The Effect And Mechanism Of Delayed First Feeding On Growth Of Myxocyprinus Asiaticus Larvae

To investigate the effect and mechanism of delayed first feeding on growth of Myxocyprinus asiaticus larvae under the condition of water temperature (19±0.5)℃, two experiments were performed. The first one was a starvation test, to examine the effect of starvation on survival, feeding, morphology and behavior habits in Myxocyprinus asiaticus larvae. The second was the delayed first feeding experiment along with different initial feeding time groups:0(control group),1,3,5,7,9and11delayed days group, denoted as D0, D1, D3, D5, D7, D9and D11, respectively. Once the delayed first feeding experiment was completed, the growth capacity, digestive enzyme activity, fat content, fatty acids and amino acids were detected, and refeeding groups (19days and29days) as well. The objectives of this study are (1) to investigated starvation resistance capability of Myxocyprinus asiaticus larvae,(2) to explore the energy distribution mechanism and physiological ecology adaptation strategies in the face of starvation,(3) to provide practical information for application in natural resources, larvae breeding, fishery enhancement and release.The main results of present study as follows:(1) Since hatching, Myxocyprinus asiaticus larvae’s firstly feeding was on the7th day and the yolk exhausted on the14th day, indicating the mixed feeding stage was about7days. The initial feeding rate in the starvation test decreased sharply since from the25th day after hatching, reaching43.33%in the27th day (P<0.05), suggesting the PNR (the point of no return) appeared on the27th day after hatching. The mortality rate were increased sharply from the22nd day to the26th day after hatching (P<0.05), reaching60.89%accumulatively on the26th day and100%on the29th day. Therefore, the Myxocyprinus asiaticus larvae could retain the capacity for initial feeding activity for19days and the time of no return from regaining food was about3days.(2) Starvation can impact significantly on the morphological and behavioral characteristics of Myxocyprinus asiaticus larvae. At15days old, some starvation larvae appeared with the head under the tail and the body unbalanced; at18-day-old, they performed a slow response and suspending more obviously with about1/3lying on the bottom of the aquarium. With prolonged starvation, the larvae showed more weight loss, chest and abdomen concave and pectoral angle. Around the PNR, the following phenomena were more prevalent:lying on the bottom of the aquarium body weight loss, and pectoral angle, along with large head, dorsal and anal fins differentiated, compartment bladder smaller, bowel tapering and straight, heart beat slow.(3) Delayed first feeding can lead significant effect on Myxocyprinus asiaticus larvae growth. With delay time increasing, both of the wet and dry weight decreased significantly (P<0.05), moisture content increased significantly, while the total length increased first and then decreased. At the end of the experiment (29days), compared to the control group, the D1and D3groups showed no significant difference in body weight and length, indicating a complete compensation effect. However, D5, D7, D9, and D11group were significantly lower than the control group level (P<0.05), showing a partial compensation effect.(4) Starvation can significantly effect the digestive enzyme activity of Myxocyprinus asiaticus larvae (P<0.05). During the1th to the7th day starving process, the lipase and amylase enzyme activity decreased significantly (P<0.05), while when prolonging the starvation time to9and11days, the lipase and trypsin enzyme activity rose significantly (P<0.05). After refeeding to the19th hatching day, the trypsin activity significantly increased and then gradually decreased with starvation time lasting, while indicated the opposite trend on the amylase activity. To the29th day larvae, both of them showed a significant upward trend (P<0.05).(5) The fat content of the body showed a significant downward trend when prolonging the starvation time (P<0.05). At the end of the experiment (29days old), each treatment group larval fat content were not significantly different from the control group, showing a complete compensation effect.(6) With increasing time of starvation, the total content of monounsaturated fatty acids (∑MUFA) and n-6series polyunsaturated fatty acids (Σn-6PUFA) sustained downward trend; the total content of polyunsaturated fatty acid (ΣPUFA) from DO to D3increased significantly then kept stable; while the ratio of∑n-3PUFA to Σn-6PUFA showed a significant increase (P<0.05). The percent ratio of docosahexaenoic acid (C22:6n-3, DHA) to arachidonic acid (C20:4n-6, ARA) was negatively correlated with larval weight, while positively with moisture content (P<0.05). After refeeding, to the19th day hatching larvae, the DHA, eicosapentaenoic acid (EPA) and ARA levels in group D7, D9and D11did not restored to the control group level (P<0.05). After a long recovery period (29days of age), the majority of the fatty acid levels were restored to the control group, except several individuals in group D9and D11.(7) Delayed first feeding had a significant impact on amino acid composition and content of Myxocyprinus asiaticus larvae (P<0.05).The alanine, isoleucine, leucine and histidine decreased significantly (P<0.05), aspartic acid and glutamic acid increased significantly (P<0.05), while glycine, lysine and arginine were first increased and then decreased along with the increasing of starvation from1to11days. The total content of essential amino acids (ΣEAA) showed a significant increase from the D1to D7, then a sharp decline and stability (P<0.05). The ratio of the total content of essential amino acids to non-essential acid ratio (ΣEAA/ΣNEAA) in D5, D7, D9and D11group were significantly lower than the level of the control group (P<0.05). After refeeding to the19th hatching day, with starvation time lasting, the essential amino acids valine, methionine, lysine and histidine were significantly decreased (P<0.05), and non-essential amino acids aspartic acid, glutamic acid, alanine, proline as well (P<0.05), while the ΣEAA/ΣNEAA showed no significant changes. At the end of the experiment (29days old), with starvation time lasting, the essential amino acids (threonine, valine, methionine, isoleucine, leucine, phenylalanine, lysine, histidine) and the non-essential amino acids (aspartic acid, serine, alanine) were significantly decreased, but proline content was significantly increased (P<0.05).The main conclusion as follows:(1) The PNR was suggested to be about27days, the capacity for retaining initial feeding activity was19days and the time of no return from regaining food was about3days for Myxocyprinus asiaticus larvae.(2) Starvation slowed significant effect on the growth and development of Myxocyprinus asiaticus larvae, along with slow growth and development, and even significant pectoral angle characteristics when compared with normal feeding larvae.(3) With increasing time of starvation, larvae could regulate their main enzyme activities to enhance the amino acids and fatty acid utilization of energy to meet the basic needs of life.(4) After refeeding, compared to the control group, the Myxocyprinus asiaticus larvae showed compensatory growth effects in feeding intensity, digestive enzyme activity, the content of amino acids and fatty acids. They obtained the amino acids and fatty acids quickly mainly through improving the feeding intensity and increasing the activity of digestive enzymes. While compared to the amino acid content, the fatty acids restored more slowly.