| Objective:The oriental river prawn?Macrobrachium nipponense?,a freshwater species of the family Palaemonidae?Decapoda?that is found in China,Japan,and Southeast Asian countries,is one of the economically important cultured prawn species in China.In recent years,as the living standards of ordinary people improve,the demand for the oriental river prawn with superior quality is steadily growing,and the prawn aquaculture has been continuously expanding.However,large-scale commercial aquaculture of the oriental river prawn with big size and superior quality was seriously affected and restricted by immature breeding technologies for large-scale production of prawn larvae,and the lower survival rate of the prawn fry.In this study,from perspectives of feeding ecology,the food selection preferences,digestive tract development,and the digestive characteristics of the prawn larvae were investigated by examining the food composition of the oriental river prawn during the its early development,and the species composition of food organisms in prawn ponds,and by analyzing the activities of digestive enzymes and the morphological characteristics of digestive tract at early developmental stages.Based on the results obtained above,the reasons for lower survival rate of the prawn fry were analyzed.This will provide scientific evidence for cultivating live feeds and improving the ingredients of artificial diets during the course of prawn fry cultivation.Methods:1.Two prawn ponds?Nos.5,6?were randomly selected for rearing prawn fry in this study.The species compositions of food organisms in the digestive tracts of prawn larvae from Z1 to post stages and those of plankton communities in the prawn ponds were simultaneously examined.The community succession and the growth-decline patterns of the aquatic food organisms in prawn ponds,and the changes in compositions of food organisms in the digestive tracts of prawn larvae were analyzed.And then,the association between food selection preferences of prawn larvae and thecommunity succession and the growth-decline patterns of food organisms was evaluated.2.The samples of prawn larvae from Z1 to post stages were collected from these two ponds,and the activities of pepsin,trypsin,lipase,amylase,and cellulase in the prawn larvae were measured.The association between activity changes of these enzymes and the compositions of food organisms in the digestive tracts was then assessed.3.The developmental characteristics of digestive organs of the oriental river prawn during the early developmental stages were identified by observing the histological and morphological changes in digestive tract development of the prawn larvae.Results:The initial feeding forthe prawn larvae was observed at Z2 stage,including Chlorophyta such as Chlorella,Cyclotella and Bacillariophyta,Rotatoria,protozoa.The planktonic species compositions differ in different ponds,and the difference can derectly effect the dietary that the prawn larvae ingest plankton.The biodiversity of fish bait for the prawn larvae in pond 6 is higher than pond 5 and Chlorella,Cyclotella are in a dominant position.So the small-sized green algae were the mostly consumed feed.Statistics analysis for biomass showed that with the development of prawn larvae,the biomass of food organisms in food pellets of prawn larvae shifted from the single-cellular microalage to larger-sized rotifers and protozoans.The community structure and abundance of food organisms in the prawn ponds affected the development and survival rate of the prawn larvae.The pepsin activities of the prawn larvae from the two ponds showed different variation trends during the course of larval development.The larvae from pond 5presented slightly higher pepsin activities at Z5 and Z7 stages,and relatively lower activities at Z4 and Z8 stages,while in larvae from pond 6,the peak pepsin activity was detected at Z6 stage.In the larvae from both ponds,the peak trypsin activities occurred both at Z9 stage,while lower activities were observed at other different developmental stages.Lipase activities achieved the highest level in Z6 larvae from pond 5,and in Z4and Z5 larvae from pond 6.Lower activities of amylase and cellulase were detected in prawn larvae during the whole developmental stages,and significantly lower than those in the liver and stomach of adult prawn?p<0.05?.However,the amylase activities were significantly higher at Z4 and Z5 stages than those at earlier stages.The observations of higher protease and lipase activities and lower amylase and cellulose activities during the larval development indicated that prawn larvae preferentially selected zooplankton as their food sources.Our results showed that the esophagus of young larvae of the oriental river prawn consisted of esophageal gland,X-shaped esophageal cavity,and muscular layer.The valve in the esophagus functioned to prevent food reflux.A cardio-pyloric valve developed in the middle of the gastral cavity of prawn larvae at Z1 stage,and a platelike chitinous layer of gastric wall formed,and high cylindrical ventral epithelium of pyloric stomach developed at Z6 stage.The primary filter and gastric mill appeared at cardiac stomach,and the secondary filter appeared at pyloric stomach in Z7 larvae.At the midgut,only a few small vacuoles of epithelial cells were found from Z1 to Z3 stages.The striated border appeared at Z3 stage.Then,more large vacuoles formed at Z4 stage.Subsequently,basal cells occurred at Z6 stage,and the epithelial layer became sunken and folded to form midgut ridges.At the hindgut,no obvious histological and morphological changes were observed during the different developmental stages.In addition,a large number of fat droplets were found in the hepatopancreas of Z2 larvae,and many B cells occurred in Z3 larvae.Moreover,the hepatopancreas was found to be composed of four types of cells?E,F,R,and B?during the development of prawn larvae.Conclusion:The initial feeding for the prawn larvae occurred at Z2 stage,and the small-sized green algae were used as initial feed.Over the development of the oriental river prawn larvae,major biomass of food composition shifted from the single-cellular alage to larger-sized rotifers and protozoans.The community structure and abundance of food organisms in the prawn ponds have much to do with the development and survival rate of the prawn larvae.The patterns of digestive enzymes suggested that the zooplankton were preferentially selected by the prawn larvae as main food sources during the whole larval development.The observation on the massive deposit of fat droplets in the larval hepatopancreas from Z2 stage is consistent with our results of the initial feeding at Z2 stage. |
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