Ontogeny of the digestive tract in mud loach Misgurnus anguillicaudatus larvae

This study on histological and mucous histochemistry characteristics of the digestive system of loach (Misgurnus anguillicaudatus) was carried out from hatching (0 day after hatching, DAH) until 45 DAH. The peculiar development of both digestion processes and air-breathing functions of the intestine was revealed. At 3 DAH, both the mouth and anus opened along with the first feed. At 4 DAH, lipid vacuoles appeared in the anterior part of the intestine and at 5 DAH the acidophilic supranuclear vacuoles appeared in the posterior part of the intestine. Mucous cells occurred in the buccopharynx and oesophagus after mouth opening and grew both in number and size as larvae grew. At 15 DAH, blood capillaries were found in the posterior part of the intestine. At 20 DAH, as a valve appeared in the intestine, the whole intestine could be divided into anterior, mid and posterior parts. With large numbers of blood capillaries and a very thin wall, a gas–blood barrier formed in the posterior intestine, which indicated that the dual roles of intestine were formed. These results suggested that the air-breathing function of the digestive tract formed from 15 to 20 DAH, which is a critical period for loach larvae.