Seasonal Variations Of Nutritional Composition, Food Resources, And Digestive Physiology In Sea Cucumber Apostichopus Japonicus

In this dissertation, the seasonal variation of nutritional composition, food resources, and structure and function of digestive tract in Apostichopus japonicus were investigated. The effects of water temperature on digestive physiological characteristics in A. japonicus were also studied. The main results were as follows:1. The current status of sea cucumber farming in China was reviewed. The nutritional composition, the digestive physiological characteristics, and aestivation of sea cucumbers were reviewed. Furthermore, the future studies in the area were suggested.2. The seasonal variations of nutritional composition of A. japonicus were investigated from July 2006 to June 2007. The basic nutrition values, amino acid, and fatty acid levels of body wall were significantly affected by seasons. The composition evaluation of the amino acid (AAS) was between 75.79 (in January) and 85.17 (in August). Saturated fatty acids (SFA) and polyunsaturated fatty acids (PUFA) yielded significant seasonal variations. SFA showed the highest contents, whereas, PUFA showed its highest value in August. Conversely, the maximum PUFA and the minimum SFA appeared in January. The principle component analysis showed the nutrition quality of A. japonicus was best in winter (November and January).3. Seasonal food resources of A. japonicus were determined by fatty-acid biomarker approach from July 2006 to June 2007. In January, the main diets of the sea cucumber were diatom, flagellate or protozoan, brown alga, and bacteria (Proteobacteria and Gram-negative bacteria). The main food resources were diatom, flagellate or protozoan, and Chlorophyta in March. The proportion of Chlorophyta was maximum in the food composition of sea cucumber in June. In July, bacteria (Cytophaga-Flavobacteria and Gram-positive bacteria) and Chlorophyta were the main food resources of A. japonicus. Cytophaga-Flavobacteria and Gram-positive bacteria were the most important food resources in August and September. In October and November, A. japonicus feeds mainly on brown alga, and bacteria (Proteobacteria and Gram-negative bacteria).4. Seasonal variations of digestive enzyme activities and digestive tract index in A. japonicus were studied from June 2006 to June 2007 in situ. All gut dimension parameters and digestive enzyme activities except for alginase yielded significant seasonal variations. The digestive tract significantly degenerated during aestivation with relative gut mass (RGM) 8.2% and Zihler's index 28.0% on Sep 5 respectively of the corresponding maximum values over the year. Carbohydrate enzymes (amylase, cellulase, and alginase) showed the lowest activity, whereas, pepsin showed its highest activity in September. Accordingly, high gut dimensions, high carbohydrase and trypsin activities, and low level of pepsin activity in A. japonicus exhibited in non-aestivation seasons. Zihler's index was closely related to the properties of food consumed by A. japonicus. RGM can be used to determine the feeding states of the sea cucumber.5. To investigate the effects of water temperature on gut mass and digestive enzyme activities in sea cucumbers A. japonicus, relative gut mass, amylase activity, lipase activity, pepsin activity, and trypsin activity were evaluated at water temperatures of 7 oC, 14 oC, 21 oC and 28 oC for a duration of 40 days. RGM of sea cucumbers in 7 oC were higher than the other treats. RGM significantly decreased after 40 days experiments at 21℃, and it markedly decreased over the whole experiments at 28℃, yet no significant effect of duration was observed on RGM at 7℃and 14℃. Principle component analysis showed that the sea cucumbers on the 40th day in 21℃group and in the former 20 days in 28℃group were in the prophase of aestivation; the sea cucumbers aestivated at about 30-40 days after the experiments started at 28℃. In the prophase of aestivation and aestivation phase, RGM and the activities of amylase, lipase, and trypsin decreased. However the pepsin activity sharply increased in the two phases. It is concluded that the temperature effect on the digestion of A. japonicus is comparatively weak within a proper range of water temperature, and aestivation behavior of the sea cucumber was accompanied by significant changes of RGM and digestive enzyme activities. Accumulated temperature may be a more important factor to trigger the aestivation.