Acidification, warming and cadmium exposure disrupt zebrafish calcium metabolism, bone formation and swimming activity.

Skeletogenesis is a tightly regulated process that is highly sensitive to abiotic factors and environmental change. Any skeletal abnormalities arising in early life can have lifelong consequences. Freshwater fish must cope with increased temperatures and declining pH, as well as with pollutants released into the environment by human activities. Our study aims to determine whether warming modulates the impacts of low pH and the environmental pollutant cadmium on zebrafish skeletal development. Zebrafish larvae were exposed to warming (31.5°C), acidification (pH 4.5) and cadmium (nominal concentration of 0.3 µmol l-1) in E3 medium from 0 till 7 days post fertilisation. Whole-body calcium content and mineralisation of craniofacial structures were reduced by low pH, cadmium, and a combination of both. Warming accelerates all physiological processes, including calcification, and was shown to partly mitigate the disruption of mineralisation induced by acidification. This attenuating effect of warming was found even after accounting for the thermal effects on development by comparing fish at the same developmental stage. In contrast, cadmium-induced disruption was not attenuated by warming. By comparing larval locomotor behaviour, it was shown that the effects of cadmium and acidification on swimming behaviour are dependent on environmental temperature, and occur mainly during the night. However, the combined effects of low pH and cadmium on swimming distance were not modulated by warming. In summary, we found that multiple stressors influence each other and impact calcium metabolism, bone development and swimming behaviour of zebrafish larvae. We found evidence for a mitigation of stressor effects in a warming context.