Effects Of Starvation On The Digestive System Of Brachionus Plicatilis

This article summarizes the current studies on morphology, classification, effects of starvation, and aquaculture application of rotifers Brachionus plicatilis. By exploring appropriate paraffin section technology for Brachionus plicatilis, combined with live observation, changes in the morphology of digestive system of rotifers under fed or starved condition are studied. Changes in activities of protease, cellulase and amylase are also discussed. The purpose of this study is to broaden our knowledge on rotifer digestive system so as to provide guidance for its future cultivation and feeding. The results are summarized as follows:1. Before paraffin section preparation, rotifers are first anaesthetized with ethanol so that the rotifers stretch naturally and lose reaction to external stimuli. The rotifers are then fixed in Bouin's fixative for 6-12h. During the process of specimen collection, anesthesation, fixation and staining, samples are all transferred by pipette under dissecting microscope. Before dehydration, the rotifers are wholly stained with hematoxylin. Alcohol concentration gradients are controlled at a minimized level when rotifers are dehydrated. Both alcohol dehydration and xylene clearing duration are controlled within 5min, and the wax penetration time within 10-15min.These technical improvements efficiently minimize material loss and section stiffness, resulting in desirable sections with rotifers in natural postures.2. Stained with hematoxylin eosin, paraffin sections of the rotifer digestive system under fed and starved condition are studied under light microscope. During starvation, the most significant changes of the digestive system are the stomach and the gastric gland. With starvation, the stomach shrinks gradually and eventually turns into a tubular-like construction. The gastric gland, initially horseshoe-shaped, becomes slender and far from the stomach with starvation. The gastric gland contracts to a dendritic shape in the end.Paraffin sections show the size of the stomach cells decreases from 20×25μm to 12×17μm after 12 days of starvation. Stained with hematoxylin eosin,the cytoplasm appears less acidophilic and contains fewer vacuoles with starvation. The cytoplasm of the gastric glands also shows less acidophilic and lighter staining. Fewer particulate matters are seen in the cytoplasm and the distance between the nucleuses becomes smaller. The size of nuclei did not change significantly but the nucleoli tend to become smaller during the starvation.3. Several digestive enzyme activities of the rotifer Brachionus plicatilis during starvation and re-feeding are studied at 25℃. The results show that activities of amylase and cellulase remain at high levels during the whole process. During starvation, activities of amylase and cellulase decrease dramatically in the first two days, and then maintain stable, while the activities of pepsin and tryptase decrease slowly. When the egg ratio is less than 0.09, the activities of all four digestive enzymes are lower than those under normal condition. After one-day re-feeding, the egg ratio increases from 0 to 0.21, the activities of tryptase and cellulase increase to normal level, and the activities of pepsin and amylase are higher than under normal condition. It is found that the activities of these four digestive enzymes are not correlated to the egg ratio when rotifers are fed. Stomach and gastric glands restore their normal shapes after one-day re-feeding.Higher activities in digestive enzymes could be achieved if rotifers are properly starved and then re-fed. When fed into larvae tanks, their digestive enzyme contents could be maintained by supplying rotifers with microalgae.