Effects Of Salinity On Survival, Growth, Osmoregulation, And Several Related Physiological Parameters Of The Lined Seahorse, Hippocampus Erectus

Hippocampus erectus is a high valued medicinal and ornamental species which has been considered as a good aquaculture candidate. Until now, we still know very little about the impact of salinity stress on H. erectus. In the natural environment, H. erectus is mainly suffered from low salinity stress due to river runoff or heavy rain. To better understand the responses of H. erectus juvenile to low salinity stress, studies on the effects of salinity on survival, growth, osmoregulation, enzymes and hormones involved in osmoregulation of the lined seahorse were conducted. 1. Effect of salinity on the survival and growth of H. erectusA factorial experiment consists of two factors, salinity change rate(5, 10 /d) and juveniles at different ages(1 day, 20 days, 40 days post release) was conducted. Seahorses survival was examined. Results indicate that there was no significant interaction between the two factors(P > 0.05). Survival was significantly(P < 0.05) different between the salinity change rates(i.e. 5 /d, 10 /d). Salinity tolerance of the juveniles at different ages was not significantly different(P > 0.05). Tests showed that the different ages(1 day, 20 days, 40 days post release) lethal salinity were 6.67, 6.33, 5.83 at deceleration of 5/d, respectively. The different ages(1 day, 20 days, 40 days post release) lethal salinity were 5.67, 5.83, 4.67 at deceleration of 10/d, respectively.According to the effect of salinity on survival of the juveniles, growth of the juvenile at 14, 21, natural seawater was investigated for 40 days. Results show that growth of the juveniloes was not significantly(P > 0.05) different at different salinities, although better growth performance found at 14 treatment. The results suggest that decreasing salinity may be an approp riate way to improve growth of the juveniles. For aquarium hobbists, maintaining the seahorse at lower salinity may reduce the cost of salts for preparing the water. 2.Effect of salinity on the osmotic pressure, ion level and NKA activities of H. erectusIn this study, the effects of acute salinity stress(10、15、20、25、32)on Na+/K+-ATPase(NKA) activity for 0 h、1 h、6 h、12 h、24 h、48 h、96 h, plasma osmotic pressure and ions including Na+, K+, Ca2+ and Cl- in H. erectus juveniles were investigated. The results of one-way ANOVA indicate that salinity significantly influenced the ions’ levels, NKA activity and osmotic pressure of H. erectus juveniles.Plasma Na+ and Cl- of the seahorse juveniles in all treatments started to decline significantly at 1 h, Plasma K+ and Ca2+ of the seahorse juveniles in all treatments started to decline significantly at 6 h, respectively. However, the osmotic pressure of the juveniles in all treatments reduced rapidly, and was significantly different(P < 0.05) between the treatments and the control at 1 h. At 96 h, significant difference(P < 0.05) was found between every two treatments except in 20 and 25 groups. The NKA activity of the gill filament in all treatments reduced first then raised up except in the salinity of 10 treatment. The NKA activity reached the lowest point in the 15 group at 12 h, and at 24 h in the treatments of 20 and 25. After the time points, all of them rised up again to some extent. Significant difference was found in NKA activities between every two groups except between 20 and 25 groups. The isotonic point of the juveniles was 317.13 m Osm·kg-1, equivalent to salinity of 12.05, isoionic point of Na+ and Cl- was 96.48 mmol·L-1 and 113.64 mmol·L-1, equivalent to salinity of 8.82 and 10.13, respectively. The results presneted here suggest that H. erectus possesses the ability to live in the salinity of >10. 3. Effect of salinity on the hormones involved in the osmoregulationIn the present study, the variation of osmoregulation hormones including GH, COR, PRL, T3 and T4 after acute salinity stress were examined. At the test, seahorse were the transferred directly from natural seawater to low-salinity waters, regularly take blood samples were measured. The results of one-way ANOVA indicate that salinity significantly(P < 0.05) influenced on GH at 24 h, 7 d, 15 d and 21 d. Salinity significantly(P < 0.05) influenced on COR at 1 h, 96 h and 7 d. Salinity significantly(P < 0.05) influenced on PRL、T3、T4 after 1 h.During this experiment, the GH level of H. erectus plasma in each treatment first rose up,then reduced. O n day 21, the GH level reduced as salinity reduced, significant difference was found between each two groups except between 10 and 15, and 25 and 32. However, for COR, significant elevation was found at 1 h after treatment. On day 7, the COR level reduced as the salinity declined. For the PRL level, significant elevation was found at 6 h after treatment. On day 7, the PRL level rose up as the salinity reduced, and significant difference was found between each two groups except between 10 and 15, adn between 25 and 32. Finally for the T3 and T4 level, obvious fluctuation was found at 6 h after treatment. On day 7, T3 in salinity of 10 and 15 groups was higher than in the other groups. T4 in salinity of 15 and 20 groups was higher than in the other groups. The results presented here suggest that H. erectus can adapted to hypotonic environment rapidly due to quick activation of GH, COR, PRL, T3 and T4 hormones involved in osmoregulation. 4. Effect of salinity on the immune index of the juvenile seahorseSOD, CAT and serum MDA, AKP, ACP of H. erectus liver were examined to measure the immunity of the seahorse juveniles in different salinities(10、15、20、25、32). The results of one-way ANOVA indicate that salinity had significant effect on SOD(P < 0.05) since 6 h. Salinity had significant effect on CAT(P < 0.05) at 12 h and 96 h. Salinity had significant effect on MDA(P < 0.05) at 6 h, 12 h, 96 h. Salinity had significant effect on ACP and AKP(P < 0.05) since 6 h.SOD in salinity of 10 significantly changed(P < 0.05)under acute salinity stress, but was not significantly different in the other groups(P > 0.05). At 96 h, SOD increased in lower salinity. Salinity had significant effect(P < 0.05)on SOD except the salinity of 25. Compared with the control group, CAT in the salinity of 10 had significant difference at 6 h, and at 12 h in salinity of 15 and 20(P < 0.05). At 96 h, there was significant difference(P < 0.05) except salinity 25. CAT increased with salinity decreased. MDA content in each group changed at 6 h. MDA in the salinity of 10 and 15, showed a significant difference(P < 0.05) at 6 h, and at 12 h in the salinity of 20 and 25(P < 0.05). At 96 h, there was significant difference(P < 0.05) between salinity groups and the control group except the salinity of 25. AKP of the juveniles in each experimental group changed significantly(P < 0.05) at 6 h. At 96 h, AKP in the salinities of 10 and 15 was not significantly different(P > 0.05), however, there was significant differences among the other groups(P < 0.05). In the salinity of 15, AKP was significantly different(P < 0.05) at 12 h while others at 6h. At 96 h, ACP gradually increased with increasing salinity. The results show that juvenile H. erectus can adapt hypotonic environment through adjusting antioxidant and dephosphorylation. In conclusion, the results presented here suggest that H. erectus juveniles could be farmed in relatively low salinity environment, thus provide fundamental information for improving the culture protocols.