| Onchidium struma is a kind of shell-less shellfish with high economic value, medicinal value and ecological value which in endangered currently due to environmental pollution, while the salinity is one of the important factors affect their growth metabolism. The objective of this study was to evaluate the effect of salinity on body composition, blood biochemical indicator, immune index, the histology and ultrastructure of the digestive system, distribution of the digestive enzymes and enzyme activity change, apparent digestibility of different feed ingredients, and the influence of osmotic regulation by raising Onchidium struma under different salinities (5,15,25,35,45), and then preliminary explored the Onchidium struma osmotic pressure regulating mechanism, provided the reference for breeding, raising and basic research of Onchidium struma.1 Effects of salinity on growth and muscle nutrients of Onchidium struma.The salinity have significantly influence on the growth performance and muscle nutrients of Onchidium struma. The results showed that:(1) Onchidium struma grew well in salinity 25～45 and the survival rate was over 90%. At the same time, the weight gain rate, specific growth rate and protein efficiency ratio at salinity 25 was significantly higher than that of the rest salinity groups (P<0.05) and the feed conversion ratio was also decreased significantly (P<0.05). (2) With the increase of salinity, the muscle moisture of Onchidium struma decreased significantly (P<0.05), crude ash showed a trend of increasing after an initial decrease, while the effect on muscle calcium and phosphorus content was not significant difference (P>0.05). The content of crude protein was the highest at salinity 25, that is 76.66%, while the content of crude fat was increased significantly in high salinity (P<0.05) and showed the lowest and the highest at salinity 25 and 45, respectively. (3) Most amino acids showed a trend of decreasing after an initial increase with the increase in salinity, and the highest was showed at salinity 25, but the difference was not significant. While the amino acids, such as glutamic acid, aspartic acid and proline, were significantly affected by salinity. (4) The effect of salinity on every kind of fatty acid was significantly different, and the n-3 PUFA and n-6 PUFA increased range from salinity 5 to 35, while n-3 PUFA/n-6 PUFA values gradually increased. Unlike most seawater shellfish, DHA content of Onchidium struma is far lower than EPA. The test results indicated that salinity had a significant effect on the Onchidium struma growth performance and biochemical composition of muscle, suitable salinity can improve Onchidium struma muscle quality and flavor. Combined with the effect of salinity on unsaturated fatty acids of Onchidium struma muscle, the suitable breeding salinity is 15-25.2 Effects of salinity on hemolymph biochemical index and stress index of Onchidium struma.The salinity have significantly influence on hemolymph biochemical index and stress index of Onchidium struma. The results showed that:(1) Blood glucose concentration significantly higher than that of high salinity at lower salinity; Albumin, globulin and total protein content showed a same trend of decline, and achieve the higher when salinity 35; While with the increase of salinity, the triglycerides content of Onchidium struma in hemolymph showed a downward trend, but had no significant effect. (2) Alkaline phosphatase activity presents downtrend with the increase of salinity, while Alanine transaminase presents parabola change with the increase of salinity, and had the highest salinity at 15. (3) Immune index is consistent with the blood biochemical indexes, when the salinity stress, inducing Onchidium struma produces a series of reaction to damage the body The test results indicated that the energy for osmotic adjustment of Onchidium struma in low salinity is more than the high salinity, and too high or too low salinity will damage Onchidium struma hepatopancreas and cause the content of Alkaline phosphatase in the hemolymph.3 Effects of salinity on digestive system organization structure of Onchidium struma.Salinity can significantly affect the pylorus stomach, cardiac stomach, liver, pancreas and intestinal structure of Onchidium struma. With the increase of salinity, pyloric stomach and cardiac cells contracted in different degree, while the hepatopancreas partly dissolved occurring in the low salinity, and cell damaged is serious in the high salinity, cell began to appear cavity in salinity 35-45. In addition, the high salinity has damage to the intestinal villi cells, the intestinal villus will atrophy. With the increase of salinity, Onchidium struma foregut fold highly reduce gradually and present flat when salinity 35～45, digestion and absorption area is significantly reduced by scanning electron microscope. The test results showed that the salinity has remarkable effect on the digestive system structure on the digestive system of high salinity tissue damage is serious, digestion and absorption rate decreased significantly of Onchidium struma, so we can speculate the appropriate breeding salinity of Onchidium struma is 15 from the perspective of the histologic analysis shows.4 Effects of salinity on the digestive enzyme activity, the digestive tract index and intestinal emptying rate of Onchidium struma.The salinity have significantly influence on the digestive enzyme activity, the digestive tract index and intestinal emptying rate of Onchidium struma. The results showed that:(1) Digestive enzymes were distributed in the pyloric stomach, cardiac stomach, hepatopancreas and intestine, while the digestive enzyme activities among these organs were significantly among these organs (P<0.05). The descending order of protease activity of different digestive organs was pyloric stomach (1462.77±13.40 U/g), hepatopancreas (1021.73±15.12 U/g), intestine (369.12±10.70 U/g) and cardiac stomach (266.82±13.87 U/g), while the amylase activity of hepatopancreas was the highest (87.11±1.31 U/mg), followed by pyloric stomach (45.72±1,06 U/mg), intestine (26.52±1.05 U/mg) and cardiac stomach (23.68±0.82 U/mg), and the hepatopancreas also own the highest cellulose activity (2.02±0.01 U/mg), followed by the intestine (1.01±0.01 U/mg), and the pyloric stomach (0.53±0.01 U/mg) was the lowest. (2) Salinity had a significant influence on the digestive enzymes activity (P<0.05). The activity of the digestive enzymes of all digestive organs had a trend of decreasing after an initial increase with the salinity rising except the protease of the hepatopancreas. The amylase and cellulose activity had the highest at salinity 15 and 25 respectively, and lowest activity in salinity 45.