From the Field to the Lab: Physiological and Behavioural Consequences of Environmental Salinity in a Coastal Frog

Salty surprises: Developmental and behavioral responses to environmental salinity reveal higher tolerance of inland rather than coastal Bufo spinosus tadpoles

Salinization is predicted to intensify due to climate change, impacting biodiversity and ecosystem functioning. Amphibians, particularly embryos and larvae, are highly susceptible to environmental salinity. Yet, local adaptation may cause differing vulnerabilities between coastal and inland populations. In this study, we investigated the physiological, behavioural, and life-history responses to environmental salinity (0, 2 and 4 g l-1) of embryos and larvae of a widespread amphibian species (spined toad, Bufo spinosus) from salt-exposed (coastal) and salt-free (inland) populations. Moderate salinity (4 g l-1) altered embryonic and larval development in both populations, causing increased malformations, decreased body size and survival, and altered behavior, but did not affect telomere length or oxidative status. Individuals exposed to low salinity (2 g l-1) performed better across most traits. However, moderate salinity had stronger negative effects on coastal individuals, indicating a lack of local adaptation and overall lower performance compared to their inland counterparts. These findings suggest that increasing salinity will have varied impacts on organisms depending on their population origins and developmental stages.

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.

Variations of salinity during reproduction and development affect ontogenetic trajectories in a coastal amphibian

Although coastal ecosystems are naturally submitted to temporal variations of salinity, salinization has been increasing over time threatening coastal biodiversity. Species that exploit such habitats can thus be exposed to brackish water at different life stages. However, the impacts of variations of salinity on wildlife remain poorly understood. This is particularly true for coastal amphibians, due to the strong dependency of early life stages (embryos and larvae) on aquatic environments. In order to investigate the effect of salinity during egg laying and embryonic and larval development of coastal amphibians, we used a full-factorial design to expose reproductive adults, eggs, and larvae of coastal spined toads (Bufo spinosus) to fresh (0 g.l-1) or brackish water (4 g.l-1). At egg laying, we evaluated parental investment in reproduction. During embryonic and larval development, we assessed effects on survival, development, and growth. We highlighted strong effects of environmental salinity on reproduction (reduced egg laying time, marginally reduced egg size, and reduced investment in reproduction). Responses to salinity were highly dependent on the developmental stages of exposure (stronger effects when individuals were exposed during embryonic development). These effects carried over when exposure occurred at egg laying or during embryonic development, highlighting the importance of the environmental conditions during early life on ontogenetic trajectories. We also highlighted partial compensation when individuals were transferred back to freshwater. Whether the magnitude of these responses can allow coastal biodiversity to overcome the observed detrimental effects of salinization remain to be assessed.