Study On The Histological Structures Of Digestive Tract And The Coordination Between Digestion And Respiration In Misgurnus Anguillicaudatus

The weatherloach(Misgurnus anguillicaudatus)belonging to the family Cobitidae,is an important freshwater fish in China.It has a very short digestive tract.In order to adapt to the extreme environment,the weatherloach specialized an accessory respieritory organs.In the process of gastrointestinal air-breathing(GAB),the swallowed gas must bypass the digesta of the digestive intestine and unidirectionally flow into the respiratory intestine.Considering the GAB potentially compromises digestive function in M.Anguillicaudatus,the histological section and scanning electron microscopy(SEM)were adopted in this study to explore the structural basis of GAB.In order to confirm the gas channel in the intestine,X-ray technology was used to observe the digesta and ventilation under excessive feeding and respiratory inhibition.We hope that this information would clarify the coordination of digestion and respiration in the intestine of the weatherloach and provide new ideas for the scientific aquaculture of loach.The main results are as follows:By using HE staining and SEM,the result indicated that the digestive tract of M.Anguillicaudatus was composed of oropharynx,esophagus,anterior intestine,middle intestine,posterior intestine and rectum.All parts cooperated with each other and constitute the structural basis of digestion,absorption and respiratory.A funnel-like valve is present between the esophagus and intestine and presumed to prevent food and air from reverse flow.The anterior intestine,markedly swollened,displays muscular wall,heavy folds but thin lamina propria,suggesting the characteristic of expansion to meet the air inflation.The major changes that appear in the mid-intestine are the considerably reduction of the diameter and the modification of the mucosal folds.It is hypothesized that the flattened folds is evidence of an important added function of compressing digesta into strips,thus creating an air channel above the digesta in the saccular spiral regions.However,the mucosa of the posterior intestine is lined with increasing number of mucin cells,providing the evidence of enclosing the feces in mucus clums to aid defecation.In addition,the arrangement of microfolds and the penetration of blood capillaries in between the columnar epithelial lining in the dorsal region of posterior intestine and rectum unequivocally suggested its respiratory role.The feeding study showed that the GAB frequency increased and appeared two peaks at 2 h and 6 h after feeding.The two peaks respectively corresponded to the fast movement contents of anterior intestine and frequent excretion.In the normal condition,the intestinal contents emptying time was 8 h.Inhibition of GAB could not only reduce intake,but also obviously slow intestinal contents moving.Besides defecation behavior reduced by 85%.X-ray pictures showed that the contents of the anterior intestine presented semiliquid state and the gas distributed above the contents in the form of two bubbles;The contents of the middle and posterior intestine presented solid state on the ventral part in long strips and the gas in the form of gas beam above the food;The rectum has been filled with gas,there is almost no food.The breathing rate increased after feeding.Compared to the normal state and inhibit GAB,we could speculate that GAB could promote the movement of the intestinal contents,at the same time it was very important to the excretion.In summary,we concluded that the anterior intestine of M.anguillicaudatus was the main digestion and absorption area;the middle intestine and posterior intestine had the function of compressing food and wrapping it with mucus so as to create an air channel.The structure of the digestive tract coordinated the functions of digestion and respiration,meanwhile GAB could effectively promote the movement of the food as well as excretion of the rectum.