| The anti-season breeding mode of Portunus trituberculatus(swimming crab)is a new breeding mode.Compared with the traditional "spring seedling and winter harvest" breeding mode,the water temperature change law of the whole breeding cycle is quite different.The swimming crab will experience the breeding process of early autumn breeding,overwintering breeding,and early summer ovarian development.Therefore,based on the main line of water temperature,this study carried out the effect of water temperature on the four key stages of embryonic development,larval development,overwintering of juvenile crabs,and ovarian development of female crabs after mating under off-season breeding mode.The specific research results are as follows:In this study,the rate of embryonic development,embryo antioxidant ability,and hatching rate of swimming crabs in different water temperatures in an off-season breeding mode(breeding in early autumn)and the digestive enzyme activity of newly hatched larvae was compared.The results showed that the duration of each development stage of swimming crab embryos was gradually reduced with increasing water temperature.The total development time was 9.43 d at 27℃ and only 6.88 d at 33℃.These effects were accompanied by an increase in the development rate from 0.11 d-1 to 0.15 d-1.The total effective accumulated temperature under the four temperature conditions was maintained at about 150 ℃·d(147.08～153.62 ℃·d).When the water temperature was 31℃,the late embryonic development was not synchronized,individual embryos had stagnated development,and even death occurred,especially at 33℃.The embryo hatching rate was approximately 70%at 27℃ and 29℃,and the rate was significantly reduced at 31℃ and 33℃(P<0.05).Specifically,the rate was only 13.89%at 33℃.With the increase of water temperature,the activities of superoxide dismutase(SOD)and glutathione peroxidase(GSHPX),total antioxidant capacity(T-AOC)and malondialdehyde(MDA)content in embryos increased first and then decreased,and reached the highest at 31℃.Catalase(CAT)activity exhibited the opposite trend which was the lowest at 29℃ with a value of only 0.17 U/mgprot and the highest at 33℃ with a value of up to 0.51 U/mgprot.The differences of digestive enzymes in newly hatched larvae were mainly manifested in the higher activities of pepsin(PEP)and α-amylase(AMS)at 31℃,and the other differences were not obvious.The activities of digestive enzymes in newly hatched larvae at 33℃ were relatively low,especially the activities of trypsin(TPS)and cellulase(CL)were significantly lower than the other three groups(P<0.05).Overall,a temperature below 31℃ is the appropriate temperature for embryonic development in autumn in swimming crabs.In the off-season breeding mode,the molting cycle,development rate,effective accumulated temperature,survival rate of the larva development,and antioxidant capacity,digestive enzyme activity of the megalopa were systematically compared under different water temperature conditions.The results showed that the molting cycle of zoaea stage Ⅰ(Z1)～zoaea stage Ⅱ(Z2)was less affected by water temperature,and it was maintained at about 3 d.During the period of Z2-megalopa(M),the molting cycle of 31℃ group was always the shortest.The total duration of development during Z1～M was the shortest in the 31℃group,which was 9.97 d,and significantly lower than that in the 27℃ and 29℃groups.At the same time,the lowest total effective accumulated temperature was 250℃·d,which was significantly lower than that in the 33℃ group(P<0.05);Through the binomial equation fitting calculation of temperature and larval development rate,it was found that the development rate was the fastest at 30.14℃;With the increase of water temperature,the survival rate of larvae in each stage decreased gradually,and the survival rate of 27℃ group was always significantly higher than that of 31℃ and 33℃ groups(P<0.05).Especially in Z4～M stage,the survival rate of 33℃ group was only 2%,which was significantly lower than that of 27℃ and 29℃(P<0.05).In terms of antioxidant performance,the activities of SOD,CAT,and GSH-PX in 29℃ and 31℃ groups were relatively lower than those in 27℃and 33℃groups,especially the SOD activity in 31℃and 33℃ groups was significantly higher than that in the other two groups(P<0.05).The T-AOC index of the 31℃ and 33℃groups was significantly higher than that of the 29℃ group(P<0.05),the content of MDA gradually increased with the increase of water temperature,but there was no significant difference(P>0.05).In terms of digestive enzyme activity,with the increase in water temperature,the activities of PEP,LPS,TPS,AMS,and CL in 31℃ and 33℃ groups were higher than those in 27℃ and 29℃ groups.In particular,the AMS activity of 31℃ and 33℃groups was significantly higher than that of 27℃ and 29℃ groups(P<0.05).Therefore,comprehensive indicators,to ensure the survival rate,the development quality of larvae between 29℃ and 31℃ is better in the anti-season breeding mode.This study analyzed the composition and content of nutrients in the muscle and hepatopancreas