Growth and diet dependant structural adaptations of the digestive tract in juvenile grass carp (Ctenopharyngodon idella, Val.)

The morphology of the intestinal epithelium is described and compared with published data. In the gut of this stomachless fish, three zones can be distinguished based on light and electron microscopic observations of absorptive cells: a rostral zone with numerous fat droplets, a second zone with large P.A.S.‐positive supranuclear vacuoles and pinocytotic vesicles, and a third zone without these characteristics. In these zones deep invaginations of the cell membrane is a feature of the basal part of the cells. The presence of pinocytotic vesicles in the second zone is considered evidence of the uptake of undigested protein‐like substances. Juvenile grass carp from 15 to 335 days, fed on either animal or vegetable food, were measured. The growth rates indicate that animal food stimulates rapid growth. The data also show a vegetable diet causes a slight increase in relative gut length, mainly in the length of the first zone.