Contribution No. 152 of the Aquaculture Department, SEAFDEC.

Midgut gland cells of Penaeus monodon postlarvae were investigated by electron microscopy after starvation and refeeding with different diets. Well nourished postlarvae could be starved for 5 days without irreversible detriment. They recovered easily on a good diet. Only R cells were affected by a short starvation period. After 13 days of food deprivation the structures of all cell types were disintegrated. The postlarvae were able to starve for a maximum of 15 days. The most diversified ultrastructure was obtained by refeeding with cod liver oil and 2s (mixed diet). Casein was not well utilized. Sucrose was the poorest diet. The midgut gland of Penaeus monodon seems to be lipid oriented. Only fat was used as storage material; glycogen was lacking. Different ultrastructures were induced in R cells by a given diet after longer starvation periods. The reestablishment of their ultrastructure by means of a definite diet seems to be impossible after a certain period of starvation (point of no return). R cells are very sensitive to different diets. B cells show slight reactions, while F and E cells remain relatively unchanged. This indicates that R cells could be used to monitor the nutritional value of prawn diets in aquaculture.