of juvenile swimming crabs cultured in a pond at the beginning,middle,and end of overwintering,to understand the nutrient metabolic patterns in the body of swimming crabs during overwintering.The results showed that the muscle yield(MY)did not change much during the overwintering period,but the hepatopancreatic index(HSI)decreased significantly(7.88%at the beginning of overwintering and only 6.23%at the end of overwintering).During the whole overwintering period,the conventional biochemical composition of muscle and hepatopancreas of juvenile crabs,especially the total carbohydrate decreased by nearly 50%,and the crude fat decreased by about 30%.In terms of fatty acid composition,the sum of saturated fatty acids(∑SFA)in juvenile crab muscle decreased slightly,while the sum of monounsaturated fatty acids(∑MUFA)increased slightly,and the sum of polyunsaturated fatty acids(∑PUFA)remained basically unchanged.In contrast,the content of fatty acids in hepatopancreas changed greatly,among which ∑SFA showed a significant decreasing trend(P<0.05),which was 44.2%at the beginning of overwintering and only 24.02%at the end.MUFA decreased slightly in the late winter,and∑PUFA increased rapidly,which was 7.62%at the beginning of overwintering,18.59%at the middle of overwintering and 32.83%at the end of overwintering.In general,the energy material of juvenile swimming crabs was mainly derived from the hepatopancreas throughout the overwintering period,and it mainly utilized carbohydrates and crude lipids in the body and was highly selective in the oxidative utilization of fatty acids,mainly utilizing SFAs for oxidative energy supply,while PUFAs,high unsaturated fatty acids(HUFAs),C20:5n3(EPA)and C22:6n3(DHA)were selectively retained.In this study,to explore the effects of different water temperature conditions on the ovarian development of swimming crabs after mating under off-season breeding mode,a systematic study was conducted using relevant biological indexes,tissue section observation,and biochemical composition determination methods.The results show that with the increase of water temperature,the gonadosomatic index(GSI)and condition factor(CF)of swimming crabs during the experiment continued to rise,as high as 5.95%and 0.62,respectively,and were significantly increased compared with 20℃ and 23℃(P<0.05).The final variation range of MY among all groups was not large,basically maintaining at about 34%,while HSI gradually decreased with the increase of water temperature and the difference was not significant(P>0.05).From the observation of ovarian sections,the diameter of oocytes at 29℃ water temperature was up to 287.44±25.25 μm,and significantly higher than the other three groups(P<0.05).At the same time,the maturity of the ovary is also getting higher and higher,and the number of lipid droplets in the oocytes and the degree of homogenization of yolk granules are also gradually improved;In terms of routine biochemical composition,the content of moisture and crude protein in muscle and the content of crude fat in hepatopancreas did not change much among the groups,but the content of crude protein in ovary at 29℃ was as high as 24.65%,which was significantly higher than that of the other three groups(P<0.05);In terms of fatty acid composition,the contents of SFA,∑SFA and∑MUFA represented by C16:0,C18:0 and C14:0 in ovarian tissue did not change significantly(P>0.05),and the contents of DPA,EPA and DHA in PUFA were less affected by water temperature.The contents of C20:4n6,EPA,DPA and DHA in hepatopancreas PUFA were low and kept below 0.5%.With the increase of temperature,the content of C16:0 in muscle SFA and C18:ln9c,C16:1 and ∑MUFA in MUFA decreased first and then increased,while the content of PUFA increased first and then decreased except C18:2n6,C22:2n6 and C18:3n6;In terms of amino acid content,the content of amino acids in the ovary gradually increased with the increase of water temperature.At 29℃,leucine and methionine in essential amino acids(EAA)and serine(Ser),glutamate(Gul),and arginine in nonessential amino acids(NEAA)were significantly higher than those at 20℃(P<0.05),and the increase rates were as high as 39.10%,52.08%,43.26%,41.40%,and 44.65%,respectively.In contrast,the content of amino acids in muscle was less affected by it,and there was no significant difference in amino acids among the four groups(P>0.05);Leu and Met in muscle were the limiting amino acids under the four groups of water temperature conditions.In addition,Met in ovarian tissue at 20℃ and 23℃ and phenylalanine(Phe)+tyrosine(Tyr)in the ovary at 26℃ were the limiting amino acids.On the whole,the essential amino acids score(EAAS)and the average score of the two tissues at 20℃ and 23℃ were close,and the score reached the highest at 29℃.In general,with the increase in water temperature,the ovarian development rate was significantly accelerated,GSI and CF were significantly improved,the nutritional composition was relatively rich and the amino acid composition was reasonable. |
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