| 1. Effects of salinity stress on the survival, growth and correlation of characters of juveniles of milkfishThe growth and development of teleost were more or less directly affected by the environmental factors. Salinity is one of the important ecological factors. The influence of salinity on the survival and growth of marine fish has been demonstrated in many studies. However, distinct results were displayed due to the different abilities of tolerance to salinity among fish species. The milkfish, which has a nice marine habitat and an efficient osmoregulation, is widely distributed throughout tropical and subtropical Indo-Pacific This species occurs naturally and is commercially cultured in fresh, brackish, and oceanic waters as well as in hypersaline lagoons. In the present study, the influences of salinity stress on the survival and growth of milkfish juveniles were assessed and the effects of morphometric attributes on body weight were also evaluated. Juveniles of milkfish(mean weight and total length: 0.215±0.083g; 2.850±0.356 cm, respectively) were reared at 0(fresh water), 10, 20, 27(sea water) and 35 ppt without acclimation, in triplicate, for 30 days’ cultivation. The survival rate was recorded and the body weight and morphometric attributes of each individual were measured. Results showed, the lowest survival rate was founded at salinity 0 group(73.33 %) when compared with other groups(> 90 %). The juvenile stage is a special stage during the life history of fish, the juvenile developed not as well as the adults, and cannot adapt to the short-term drastic changes of external osmolality, which may be one of the reasons of the high mortality rate at salinity 0 group. O n the other hand, however, the highest rates of weight gain rate(573.94±231.58) % and specific- growth rate(6.23±1.16) % were obtained at salinity 0 group(P < 0.01). Meanwhile, there was no significantly difference among other groups(P>0.05). The higher weight gain rate and specific-growth rate were also found in the intermediary groups with salinity 20 and 27. It has been suggested that the marine fish often has a better growth rate in water with lower salinity for the reason of fish spending less energy on osmoregulation in such conditions. Although it has not been reported that milkfish require a fresh water environment for any part of its life cycle, it did showed well growth performance at fresh water in many studies. Furthermore, recent studies indicated that the true proportion of energetic cost of osmoregulation remains under debate as thought before. The total length(x1), body length(x2), tail length(x3), trunk length(x4), head length(x5), postorbital length of head(x6), eye diameter(x7), snout length(x8), caudal peduncle height(x9), and body height(x10) were measured by QCapture Pro 6.0 software after the cultivation. The correlation analysis, path analysis and determination coefficients were calculated by SPSS 13.0 software. Significant differences(P < 0.01) were found in all correlation coefficients between each morphometric attributes and the weight. The results of path analysis showed that the traits which had the strongest effect on body weight were total length, body length and body height, and the direct effects of morphometric attributes on body weight were different at salinity stress. The analysis of determinant coefficients revealed that the determinant coefficient of body length was the largest(d = 0.94930) in the salinity 0 group. For salinity 10 group, the determinant coefficients of total length and body height were larger compared with other traits, among which total length had a predominant determinative effect(d = 0.33841). For salinity 20 group, the determinant coefficients of total length, body length, and body height were larger, among which body length had a predominant determinative effect(d = 12.15845). For salinity 27 group,the determinant coefficient of total length was the largest(d = 0.89838). For salinity 35 group,the determinant coefficients of body length, eye diameter, snout length, and body height were larger, among which body length had a predominant determinative effect(d = 0.32091). In addition, the best multiple linear regression equations were established at salinity stress, which will be helpful to the cultivation of milkfish. 2. The morphological structure of mitochondrion-rich cells in the gills of juveniles of the milkfish and its changes in various salinitiesThe distribution and changes of the mitochondrion-rich cells in gills and changes in gills of juveniles of seawater-acclimated milkfish under different salinities(0、10、20、27 and 35 g/L)were studied by light micrograph, transmission electron micrograph and scanning electron micrograph. mitochondrion-rich cells with large number of mitochondria and large nucleus were oval or ovoid in gills. Two types of mitochondrion-rich cells in gills of juveniles of milkfish were observed when acclimated to various salinities:(1) Type A mitochondrion-rich cells with apical crypts and larger mitochondria;(2) Type B mitochondrion-rich cells which were a single ce ll with smaller mitochondria. With the reduction of salinity, the number and size of mitochondrion-rich cells and mitochondria was reduced, electron density was decreased, and the apical opening of mitochondria was narrower, or closed. When milkfish were transferred to fresh water, hypertrophy was observed on branchial lamellae, and type B mitochondrion-rich cells with large area apical membrane appeared as a flat or wavy disk with a mesh- like structure on its surface were observed on branchial lamellae. The results indicated that type A mitochondrion-rich cells would be rich and well development in hypertonic environment. Type B mitochondrion-rich cells would be rich in hypotonic environment. So type A and type B mitochondrion-rich cells are considered to be seawater-acclimation type and freshwater-acclimation type respectively, for their structural characteristics adapted in ion intake and secretion functions. Meanwhile, the existing of different types of mitochondrion-rich cells in gills can make euryhaline marine teleost adapt to a wider range changes of salinities. 3. Survival, growth and changes in the gills of juveniles of the milkfish under fresh water acclimationThe survival, growth and changes in gills of juveniles of milkfish under fresh water acclimation were studied. At the end of the experiment, the survival rate, weight gain rate and Specific- growth rate are recorded, the gill of juveniles are dissected and observed. The results were compared and analyzed with the juveniles under fresh water without acclimation and sea water. The results showed that, the highest survival rate(100±0)%, weight gain rate(664.96±113.07)% and specific-growth rate(6.77±0.52)% of juveniles were obtained under fresh water acclimation. Large numbers of mitochondrion-rich cells were observed on branchial lamellae of juveniles under freshwater. It may be one of reasons for the high survival rate of juveniles of C.chaos under the condition of fresh water. 4. The identification and location of endocrine cells in the digestive tract of milkfish by immunohistochemical techniqueSeveral neuropeptides and hormones have been identified and located in the digestive tract of milkfish by immunohistochemical technique. The esophagus, stomach(cardiacus, fundic and pyloric regions), pyloric caeca and intestine(anterior, middle, posterior regions and rectum) were divided from the digestive tract of milkfish. The distribution and frequency of the endocrine cells produced these neuropeptides and hormones including gastrin(GAS), cholecystokinin(CCK), leu-enkephalin(L-ENK), neuropeptide Y(NPY) and somatostatin(SST) were described in these segments. Endocrine cells containing all neuropeptides and hormones immunoreactivities were present in the anterior intestine region, but no reaction was found in oesophagus. Only a few L-ENK- and SST-immunoreactive cells were detected in the p yloric caeca. Large numbers of SST-immunoreactive cells were observed in the stomach with no immunoreactivities to other neuropeptides and hormones. Two different types of endocrine cells were founded in the intestine and pyloric caeca. O ne type of cells with a characteristic elongated spindle was located in the epithelium of the mucosal folds. Another type with characteristic round and spherical shapes was located in the middle or basal of mucosal folds. These results indicate that the anterior intestine region may be the main region of food digestion and absorption, and the neuropeptides and hormones regulate the food intake process by different ways in milkfish. |
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