Morphological And Histological Studies On The Digestive System Of Larval Eriocheir Sinensis

Larval Eriocheir sinensis were cultured from the stage of zoeaⅠto the first larval crab from April to July in 2004.For every stage,larval Eriocheir sinensis were fixed in Bouin's fluid and 4.0%glutaraldehyde separately and their digestive tract and midgut gland were studied using light and transmission electronic microscopy respectively.The hepatopancreatic ultrastructure of ZoeaⅠjust after spawned has been compared with that of ZoeaⅠafter 12 hour starvation and after 12 hour feeding with rotifer using transmission microscopes.The results obtained through morphology and light microscopy show that the mean length of larval digestive tract in each stage is respectively 0.845mm,1.198mm,1.507mm,1.871mm,2.279mm,2.562mm and 3.014mm.The growth rate of digestive tract of larva from zoeaⅡto the first larval crab is 41.78%,25.79%,24.15%,21.81%,12.42%and 17.64% respectively.The larval digestive system consists of digestive tract and digestive gland.The digestive tract is divided into three major divisions: foregut,midgut and hindgut.The foregut and hindgut are of ectodermal origin and the midgut arises from endoderm.Foregut includes mouth, esophagus and stomach.Midgut includes midgut tubule,midgut gland, anterior midgut cecum and posterior midgut cecum.The hindgut is thick, there is a hole called anus in hindgut terminal,which lies in the center of abdominal segment.The wall of digestive tract consists of mucous layer, connective tissue,muscular layer and outer membrane.With the development of larval stages,the esophagus ridges have more and more folding,the muscles of esophagus are increasing.The gastric mill,the gland filter,the size proportion of cardiac to pyloric stomach and cardiac-pyloric valve all have obvious changes when the larvae grow up, adapting to their diets need and filtering function of different developmental stages.The most developed division of digestive tract is midgut.The average proportion of length of midgut to digestive tract is 53.63%in all larval stages.The morphology of epithelial cells of midgut wall varies in each stage.The length of the hindgut which is shorter than the midgut at stages of zoea and megalopa shortens rapidly with the degeneration of abdominal region after the megalopa.The three accessory structure of midgut vary obviously in length and structure with the development of the larva.A pair of anterior midgut cecum which lie abdominal to the midgut increase in length and amount with development. The anterior midgut cecum which are lobe rudiments at the stage of ZoeaⅠbecome full of carapace at the stage of the first larval crab.The posterior midgut cecum which are granules at the stage of zoeaⅠoccur at the stage of zoeaⅡ,then extending and winding gradually.It could be observed at the stage of the first larval crab.The midgut gland occur at the stage of zoeaⅠ,then increasing in length gradually with the larval development.The results obtained through transmission electronic microscopy show that the hepatopancreatic cells can be divided into four types:embryonic cells(E-cells),fibrillar cells(F-cells),resorptive cells(R-cells) and secretary cells(B-cells).E-cells are stem cells that have the function of division and mitosis and can be differentiated into other three types of cells.B-cells are mainly involved in extracellular and intracellular digestion.F-cells can synthesize digestive enzymes. R-cells take part in resorpting and storing nutrition.The differentiation sequences of four cell types of hepatopancreas in larval 1riocheir sinensis are as follow:R←-E→F→B.Compared ZoeaⅠafter 12 hour starvation with that of just after spawned,the ultrastructure of midgut gland has significant difference in F-cells,especially in R-cells.F-cells become more with mitochondria swelling and irregular in shape,while R-cells decrease obviously with mitochondria swelling or disintegrating to disappear.Compared with ZoeaⅠjust after spawned,the ultrastructure of midgut gland of ZoeaⅠafter 12 hour feeding with rotifers cultured by Thalassiosira sp has significant difference in R-cells.R-cells become more and have more round lipid droplets.The mitochondrias in R-cells become oval or disorganizing. The smooth endoplasmic reticulum become inflate and distribute in cytoplasm.More and more microbodies which aggregate in the apical cytoplasm could be observed in R-cells.The changes of organelles in R-cells can indicate the status of nutrition and can be used to monitor the nutritional status of larval Eriocheir sinensis.