Osmoregulation, growth, and metamorphosis of Fejervarya limnocharis (Amphibia: Anura) tadpoles in saline environments
2009
Hochschulschrift
Zugriff:
97
The phenomenon that amphibians breed in brackish water is more commonly than originally thought, and they exhibit interesting physiological and ecological adjustments to increased salinity stress. I used Fejervarya limnocharis tadpoles living in brackish water and designed three experiments to study effects of salinity on tadpole survival, growth, development, and metamorphosis and to identify the possible osmoregulatory mechanisms. In the first experiment, I studied salinity tolerance and salinity effects on growth, development, and metamorphosis of F. limnocharis tadpoles. Specifically, I examined if tadpoles exhibited adaptive developmental plasticity when exposed to different salinities. Tadpoles collected on Green and Orchid Islands off southeastern Taiwan were assigned to different salinities: 0, 3, 5, 7, 9, 11, and 13 parts per thousand (ppt). Daily survival, weekly growth, and development of tadpoles were recorded until metamorphosis. More than 50% survived in 9 ppt for over a month. A few tadpoles survived 11 ppt for 20 days, suggesting saline tolerance of F. limnocharis tadpoles is better than most tadpole species studied to date. Tadpoles at 9 ppt had lower survivorship and retarded growth and development (from Gosner stage 26 to 35) compared to other treatments. Tadpoles metamorphosed early at a smaller size when salinity increased, suggesting existence of adaptive developmental plasticity in F. limnocharis in response to osmotic stress. In the second experiment, I studied how changes in salinity affected tadpole survival, growth, development, and metamorphosis. Tadpoles of Gosner stage 26 were initially assigned to either low or high salinities (3 ppt and 9 ppt, respectively). I switched salinity when tadpoles reached Gosner stage 30 (early salinity switch) or 38 (late salinity switch). I had a total of six treatments: constant low salinity (LLL), low salinity with early switch to high salinity (LHH), low salinity with late switch to high salinity (LLH), constant high salinity (HHH), high salinity with early switch to low salinity (HLL), and high salinity with late switch to low salinity (HHL). I recorded tadpole daily survivorship, weekly growth, and development until metamorphosis. Tadpoles that initially assigned to low salinity (LLL, LLH, and LHH treatments) had high survival rates whereas tadpoles that reared at high salinity before Gosner stage 38 (HHH and HHL treatments) had poor survival, even if later switched to low salinity. Growth and development of tadpoles after transfer to low salinity was partially reversible (HLL vs. LLL), indicating tadpoles did not completely recover growth and development after osmotic stress release. In addition, tadpoles of LHH grew and developed better than that of HLL, suggesting initial salinities were important for the growth and development. Tadpoles in the LLH treatment underwent metamorphosis more quickly and at a smaller size than those in the LLL treatment, indicating that tadpoles exhibited phenotypic plasticity in size and age at metamorphosis in response to salinity variation. In contrast, there were no differences in metamorphic traits among the initially high salinity treatments, suggesting the timing of switching from high salinity to low salinity did not affect tadpole metamorphic traits. My results suggest that initial salinities were important for the survival, growth, development, and metamorphic traits of F. limnocharis tadpoles. Furthermore, the growth and development of tadpoles are partially reversible after salinity stress is released. My third experiment was designed to study the effects of salinity switching on tadpole survival, osmolality, water content, and gill Na+, K+-ATPase expression. Tadpoles collected from Green Island were reared in 3 ppt for at least one week before transfer to: 1) 11 ppt; 2) 7 ppt for 24 h and then 11 ppt.; and 3) 7 ppt for 48 h and then 11 ppt. Results showed that tadpoles did not survive >12 h if directly transferred to 11 ppt whereas tadpoles pre-acclimated for 48 h in 7 ppt survived at least 48 h. Then, I reared tadpoles in 3 ppt for at least one week before transfer to: 1) 3 ppt (control); 2) 11 ppt; and 3) 7 ppt for 48 h and then 11 ppt. In the first 6 h after transfer to 11 ppt, tadpole osmolality sharply increased and tadpole water content sharply decreased. Tadpoles pre-acclimated for 48 h in 7 ppt were able to maintain lower and more stable osmolality 3 h after transfer. These tadpoles immediately lost water content, but over the next 6 h gradually regained water and stabilized. These tadpoles had a higher level of relative protein abundance than other treatments. Results suggest that tadpoles that were pre-acclimated to 7 ppt for 48 h was sufficient to activate NKA expression, resulting in increased survivorship and reduced dehydration when tadpoles were transferred to 11 ppt. In conclusion, my results documented high saline tolerance of F. limnocharis tadpoles and demonstrate that salinity could induce adaptive developmental plasticity in tadpoles. Phenotypic plasticity in age and size at metamorphosis in response to salinity may provide a way for tadpoles to adapt to the unpredictable variation in salinity in coastal rock pools. In addition, my results also showed that initial salinities were important for the survival, growth, development, and metamorphic traits of tadpoles. Finally, my results showed pre-acclimation to a medium salinity for 48 h was sufficient to activate osmoregulatory mechanisms that increased survivorship and reduced dehydration when tadpoles were transferred to a high salinity. My results not only shed light on how amphibians adapt to salinity when they face natural or anthropogenic salinisation of wetlands, but also provide important information for the future study of physiological mechanisms of tadpole osmoregulation.
Titel: |
Osmoregulation, growth, and metamorphosis of Fejervarya limnocharis (Amphibia: Anura) tadpoles in saline environments
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Autor/in / Beteiligte Person: | Wu, Chi-Shiun ; 巫奇勳 |
Link: | |
Veröffentlichung: | 2009 |
Medientyp: | Hochschulschrift |
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