvenile mud crab Scylla paramamosain (0.026 ± 0.010 g) were cultured at two earthen ponds with the respective salinity level of 25‰ and 5‰ for four months. The proximate composition, amino acid content and fatty acid profile in edible parts (muscle and hepatopancreas) of male and female crabs reared at different salinities and two rearing periods (three-month stage: 60–120 g and four-month stage: 150–300 g) were investigated and compared. Crabs at high salinity had significantly lower moisture contents in muscle and hepatopancreas, but had higher protein contents in muscle and higher ratio of n-3/n-6 highly unsaturated fatty acids (PUFA) in both edible parts, regardless of gender and rearing period. Additionally, significant differences in biochemical composition and nutritional value were also found between male and female crabs. For example, the contents of crude protein and most amino acids were higher in muscle and hepatopancreas of females, whereas males contained higher n-3 PUFA contents in both edible parts. Further, the contents of delicious amino acids (DAA) and essential amino acids (EAA) in muscle and the crude lipid content in hepatopancreas exhibited a higher trend at three-month stage than that at four-month stage. The results suggested that S. paramamosain reared at high salinity had better nutritional value than that at low salinity, while female and male crabs possess their respective advantages in terms of different nutritional composition. In addition, crabs had a better nutritional quality at three-month stage, even though they were at a smaller size.

Effect of dietary cholesterol levels on growth performance, body composition and gene expression of juvenile mud crabScylla paramamosain

A 56-day feeding trial was conducted to investigate the effect of dietary cholesterol (CHOL) levels on growth performance, body composition and gene expression of juvenile mud crab (Scylla paramamosain). Four isonitrogenous and isoenergetic diets were formulated with 0.4%, 0.8%, 1.2% and 1.6% CHOL supplemented, and the final dietary CHOL concentrations were 0.72%, 1.11%, 1.49% and 1.83% respectively. Each dietary treatment was performed with three replicates (28 crabs per replicate, initial body weight 0.04 g). At the termination of the experiment, although the survival had no statistic difference in all treatments, the mud crabs fed the lowest CHOL diet had the lowest survival rate. The weight gain (WG) of mud crab significantly increased with dietary CHOL level up to 1.11% and then significantly decreased with dietary CHOL level further increased. The total-cholesterol (T-CHOL) content in whole body had an increasing trend with the dietary CHOL level increased. Besides, dietary CHOL supplement generally increased the hepatic superoxide dismutase (SOD) activity, and the mud crabs fed diets CHOL1.11 and CHOL1.49 showed significantly higher value than those fed other diets. The hepatic aspartate aminotransferase (AST) activity decreased slightly with dietary CHOL level up to 1.11% and then significantly increased with CHOL level further increased. The mRNA expression of ecdysone receptor (EcR) gene in the eyestalk obviously increased with dietary CHOL level up to 1.11% and then significantly decreased with dietary CHOL further increased. These results suggested that about 1.11% dietary CHOL seem fulfil to maintain good growth performance and healthy condition for juvenile S. paramamosain.

Identification of a species diagnostic character for instar and juvenile mud crabs (Genus Scylla)

A species diagnostic character in instar and juvenile Scylla spp. allows for more focused use of resources in farms, enhanced research in wild populations and improved culture practices into preferred species. Species identification using molecular methods entails extraction of soft tissue that can be fatal to early developmental morphs. This prevents downstream applications that require live samples. In this study, a potential species diagnostic character from the frontal lobe spine shapes of 177 wild-caught Scylla crablets was generated. The species of the samples were identified using the molecular internal transcribed spacer 1 and 16S rDNA markers. Fourier transformation was employed on the images of their spines using the software SHAPE and the results underwent discriminant and principal component analyses. This method was able to assign 92.4% of Scylla serrata, 96.2% of S. olivacea and 90.1% of S. tranquebarica to the right species. The frontal lobe spine shapes of each species group were then traced and used as a means for species identification on 50 cultured and 100 wild-caught Scylla crablets through Resemble.js. This method was then able to assign 85.9% of the instars and 84.7% to the correct species, verified using molecular markers. Results of this study show great potential for the use of the frontal lobe spines as a morphological diagnostic character for instars and juveniles. Increased accuracy can be achieved by expanding the reference shape database and inclusion of more Scylla populations across the region.

低盐度胁迫对拟穴青蟹初期稚蟹成活率的影响

低盐度胁迫对拟穴青蟹初期稚蟹成活率的影响

Intertidal mangrove boundary zones as nursery grounds for the mud crab Scylla serrata

The demand for mud crab Scylla serrata (Forsskål 1775) in the global market has increased, hence there is growing momentum to farm the species in Africa. Aquaculture production in Kenya and elsewhere in East Africa currently relies on juvenile seeds sourced from the wild. Wild-seed collection calls for management of the juvenile crab industry founded on knowledge of the species’ ecology, so as to achieve a sustainable seed supply and recruitment to the capture fishery. This study investigated the tidal, diurnal and seasonal occurrence of juvenile crabs in three habitats (intertidal-flat boundary zones, inside the mangroves, and in channels) in small creeks (Mida, Kilifi and Mtwapa) and Gazi Bay, on the coast of Kenya. Sampling was done with scoop nets and seining at receding tides and via burrow searches at low spring tides (day and night). Juveniles in the mangrove/intertidal-flat boundary zone were found sheltered under mangrove leaves or debris, or in shallow burrows during low spring tides, whereas at receding tides they could be seen moving out with the tide or searching for sheltering substrate or burrows. Catch per unit effort at high-abundance sites varied between 59 and 68 crabs fisher−1 day−1. More juvenile crabs, sized 10–80 mm internal carapace width (ICW), occurred at night during the receding tide. Net-seining was effective in the collection of juvenile mud crabs <30 mm ICW, whereas burrow sampling was effective for gathering larger crabs. However, collection of juveniles by seining reduced the quality of the crabs caught due to frequent loss of chelipeds, as compared with retrieving individual crabs by searching burrows. Some juveniles collected in the intertidal-flat boundary-zone habitat were either in the process of moulting or had just moulted, indicating the significance of this habitat for mud crabs at this physically delicate life stage.