| Discus fish(Symphysodon haraldi)is a popular tropical freshwater ornamental fish hat has long held a dominant position in the aquarium market due to its bright and colorful body and unique patterns.However,with the development of discus fish culture,there has been a phenomenon of growth differences within the same batch of fish,where fish of the same age exhibit significant differences in weight.This phenomenon has brought a lot of adverse effects to the aquaculture industry during the breeding process.Therefore,it is necessary to analyze the potential process of growth heterogeneity in discus fish,and provide corresponding explanations to improve the growth of discus fish and promote the development of the aquaculture.this study selected five pairs of the same species of discus fish as parents and cultivated their offspring for 60 days.The offspring were divided into two extreme groups with significant differences in body weight: the slow-growing group(S group)with a body weight of 0.15±0.01 g and the fast-growing group(F group)with a body weight of 0.35±0.03 g.After 30 days of cultivation,the growth,nutrient digestion,carbon and nitrogen balance,and transcriptome analysis of the visceral tissue of the discus fish were measured.The results of the experiment are as follows:(1)Determination of Growth and Digestive Capacity in Allometric Growth Discus FishAfter the aquaculture experiment,there still existed significant differences in weight between the large and small individuals(P < 0.05),and the difference ratio did not decrease but rather increased.In terms of growth,the survival rate,specific growth rate,weight gain rate,average daily weight gain,average daily feed intake,and feed efficiency of the large individuals were higher than those of the small individuals,with significant differences observed in the average daily weight gain,average daily feed intake,and feed efficiency(P < 0.05).In terms of digestion,the dry matter digestion rate,carbon apparent digestion rate,and nitrogen apparent digestion rate of the large individuals were significantly higher than those of the small individuals(P < 0.05).The activity of α-amylase and gastric protease in the visceral organs of the large individuals were also significantly higher than those of the small individuals(P < 0.05).(2)Measurement of Oxygen Consumption,Excretion Rate,and Carbon-nitrogen Balance in Allometric Growth Discus FishBased on the measurements,the oxygen consumption rate,ammonia excretion rate,phosphorus excretion rate,and oxygen-to-nitrogen ratio of the large individuals were all lower than those of the small individuals.The difference in oxygen consumption rate was significant(P < 0.05).In terms of carbon balance,the large individuals had significantly less carbon intake,fecal carbon,respiratory carbon,growth carbon,and absorbed carbon than the small individuals in the S group(P <0.05).As for nitrogen balance,compared to the small individuals,the large individuals had significantly lower nitrogen intake,fecal nitrogen,excreted nitrogen,and absorbed nitrogen(P < 0.05).Based on the measurement results,carbon and nitrogen balance equations were constructed for the discus fish.Through transformation and analysis,it was found that the utilization of carbon and nitrogen by the larger fish was higher than that of the smaller fish,and both larger and smaller fish had a higher utilization of carbon than nitrogen.In terms of carbon balance,the carbon ingested by both groups was mainly used for individual growth,and the largest carbon consumption was exhaled through respiration.When consuming the same amount of carbon,the larger fish absorbed more carbon,which was mainly used for growth,and excreted and consumed less carbon through feces and respiration than the smaller fish.Conversely,although the smaller fish excreted and consumed less carbon compared to the larger fish,they excreted and consumed more carbon through feces and respiration.In terms of nitrogen balance,most of the ingested nitrogen by both the larger and smaller fish was excreted,with only a small amount used for growth,and a tiny amount of nitrogen was excreted with feces.When consuming the same amount of nitrogen,the larger fish had a stronger ability to absorb carbon,more nitrogen was used for growth,and less feces were excreted than the smaller fish.By calculating the proportion of each component in dietary carbon and nitrogen,the differences between them are more clearly reflected.In terms of carbon balance,the proportion of dietary carbon,fecal carbon,respiratory carbon,growth carbon,and absorbed carbon in large fish is smaller than that in small fish,and the difference is significant(P < 0.05).The excreted carbon in small fish is also greater than that in large fish,but the difference is not significant(P > 0.05).In terms of nitrogen balance,compared with small fish,the proportion of dietary nitrogen,fecal nitrogen,excreted nitrogen,and absorbed nitrogen in large fish is smaller,and the difference is significant(P < 0.05).However,the difference in growth nitrogen between small and large fish is not significant(P > 0.05).(3)Transcriptome Analysis of Allometric Growth Discus FishFor the study of individual growth differences in the Discus fish,we used Illumina high-throughput sequencing technology to sequence the transcriptomes of six samples.A total of 61.32 Gb of Clean Data was obtained,with each sample having Clean Data amounts exceeding 8.33 Gb and Q30 base percentages exceeding92.65%.Next,we aligned the data from these samples with the full genome of the Discus fish,with alignment rates ranging from 84.66% to 90.22%.We then conducted gene expression analysis,comparing the expression levels of genes in different samples using different differential comparison groups,and annotated and enriched the differential genes.In the differential gene analysis,a total of 216 differential genes were identified in the Discus fish differential grouping,with 68 up-regulated genes and 148 down-regulated genes.GO,KEGG annotation,and enrichment analysis were performed on the differential genes,revealing that differentially expressed genes are involved in processes such as myocardial contraction,calcium signaling,glycolysis/gluconeogenesis,PPAR,thyroid hormone,purine metabolism,insulin secretion,galactose metabolism,and the pentose phosphate pathway.Among them,the significantly upregulated genes in the F group included ann,mid1,pur,fat,elo,acy,and dc K,and the PPAR signaling pathway and purine metabolism may be related to the rapid growth of the F group.The results indicate that compared to the smallsized discus fish,the large-sized individuals have stronger growth and digestion abilities and higher utilization of carbon and nitrogen,which may be related to the activation of the PPAR signaling pathway and purine metabolism.In summary,we have explained the phenomenon of weight differences in discus fish from multiple aspects such as growth,digestion and absorption,excretion,carbon-nitrogen allocation,and gene expression.We have preliminarily revealed the underlying reasons and provided reference for improving the phenomenon of growth differences in discus fish and further selection breeding. |
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