| The red-eared turle (Trachemys scripta elegans) is one of the top100endangeredinvasive species. Now it has successfully invaded all over the world except Antarctica, alsoincluding in China in the wild of. It reported that T. s. scripta could live in salt watercanal,saline ponds and elevated salinity in the Lake Pontchartrain Basin in which theenvironmental salinity has increased due to saltwater intrusion in American，and ourfieldinvestigations showed that T. s. scripta was caught in the estuarine to Nandujiang Riverof low salinity (salinity:0.1-26‰) in Hainan province, which shows that T. s. scripta has acertain tolerance to salinity. But the salinity tolerance and osmoregulation in T. s. scripta isunclear. In this paper, we analyzed the digestive enzyme activities, histological structure ofdigestive system, blood osmotic pressure, physiological and biochemical indicators in bloodand urine, and free amino acid at salinity stress, in order to discuss the salinity tolerance andosmoregulation in T. s. scripta. The main results show as follows:1. Salinity effect on T. s. scripta survival rate and digestive physiologyT. s. scripta at an initial average weight of125.60±19.84g were divided into differentsalinities groups of5,15,25,35‰and freshwater. The results showed that when the ambientsalinity was lower than25, T. s. elegans could survive for25d; in salinity25more than2turtles could survive for at least90d, salinity of15and5groups could live above120d.The pepsin activities increased firstly then decreased and reached the maximum atsalinity5, hepatic lipase activity decreased firstly and increased secondly then decreased,reached the maximum at salinity15. Intestinal lipase and amylase activities were weakenedwith increasing salinity. Therefore, we conclude the digestive enzyme activities of T. s.elegans was affected by the ambient salinity, and thus affecting its absorbtion and even thegrowth.After exposed to salinity，the liver and intestine suffered serious damage. Such as livercytoplasm was loose, blood vessels were damaged and hepatolysis,central veins wereexpanded, tube wall edge was not complete, red nucleus were offset or seedless, with theincrease of salinity in the bile duct and the essence of central vein membrane dissolved, and the dissolved range increases, the cytoplasm vacuoles also increase. Under salinity stress, thethickness of circular muscle increased significantly in salinity group.Compared with thecontrol group, small intestinal villus length increased firstly then decreased, with the increaseof salinity small intestinal villus appear obvious deformation, adhesion and bending fracture.So the histological structure of liver and small intestine in T. s. elegans was affected byambient salinity.2. The influence of chronic salinity stress on osmotic pressure and the physiologicaladaptation mechanism in T. s. elegansThe blood osmotic pressure increased with ambient salinity increased, that in the groupof salinity25would reach400mOsm/kg, was about1.5-fold of that in the control group. Thesignificant increase in the blood osmotic pressure is observed and appears to be dueexclusively to an increase in Na+and Cl-concentrations. At30d-stress, the Na+, Cl-, and K+concentration in urine increased rapidly with the ambient salinity increased, which indicatedthat the entry of NaCl mainly be balanced through a renal route. With time elapsed, the urineNa+and Cl-concentration in group of salinity15and25decreased, therefore, levels of Na+and Cl-in the blood inevitably build up to harmful levels, which indicated that the T. S.elegans was able to bear the ambient salinity under15and appeared to be dead under salinity25. The main hormones of blood aldosterone increased firstly then decreased, which showedthat an increased sodium intake resulting in a diminished aldosterone production and inreverse a loss of sodium by diuresis is a very potent stimulus for aldosterone secretion. Themain ion transport enzymes-Na+-K+-ATPase activities of intestine, liver and muscle increasedfirstly then decreased, and they all reached the maximum at salinity5; the Na+-K+-ATPaseactivity of stomach was weakened with increasing salinity significantly. Higher blood osmoticpressure was not only contributed by higher blood ions concentration, but also by higherblood urea. The blood and urine urea concentration increased with ambient salinity increased.Compared with the control group, blood glucose except that in the group of salinity5increased significantly and other groups decreased with time elapsed. T. s. scripta is anosmoregulator that limits the entry of Na+and Cl-, but can also tolerate certain degrees ofincreases in Na+and Cl-in the plasma. When the ambient salinity was lower than15, T. s.elegans could increase blood osmotic pressure by balancing the entry of NaCl with the secretion of aldosterone decreased, and by accumulating blood urea for osmoregulationeffectors, and survive for at least three months.3. Under acute salinity stress T. s. elegans utilization of nutrients and amino acids, regulationof osmotic pressureAfter exposure to salinity environment, the blood glucose concentration increasedsignificantly in salinity group; after24h, the content of hepatic glycogen reduced significantlyand soluble protein content increased significantly since48h, it shows that blood glucose isthe first and most important energy in the process of osmotic adjustment. Liver of solubleprotein content decreased significantly when salinity5groups, T. s. elegans use protein byproviding free amino acids to maintain osmotic pressure under high salinity.We analyzed free amino acid content at salinity stress in T. s. elegans. The resultsshowed that total FAA and some FAA content tended to increase after T. s. elegans.encountered salt water. Alanine, histidine, asparagine, glutamic acid and glutamine had thehighest proportion of total FAA in muscles. The content of alanine, arginine, proline, glycine,winding isoleucine, phenylalanine, lysine, aspartic acid and histidine tended to increasesignificantly with the increase of salinity and the extension of stress time. All these resultsindicated that T. s. elegans. could maintain a stable osmotic pressure during acute salinitystress by increasing the content of FAA and alanine, arginine, proline, glycine, windingisoleucine, phenylalanine, lysine, aspartic acid and histidine are the most important in thisprocess. |
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