Effect of salinity acclimation on osmoregulation, oxidative stress, and metabolic enzymes in the invasive Xenopus laevis

Physiological Responses of Juvenile Bullfrogs (Aquarana catesbeiana) to Salinity Stress

Bullfrogs (Aquarana catesbeiana) are increasingly farmed for their high nutritional value and adaptability to intensive aquaculture systems. However, salinity stress due to environmental changes and habitat salinization poses a significant challenge for both wild and farmed bullfrogs. This study examines the physiological responses of juvenile bullfrogs to varying salinity levels (0, 2, 4, 6 ppt) to better understand their capacity for osmoregulation and adaptation to salinization. Juvenile bullfrogs underwent salinity treatments for one week, and various physiological parameters, including digestive enzyme activity, antioxidant enzyme activity, and serum biochemical indicators, were measured. The study revealed that moderate salinity (2-4 ppt) enhanced pepsin and amylase activity while maintaining high survival rates. However, higher salinity levels (6 ppt) impaired antioxidant defense mechanisms and liver tissue, increasing oxidative stress markers such as malondialdehyde (MDA). The results suggest that bullfrogs possess a degree of salt tolerance, which may predict their resilience to future landscape salinization driven by environmental changes. This research provides valuable insights into the osmoregulatory mechanisms of amphibians under salinity stress, addressing a critical gap in knowledge essential for both conservation and aquaculture management.

Physiological and behavioral responses to novel saline conditions in an invasive treefrog

Salinity can be an environmental stressor for anurans, as their highly permeable skin makes them prone to osmotic stress when exposed to saline conditions. However, certain anuran species have colonized areas near saltwater habitats, suggesting an ability to acclimate to saline conditions. Here, we evaluated physiological and behavioral responses to saline conditions in adult Cuban treefrogs (Osteopilus septentrionalis), an invasive anuran found throughout Florida. To examine their response to salinity, adult frogs were maintained in two treatments simulating a freshwater (0.5 ppt) or brackish (8.0 ppt) environment for 6 weeks. To assess their physiological response to this potential stressor, all frogs were submerged in a brackish solution to quantify individual weight change every 2 weeks. We found that frogs maintained in brackish solution lost more weight at Weeks 2 and 6 when compared to Week 0, suggesting that salinity may be an environmental stressor for Cuban treefrogs. Yet, the weight change at Week 4 was similar to the pre-exposure period, which may indicate that constant exposure to salinity may alter their physiological response to saline conditions. To supplement the physiological analyses, we investigated avoidance behavior toward saline conditions by offering individuals a choice between freshwater or brackish environments. Our results showed that Cuban treefrogs chose freshwater environments more frequently and may thus avoid saline ones. This study reveals that salinity may induce plastic and avoidance responses in Cuban treefrogs, potentially allowing them to expand their range into areas typically stressful for most anurans.

Biochemical and osmoregulatory responses of the African clawed frog experimentally exposed to salt and pesticide

Salinization and pollution are two main environmental stressors leading deterioration to water quality and degradation of aquatic ecosystems. Amphibians are a highly sensitive group of vertebrates to environmental disturbance of aquatic ecosystems. However, studies on the combined effect of salinization and pollution on the physiology of amphibians are limited. In this study, we measured the standard metabolic rate (SMR) and biochemical parameters of adult males of the invasive frog Xenopus laevis after 45 days of exposure to contrasting salinity environments (400 and 150 mOsm NaCl) with either 1.0 μg/L of the organophosphate pesticide chlorpyrifos (CPF) or pesticide-free medium. Our results revealed a decrease in SMR of animals exposed to the pesticide and in the ability to concentrate the plasma in animals exposed simultaneously to both stressors. The lack of ability to increase plasma concentration in animals exposed to both salt water and CPF, suggests that osmoregulatory response is decreased by pesticide exposure. In addition, we found an increase of liver citrate synthase activity in response to salt stress. Likewise, the liver acetylcholinesterase (AChE) activity decreased by 50% in frogs exposed to salt water and CPF and 40% in those exposed only to CPF, which suggest an additive effect of salinity on inhibition of AChE. Finally, oxidative stress increased as shown by the higher lipid peroxidation and concentration of aqueous peroxides found in the group exposed to salt water and pesticide. Thus, our results revealed that X. laevis physiology is compromised by salinization and pesticide exposure to both environmental stressors join.