Structure and function of the digestive tract of the grasscarp

CONCLUSIONS<p/>1. The grasscarp is a stomachless teleost. The intestine does not contain multicellular glands.<p/>2. The relative length of the grasscarp intestine increases from 0,7 x body length in young larvae to 2 x body length in adults. The gut length is the only morphological characteristic to change when the grasscarp turns from carnivorous to herbivorous feeding.<p/>3. After feeding the grasscarp with vegetable food, the increase in gut length is higher than in fishes fed with animal food.<p/>4. Animal food stimulates rapid growth in the grasscarp, also after they are able to ingest plant material. Grasscarp may represent a serious threat to other species as a predator as well as a competitor for food.<p/>5. The intestine of the grasscarp shows a similar regional differentiation as found in other Cyprinids, with a proximo-distal gradient in alkaline phosphatase activity.<p/>6. The anterior gut segment is involved in the absorption of lipids and proteins (and probably also of sugars), which are absorbed in enterocytes after hydrolysis in the lumen. Also in regard to the morphological characteristics of lipid absorption, the epithelium shows resemblance with the epithelium in the small intestine of mammals.<p/>7. A "second gut segment", running from ± 60-85 % of gut length, is characterized by enterocytes with a high pinocytotic activity. These cells are capable of absorbing protein macromolecules like horseradish peroxidase and ferritin.<p/>8. Protein digestion is efficient despite the absence of a stomach. Quantitatively relevant amounts of protein are not absorbed by the "second segment".<p/>9. The "second segment" is not related to the lack of peptic activity in stomachless fishes, since it is also present in <em>Clarias lazera</em> after the stomach has developed and has become functionally active.<p/>10. The stomach of <em>Clarias lazera</em> contains only one type of corpus glandular cells with the morphological characteristics of chief- as well as parietal cells.<p/>11. The intestinal epithelium of the grasscarp represents a cell renewal system and is completely renewed within 10-15 days.<p/>12. In contrast to mammals the proliferative pool of cells consists of functional cells in<br/>grasscarp larvae and juveniles and in <em>Clarias lazera</em> larvae