Towards development of a formulated diet for blue swimmer crab, Portunus pelagicus with emphasis on lipid nutrition

Mat Noordin, Noordiyana (2011) Towards development of a formulated diet for blue swimmer crab, Portunus pelagicus with emphasis on lipid nutrition. PhD thesis, James Cook University.

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Market demands on the blue swimmer crab, Portunus pelagicus, have increased substantially worldwide over past decades and have expanded from traditional hard-shell crabs to soft-shell crabs as well as pasteurized crabmeat. Increasing fisheries pressure has led to declining wild population and aquaculture has been eyed as the alternative to supply the market. As an emerging aquaculture species, comprehensive and quantitative understanding of nutritional requirements of P. pelagicus is necessary to the development of formulated diets for the species to support sustainable growth of the industry.

As a crucial nutrient, lipid provides cultured crustaceans with concentrated energy, components of cell membrane and as precursors for various hormones. A series of experiments were hence conducted to evaluate the necessity as well as optimal levels of various lipid components for P. pelagicus early juveniles. Semi-purified diets were formulated to contain different levels (% diet dry weight) of a particular lipid component and the diets were fed to newly molted first stage crabs (C1) till the crabs reached C3 or C4 stage with survival, development and growth of crabs subjected to different dietary conditions closely monitored. To prevent cannibalism, all experimental crabs were kept individually for all experiments.

A starvation and re-feeding experiment was first conducted to investigate their effects on survival and development of newly settled C1 crabs as well as their fatty acid profiles. Meanwhile, hepatopancreas histology was undertaken to observe morphological changes induced. Three treatments: 1) continuous starvation (S); 2) starved for 7 days and then re-fed (S-F); and 3) continuous feeding (F), were set up. In S treatment, crab survival declined rapidly (from 96.6% to 47.9%) between day 7 to 10, leading to the termination of the treatment on day 10. The PNR50 (point-of-no return) for the C1 crabs is therefore between day 7 to 10. There was no significant differences on crab survival between S-F and F treatments, however, newly molted C2 and C3 of S-F treatment had significant lower dry weights (p<0.05). Interestingly, despite their significant lower dry weight, S-F crabs had a significant shorter mean intermolt period from C2 to C3 than that of F crabs (p<0.05). This suggests that following prolonged starvation, re-fed crabs prioritized development over growth. Fatty acid profiles of starved crabs showed a trend of decreased PUFA but increased HUFA (as % total fatty acids), suggesting high retention of HUFA. S-F crabs managed to recover their fatty acid profiles after being re-fed and were similar to F crabs as newly molted C2. Histology observation showed a continuous shrinking of hepatopancreas cells during starvation, however, as newly molted C2 and C3, no obvious differences was noticed between the re-fed and continuously fed crabs.

A series of 8 feeding experiments were then subsequently conducted to assess dietary cholesterol and phospholipid (PL) requirements, cholesterol and PL interactions, neutral lipids (triglyceride) requirements, fish oil to soybean oil ratio, arachidonic acid (ARA) requirements and polar lipids (PL) to neutral lipids (triglyceride) ratio for P. pelagicus early juveniles. The cholesterol experiment showed that out of 7 levels of dietary supplemental cholesterol (0, 2.5, 5.0, 7.5, 10, 12.5 and 15 g kg⁻¹) tested, 10 g kg⁻¹ consistently yield the best survival, development and growth with both the low and high end of cholesterol levels gave inferior development and growth results. In addition, the highest incidents of molt death syndrome (MDS), 20.7% were found for crabs fed the diet with the highest level of cholesterol supplementation at 15 g kg⁻¹.

PL is known to play an important role in lipid and carbohydrate metabolism as well as enhancing absorption of ingested fats, including triglycerides and cholesterol. For second experiment, six iso-lipidic diets with PL supplemented at 0, 30, 60, 90, 120 and 150 g kg⁻¹ were tested and the results demonstrated that the diet without PL supplementation significantly impaired survival (with high incidents of post molt death), development and growth. PL supplemented at 120 and 150 g kg⁻¹ yielded the best survival, development and growth that were often differed significantly to other treatments. Salinity stress test (salinity dropped abruptly form 30 to 7‰ for 7 h) using C4 crabs also showed that while the majority of crabs from PL 120 and 150 g kg⁻¹ treatments survived to the end of stress test, all crabs from other treatments dead within 5 h.

As interactive effects between cholesterol and PL have been demonstrated in other crustaceans, two experiments were designed to test this for P. pelagicus. In the first experiment, 10 iso-energetic diets were formulated, including 9 diets in a 3x3 factorial design with various combinations of 3 levels of PL (30, 60 and 90 g kg⁻¹) and cholesterol (0, 5 and 10 g kg⁻¹), respectively and a basal diet (deficient in both PL and cholesterol). It was demonstrated that the addition of PL at 90 g kg⁻¹ could compensate lack of dietary cholesterol by substantially improved crab survival. As the first experiment showed that crabs fed the diet with the highest supplemented PL (90 g kg⁻¹) and cholesterol (10 g kg⁻¹) yielded the best results, a second experiment was conducted to investigate if higher PL and cholesterol levels would lead to even better performance. Four diets with combinations of 2 cholesterol (10 and 15 g kg -1) and PL levels (90 and 120 g kg⁻¹) were fed to C1 crabs and the results showed that the further increase in cholesterol and PL did not lead to significant improved survival, development and growth.

