The Mediterranean Basin is facing an increasing frequency and intensity of wildfires driven by climate change, raising concern about their off-site impacts on marine ecosystems. This study provides for the first time the effects of wildfire ash on the early developmental stages of the sea urchin Paracentrotus lividus, a keystone Mediterranean species. Environmentally relevant aqueous ash extracts (AEAs; 5 g/L) and their dilutions were tested through spermio- and embryotoxicity assays, assessing responses across multiple levels of biological organization. Fertilization success, larval development and morphology, apoptosis and enzymatic activity were evaluated up to 72 h of exposure. In addition, the Index of Contaminant Impact (ICI) was estimated by classifying the type and frequency of larval morphological anomalies, allowing the prediction of ash concentrations that may pose an ecotoxicological risk to sea urchins. Although fertilization was not affected, early development was significantly impaired, revealing a higher sensitivity of fertilized egg exposure rather than sperm. Choline acetyltransferase (ChAT) inhibition, increased acetylcholinesterase (AChE) activity, and enhanced apoptosis were observed, indicating the activation of inflammatory and stress-related pathways. Alterations in larval morphology and development were found from 25% AEA dilution upwards, being responsible from slight to moderate impact. Overall, these results suggest that wildfire ash represents an emerging threat to the early life stages of coastal marine species such as echinoderms. Furthermore, they support the use of sea urchins as sensitive bioindicators of wildfire-related marine pollution.
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