Marine ecotoxicological bioassays commonly rely on a number of established model organisms, which may partially represent the complexity of marine biodiversity. Expanding the range to ecologically relevant species is therefore essential to advance environmental risk assessment. Despite their key ecological role, previously overlooked, jellyfish's early life stage (ephyra) has only recently emerged as a sensitive, laboratory-feasible candidate to study pollutants in the marine environment.

This study explores the potential of the holopelagic jellyfish Pelagia noctiluca (Forsskål, 1775) as a suitable test species in marine ecotoxicology. Following set-up and optimization of bench-scale test conditions, ephyrae were exposed to increasing concentrations of surfactants (0.1-50 mg/L SDS and 0.0001-10% Tween 20) as reference toxicants in an acute toxicity test. After 24 and 48 h, lethal and sublethal endpoints were evaluated, including alterations in swimming activity both as frequency of lappet pulsations and as ad hoc stress-related movement categories.

LC₅₀ values obtained for SDS suggested high sensitivity of P. noctiluca ephyrae compared with other marine invertebrate models, supporting the reliability of the proposed test. Moreover, analyses of swimming activity showed that alterations in swimming quality can be used to effectively describe sublethal stress, demonstrating the value of behavioural endpoints for a more comprehensive assessment of contaminant effects on marine organisms.

We further assessed the short-term effects of microplastics as emerging contaminants on ephyrae, testing both pristine polystyrene microspheres and environmentally relevant micro-sized fragments of high-density polyethylene. Our results demonstrated the suitability of P. noctiluca ephyrae for investigating interactions with particulate contaminants, enabling particle internalisation and tissue localisation through microscopy-based approaches.

Overall, our study underlines how P. noctiluca ephyrae are a promising ecotoxicological model and establishes a reproducible methodological protocol to assess the effects of anthropogenic stressors on pelagic organisms, promoting the integration of Scyphozoan early life stages into future ecotoxicological investigations.