Rising sea levels, drought and land-use change increase salt levels in freshwater coastal environments. Maintaining an optimal amount of salt in the body is crucial for animals but incredibly hard to do, particularly for those adapted to low-salt freshwater environments that suddenly face a rapid salt increase. Because of their highly sensitive skin, toads, which reproduce and develop in fresh water, are often especially vulnerable to salty water. Léa Lorrain-Soligon, from the Centre d'Etudes Biologiques de Chizé, France, collaborated with colleagues from the same institution and Ligue pour la Protection des Oiseaux, France, to investigate the impacts of salty water on coastal toads (Bufo spinosus) across multiple life stages, from egg laying through to early tadpole development.
It takes a lot of energy to survive in salty water, even for animals that are used to it, so the scientists wondered whether dealing with extra salt would drain the toads’ energy, leaving less available to reproduce. To explore this, the team captured breeding pairs of male and female toads and set them up in tanks containing high or low (freshwater) amounts of salt. Then, they measured the size and number of eggs the toads laid in the two water conditions. While the toads laid the same number of eggs regardless of the water type, those in saltier water ended up with much smaller eggs, indicating that the higher energy costs of dealing with the salt had, indeed, led to lower-quality eggs.
Next, the team wanted to know whether the larger size of the eggs laid in fresh water would translate into healthier, more robust tadpoles. They split the two batches of eggs (laid in salty or fresh water) into four groups: half were kept in the same water, and half were moved to the opposite water type. Not surprisingly, eggs left in salty water struggled. But there was a catch: several eggs that were moved from salty to fresh water were healthier and the tadpoles grew better after hatching. Interestingly, the opposite could not be said for the eggs laid in fresh water, then placed in salty water before hatching. Those tadpoles were much smaller and more likely to hatch with deformities than any other group. These results demonstrated that exposure to salt is tough on reproducing toads. Still, if the toads can find a way to lay their eggs in fresh water, the offspring have a high chance of surviving.
Once the eggs had hatched, the researchers took the young tadpoles and, once again, split them into salty or fresh water, leading to eight different groups that had experienced various combinations of fresh and salty water throughout development. The scientists measured how well the tadpoles grew under the different combinations of fresh and salt water exposure and found that the tadpoles kept in fresh water their entire lives were the best off – they grew fastest and were bigger than those grown in salty water. Interestingly, tadpoles that experienced salt early on but were subsequently raised in fresh water were almost able to catch up to the larger size of the freshwater-only group, presumably by boosting their growth rate to compensate for their smaller size.
These findings underscore the significant challenge salt poses to toads, particularly during critical early life stages. The study's unique and intricate design uncovered something remarkable: tadpoles could partially rebound from early salt exposure when placed back in fresh water. Clearly, the timing of salt exposure is a key factor impacting toad survival rates across life stages. Results like these help us identify when, where and how salt affects vulnerable animals, which, in turn, tells us when to intervene to help them.