(3) The salinity have significantly influence on the part of digestive tract index (ratio of intestinal length to body length, ratio of intestine weight to body weight and ratio of cardia stomach weight to body weight) of Onchidium struma, while the effect on other digestive tract index (ratio of visceral weight to body weight, ratio of hepatopancreas weight to body weight and ratio of pyloric stomach weight to body weight) was not significant difference (P>0.05). (4) there are some differences in intestinal emptying rate of Onchidium struma under different salinity conditions. The intestinal emptying rate was the fastest at salinity 15～25, and the slowest at salinity 45. The results indicated that low salinity (salinity 5～25) could increase the three digestive enzymes activities in the digestive organs of Onchidium struma. It has a high digestive enzymes activity at 15～25 salinity, which will be beneficial to the digestion and absorption and thereby promote its growth. However, the activity of digestive enzymes would be inhibited when salinity above 25. Meanwhile,at salinity 15-25, the digestive tract index of Onchidium struma was lower and intestinal emptying rate of Onchidium struma was faster than other salinity group.5 Effects of salinity on Na+/K+-ATPase activity, the osmolality of pericardial cavity fluid and peritoneal fluid and ion content in Onchidium struma.To explore the effects of salinity on the Na+/K+-ATPase activity, the osmolality of pericardial cavity fluid and peritoneal fluid and ion content (Na+, K+and Cl-) in Onchidium struma, six hundred healthy specimens (wet weight 10.24±2.16 g) were selected and had been raised for 30 days with simulated ecological environments that in different salinity conditions (5,15,25,35 and 45). The results shows:1) The salinity and osmolality of peritoneal fluid and pericardial cavity fluid of Onchidium struma are increased with increasing ambient salinity, and the increased amplitude of peritoneal fluid all greater than the pericardial cavity fluid.2) The content of Na+, Cl- and K+ of pericardial cavity fluid consistent with the changes of ambient salinity and these are all increased firstly then decreased. The concentrations of Na+, Cl- and K+ reached the maximum at salinity 25 and salinity has a significant impact on Na+ and Cl- but with no significant effect on K+.3) There are significant differences on Na+/K+-ATPase activity of different parts in Onchidium struma. The Na+/K+-ATPase in cardia stomach, intestine, epidermis, muscle and pericardial cavity film of Onchidium struma have higher activity. In addition, the Na+/K+-ATPase activity of different parts activated by salinity with some tissue-specificity, there are higher activity of hepatopancreas, cardia stomach and epidermis in low salinity while the pyloric stomach, intestine, kidney and pericardial cavities film in high salinity. These results indicates that the salinity can significantly impact the osmolality and ion concentration of peritoneal fluid and pericardial cavity fluid as well as the Na+/K+-ATPase activity of Onchidium struma. The cardia stomach, intestine and muscle are main organs involved in its ion transport and osmoregulation with strong osmotic adjustment capability.6 Apparent digestibility of nine conventional feed ingredients for Onchidium struma.The apparent digestibility of dry matter, crude protein, crude fat, crude fiber, amino acids, calcium and phosphorus of nine feed ingredients for Onchidium struma were determined. Feed ingredients included fish meal, meat and bone meal, soybean meal, rapeseed meal, cottonseed meal, corn gluten meal, cottonseed gluten meal, wheat meal and corn meal. Taking spirulina feed as the basal diet, test diets consisted of 70% basal diet and 30% test feed ingredients, and added 0.5% Cr2O3 as an indicator. Two thousand four hundred Onchidium struma with the average body weight of (16.82±2.14) g were randomly divided into 10 groups with 3 replicates per group and 40 individuals per replicate, and one control group randomly selected from the 10 ones was fed basal diet. The results showed that the apparent digestibility for dry matter, crude protein, crude fat, crude fiber, total amino acids, essential amino acids, calcium and phosphorus of nine feed ingredients were 33.24%～55.29%,27.30%～79.35%,36.91%～69.81%,34.89%～ 65.04%,25.35%～75.78%,21.54%-72.57%,44.98%-74.64% and 10.42%-32.15%, respectively. The results indicated that corn gluten meal showed the best apparent digestibility for all nutrition in the seven protein feed, followed by cottonseed gluten meal and cottonseed meal. In the two energy feed, corn meal showed higher apparent digestibility than wheat meal, but lower than the corn gluten meal. Thus, it can be seen that corn gluten meal can be used as high quality feed raw material, and it can partially replace spirulina feed in Onchidium struma feeds. At the same time, we can also reasonable collocate and moderate use corn meal and cottonseed gluten meal etc., which were likewise critical feed raw materials. |
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