Historically, marine ecotoxicology has relied on a limited range of model organisms, primarily sea urchins (Echinoidea), resulting in an incomplete representation of the functional biodiversity of coastal ecosystems. This study combines a thorough 20-year literature review of underutilised echinoderm classes (Asteroidea and Ophiuroidea) in ecotoxicology with an original experimental investigation into the biological effects of gadolinium (Gd), a rare earth element of significant concern due to its release from medical contrast agents.

By systematically analysing 74 peer-reviewed publications (2005-2025), we first mapped taxonomic and methodological gaps. Crucially, the review highlighted that brittle stars possess several welfare-friendly endpoints (e.g., behavioural and regenerative traits) that are highly valuable within a modern 3Rs vision (Replacement, Reduction, Refinement), despite a strong historical bias in literature towards trace elements and destructive bioaccumulation endpoints. To address these knowledge gaps, a 7-day laboratory exposure experiment was conducted using two Mediterranean brittle star species, Ophiactis virens and Amphipholis squamata, exposed to a Gd concentration gradient (from 1 to 750 µg/L). Sub-lethal biochemical responses were investigated using a range of cellular biomarkers, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), glutathione reductase (GR), and lactate dehydrogenase (LDH).

Experimentally, exposure to Gd triggered early and significant alterations in oxidative stress biomarkers in both species. Distinct species-specific sensitivity profiles and significant metabolic changes were observed even at environmentally realistic concentrations. Combining the review's theoretical framework with novel experimental data demonstrates that brittle stars represent robust, highly sensitive models capable of diversifying benthic bioindicators within an ethically refined ecotoxicological framework.