With optimal cholesterol and PL level established, 3 experiments on neutral lipids requirements were executed. Firstly, effects of different levels of total neutral lipids (triglyceride) were assessed with 6 triglyceride levels at 0, 20, 40, 60, 80 and 100 g kg⁻¹ (fish oil to corn oil ratio fixed at 2:1) being included in the diets. The results demonstrated that level of triglycerides did not significantly affect survival of the crabs (p>0.05), however, crabs fed the diet supplemented with 40 g kg⁻¹ triglycerides achieved significant better growth than all other treatment except the diet supplemented with 20 g kg⁻¹ triglycerides. The 40 g kg⁻¹ triglycerides treatment also had the highest survival among all diet treatments. A subsequent experiment set triglyceride at the optimal level of 40 g kg⁻¹ with varied fish oil (FO) to soybean oil (SO) ratio at 1:0, 3:1, 2:1, 1:1, 1:2, 1:3 and 0:1, respectively, to investigate their effects on performance of P. pelagicus early juveniles. The results showed that the diet without supplemented fish oil (SO:FO at 1:0) impaired survival of the crabs with the longest development time to C4 and the lowest growth. The greatest Specific Growth Rate (SGR) for dry weight and carapace width was achieved when crabs fed the diet with SO:FO ratio at 1:1, which was significantly better than that of the treatments with SO:FO ratio of 1:0 and 2:1. The results suggest fish oil can be partially replaced by plant oil and indicates the need for a balanced n-3 and n-6 fatty acid profile in P. pelagicus diets. Fatty acid analysis on both fish and soybean oil showed low level of arachidonic acid (ARA), an important n-6 HUFA. A further experiment was then carried out to investigate whether additional ARA on top of the level provided by triglyceride supplementation (40 g kg⁻¹ at SO: FO ratio 1:1) might promote performance of P. pelagicus early juveniles. Six iso-lipidic diets were formulated to contain additional ARA levels at 0, 2.0, 4.0, 6.0, 8.0 and 10.0 g kg⁻¹, respectively. Another two diets were formulated to contain ARA at 5.0 and 10 g kg⁻¹, but with triglyceride supplementation of macadamia oil (contain mostly SFA and MUFA). Survival of the crabs ranged from 50.0% to 76.7% with no significant differences detected among treatments with similar result obtained for development. However, the crabs fed the diet without additional ARA supplementation (0 g kg⁻¹) had significantly higher dry weight than all other treatments with supplemented ARA (p<0.01), the diet also resulted in the shortest development time from C1 to C4 and the biggest carapace size. The results indicated that further provision of ARA other than that already provided by triglyceride supplementation of soybean oil and fish oil at a fix ratio of 1:1 is not necessary.

A final experiment investigated effects of supplementing P. pelagicus diets with different polar lipids (PL) and neutral lipids (triglyceride; SO: FO ratio at 1:1) ratios. Five iso-energetic and iso-lipidic diets were formulated to contain PL and triglyceride mixture (TG) at 1:0, 2:1, 1:1, 1:2 and 0:1, respectively. The results showed that the diet without PL supplemented with triglyceride only (PL:TG ratio 0:1) impaired crab survival. The treatment also had a significantly longer mean development time to C4 when compared to crabs fed diets with PL:TG ratios at 1:0, 2:1 and 1:1 (p<0.01). Meanwhile, crabs fed the diet with a PL:TG ratio of 1:0 had the greatest dry weight and carapace size, which were significantly higher than crabs fed diets formulated with PL:TG ratios at 0:1 and 1:2. Salinity stress test (salinity dropped abruptly form 30 to 7‰ for 9 h) using C4 crabs demonstrated that crabs fed diets with PL to TG ratios (PL:TG = 1:1 and 1:2) suffered 100% mortality while > 50% crabs fed diets formulated with higher PL to TG ratios (PL:TG = 1:0 and 2:1) survived to the end of the stress test.

This study has covered the majority optimal lipid requirements for P. pelagicus, an emerging aquaculture species. The optimal level of lipids constituents in this study were determined using parameters set for aquaculture, including survival, development and growth. The use of practical ingredients makes the outcomes of the study more readily for industry implementation.

Item ID: 39433
Item Type: Thesis (PhD)
Keywords: aquaculture; blue manna crab; blue swimmer crab; cholesterol; cultured crustaceans; diet; dietry requirements; fats; feeding; flower crab; formulated diet; growth; lipid nutrition; neutral lipids; nutrition; phospholipids; polar lipids; Portunus pelagicus; sand crab; triglycerides
Date Deposited: 06 Aug 2015 00:04
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070401 Aquaculture @ 100%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830199 Fisheries - Aquaculture not elsewhere classified @ 100%
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