| 1. Tolerance of southern flounder to low salinity and acclimation to fresh waterSouthern flounder was an euryhalinous fish which could even live in fresh water. In this paper, the tolerance of the larvae and juveniles to brine or fresh water at different ages and sizes were studied. The larvae being hatched within a day could live for only 5 days under water salinity of 5‰. The 30-day larvae during metamorphosis all died within 4h10min after being put into fresh water directly. The survival rates of acute-acclimation-experiment of 55-day, 70-day and 90-day age groups in 72h were 60.00%, 96.25% and 96.00% respectively, while the survival rates of them in 24h-acclimation-experiment were 80.00% within 7 days, 86.67% and 98.95% within 9 days. It could have more ill effects on survival and feeding in the acute-acclimation-experiment than the 24h-acclimation-experiment. The capability of osmoregulation and the tolerance to fresh water of southern flounder increased with age. At the same age, there was no remarkable difference in the survival rates of different total length (TL). The survival rate could be above 98% when acclimating them slowly to fresh water at the 90th day (after being hatched) or later.2. Study on tissue structure alternations of branchial filaments, skin and kidney during the salinity variationSouthern flounder was a rare and economical marine teleost, and it could be cultured in freshwater. This paper studied its tissue structure alternations of branchial filaments, skin and kidney during the salinity variation (from sea water to fresh water). Two groups, which were acute salinity variation group (acute group) and 24h salinity variation group (24h group), were designed. Fish were transferred from 30ppt～31ppt of sea water to fresh water in two ways, one of which was fish being transferred from sea water to fresh water directly and the other was fish being transferred from sea water to fresh water gradually in 24h. Tissue samples were collected and fixed during transformation. Slices were made and investigated soon. During salinity variation, branchial filaments and skin excrete more mucus than before and cells become more active. Glomerulus gets bigger at the end. The electron density at the edge of the epidemic cells in branch leaf of normal southern flounder was almost symmetrical. Chloride cell located in the base and surface of branch leaf, some were alone and some were assembled by 2~3 cells. There were a lot of oval mitochondrions in chloride cell, and the cristae were strip. There were several lipid droplets and salt granules in chloride cells. At the 2nd hour of acute salinity variation, plenty of goblet cells appeared and much mucus was excreted. At the 3rd day, branch leaves in the branchial filaments were conglutinated and resumed at the 8th day. However, the gill of 24h salinity variation secreted much mucus at the 36th hour, and severe conglutination appeared in branch leaves. At the 5th day, configuration of gill began to resume. The electron density at the edge of the epidemic cells in branch leaf of southern flounder during 24h salinity variation increased and the activity of chloride cell decreaed. Generally, the skin of southern flounder could be divided to three layers which were epidermis, hypodermis and muscle layer. There were 3 to 7or8 layers of epidermic cells and in which there were few goblet cells and granular cells. Hypodermis contained about 20 layers of interlaced collagenous fiber. Under the squama, there were 2~5 stratums of epidermic cells and they were connected closed by many desmosomes between them. At the 2nd hour of acute salinity variation, the number of goblet cells increased in epidermis, and at the 4th day, many granular cells appeared and few epidermic cells desquamated. Epidermis resumed at the 12th day. However, the epidermic cells under the squama were active and had some vacuoles around the nucleolus. The diameter of glomerulus in normal cultured southern flounder were 40～50μm. There were developed microvilli on the dissociative side of epidermic cell of proximal convoluted tubule (PCT). The electron density in nucleolus was symmetrical, and there were many round mitochondrions with loop or irregular cristae. Developed membrane drape was near basement membrane. The borderline between two epidermic cells of PCT was mistiness while it was distinct in distal convoluted tubule (DCT). Epidermic cell of collecting tubule was low-columned and the diameter of tubule was bigger than PCT or DCT. No remarkable change was investigated expect the increasing diameter of glomerulus at 12th day of salinity variation. Metabolism of cells in DCT was more active. Results indicated that, alternation of tissue configuration and structure of southern flounder was greater from the beginning to 4th day of salinity variation, and the tissue structure resumed at the 12th day generally.3. Effects of salinity variation on some enzymes of southern flounderEffects of salinity variation on some enzymes, such as Na+-K+-ATPase in gills, Superoxide dismutase (SOD) and glutamic oxaloacetic transaminade (GOT) in serum of southern flounder Paralichthys lethostigma were assessed. Two groups, which were acute salinity variation group (acute group) and 24h salinity variation group (24h group), were designed. Gill and serum samples were collected during the transformation and stored below -80℃before being measured. Results showed that three enzymes of samples in both groups differed significantly from the control (P<0.05) except SOD activity in acute group because it was restrained by severe change of salinity (P>0.05). Na+-K+-ATPase activity increased and formed two peaks. This indicated the active regulation during the transformation. Compared to 24h group, SOD activity of acute group seemed being restrained. GOT activity degraded in 1st day in both groups. GOT activity of 24h group reached the peak value earlier than that of acute group. After that, the activity of both groups decreased gradually and below the initial level in the 12th day (P<0.01). The peak values of SOD and GOT activities were higher in 24h group than acute group. The exposure of acute salinity variation appeared more adverse. The activity of three enzymes inferred an adjustive period during the salinity variation which was from beginning to the 8th day.4. Study on alternation of osmolality and ionic concentration in blood and urine of southern flounder during fresh water adaptationAlternation of osmolality and ionic concentration in blood and urine of southern flounder during fresh water adaptation was studied. Two groups, which were acute acclimation group and 24h acclimation group, were designed. Blood and urine samples were collected carefully and seriously. The osmolality and ionic concentrations were measured and analyzed. Results showed that the osmolality of urine declined evidently and fleetly while the osmolality of serum declined gradually and then came back. There was an adjustive period of southern flounder to adapt fresh water which was the time from the beginning to the 8th day. During the adjustive period, the sodium concentration and potassium concentration of urine achieved the peak value from the 4th day to 6th day, while calcium concentration decreased remarkblely and rapidly (P<0.01) which might related to osmolality decrease of urine. The sodium concentration and potassium concentration of serum decreased slightly at last, but calcium concentration declined during the adjustive period and then came back to the initial level. The alternations of ionic concentration indicated that optimal environmental parameters should be supplied during the adjustive period.5. Results and suggestionsTolerance to low salinity of larvae and juvenile southern flounder, technology of acclimation to fresh water and effects on southern flounder by salinity variation (sea water to freshwater) were studied in this paper. Results showed that the fresh water acclimation of 90d-old flounder could obtain the optimal impact. To adapt the fresh water environment, they secret mucus, change the activity of osmotic enzymes and control the absorption and excretion of ions during salinity variation. Based on the research, the optimal culture condition should be supplied for at least 8 days during the freshwater acclimation. No flounder was cultured inland in our country, but according to its great adaptation of freshwater, we can extend the culture district of southern flounder to inland.
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