Sea star wasting disease in the keystone predator Asterias rubens from the Baltic Sea

Sea star wasting disease (SSWD) is one of the most severe marine epidemics recorded, affecting numerous asteroid species and causing widespread population declines. Although a bacterial pathogen has recently been proposed for one species, its generality across taxa and regions remains unresolved. Here, we report the first year-round field assessment of SSWD in the Baltic Sea, a rapidly warming, low-salinity ecosystem hosting a single keystone sea star predator, Asterias rubens. Using field surveys, image-based monitoring, and laboratory experiments, we characterised disease dynamics and potential drivers in Kiel Fjord (western Baltic Sea). SSWD-like symptoms were present throughout 2024, with mean prevalence exceeding 40% across seasons and elevated levels during summer and early autumn, when sea surface temperatures approached 22 {degrees}C and salinity fluctuated between 11 and 17. The mean body radius of asymptomatic individuals declined from 5.6 cm in spring to 2.3 cm in early summer before partially recovering in autumn, consistent with high recruitment of juveniles and selective loss of larger, symptomatic individuals. In a complementary laboratory experiment, survival analyses identified body size as the strongest predictor of SSWD-associated mortality (hazard ratio = 50.8, p < 0.001), with large individuals far more likely to die than small ones. This size-selective mortality, together with environmental constraints on recruitment, suggests that SSWD may be reshaping population size structure and reducing predation pressure on blue mussels, with potential consequences for benthic community dynamics. Continued monitoring will be essential to assess the long-term impacts of SSWD on A. rubens populations and associated benthic ecosystems under ongoing climate change.