A sea of soap: toxicity of anionic surfactant to Clytia hemisphaerica (Hydrozoa)

: The increasing threat of anthropogenic pollution to marine ecosystems underscores the need for robust risk assessment through ecotoxicology. While standard bioassays rely on model organisms chosen for their sensitivity and ease of laboratory use, there is growing interest in ecologically relevant species such as jellyfish, to reflect marine environmental complexity. Among jellyfish, hydrozoans represent valuable models in neuroscience and developmental and evolutionary biology; however, they are still unrepresented in ecotoxicology. Here, we propose the hydrozoan Clytia hemisphaerica as a novel model for acute toxicity testing. Following the optimization of test conditions, juvenile medusae were exposed to an anionic surfactant, Sodium Dodecyl Sulfate (SDS), assessing mortality and sub-lethal endpoints, including behaviour and feeding, after 24 and 48 h. SDS exposure triggered a concentration-dependent increase in mortality, with LC50 values of 16.3 mg/L at 24 h and 9.2 mg/L at 48 h. Feeding rate emerged as the most sensitive sub-lethal indicator (EC50(48 h) = 5.2 mg/L). Notably, jellyfish transferred to clean seawater after exposure showed partial recovery of motility and feeding ability, highlighting potential resilience to short-term chemical stress. This study is the first to report SDS toxicity in a hydrozoan and provides a proof-of-concept demonstration of the feasibility of using C. hemisphaerica in marine ecotoxicological assays. Our results emphasize the value of including behavioural endpoints, such as swimming and feeding, to achieve a more comprehensive assessment of pollutant impacts on jellyfish.