Low salinities impair heat tolerance and alter reproductive traits with no implications for survival in a marine annelid from coastal habitats

Climate change is expected to intensify salinity and temperature fluctuations in estuarine and coastal habitats. However, the impact of salinity variation on ectotherms’ thermal tolerance remains underexplored, limiting our understanding of the interplay between these stressors. This study investigated salinity-dependent changes in life history traits (survival, growth rate, fecundity, and egg size) over a 33-day exposure to a salinity gradient (12, 17, 22, 27, 32, 35, 40, 45, 50) in the marine annelid Ophryotrocha labronica. Individuals that survived the salinity treatments were assessed for their upper and lower thermal tolerance limits. Female individuals, the target of the study, exhibited a relatively broad optimal salinity range for reproduction and growth (22–35), with comparable survival rates across the remaining salinity treatments (12–45). Declining performance was observed towards the extremes of the gradient, with the highest salinity (50) proving lethal within 2–3 days of exposure. The decline in fecundity observed at the lowest (12) salinity was associated with a reduction in upper thermal tolerance limits and growth rates, as well as an increase in egg size. Cold tolerance remained consistent across treatments, while heat tolerance remained unchanged at the highest salinities. Our study demonstrates that salinity variation affects the heat tolerance and reproductive traits of O. labronica, with adjustments primarily driven by low salinities. These adaptations may contribute to the species’ extended survival under suboptimal conditions. Our findings contribute to the debate on the role of salinity-dependent thermal tolerance in determining the ability of marine organisms to cope with changing temperature conditions.