Juvenile Cassiopea andromeda medusae are resistant to multiple thermal stress events

Oceans are undergoing successive heatwaves. Several invertebrate taxa associate with dinoflagellates and are susceptible to bleaching caused by heat stress. Although the impacts of a single bleaching event have been well documented, the consequences of successive events are less understood. We investigated the effects of multiple thermal stress events on juvenile Cassiopea andromeda, while also addressing the roles of symbiont concentration and heterotrophic diet regimen. We exposed medusae with two distinct symbiont concentrations (high and low) and under two feeding frequencies (Artemia offered daily and every 3 days) to three thermal stress events and at three temperatures (control of 27 °C, and treatments of 30 and 33 °C) while recording proportional changes in chlorophyll-a and jellyfish growth. Results show that 30 and 33 °C were not enough to trigger bleaching and did not affect growth. Symbiont concentration also did not affect growth, but medusae with higher symbiont concentration displayed higher chlorophyll-a loss. Diet regimen had little impact on chlorophyll-a variation, but had a dramatic effect on growth, as medusae fed daily grew, while those fed every 3 days shrank. These findings show that it is not possible to evaluate if C. andromeda becomes more resilient after successive thermal stress episodes, because even 33 °C did not generate enough stress. Therefore, C. andromeda juveniles are thermotolerant and may not serve as a good model for investigations on the resilience of coral reef zooxanthellate fauna to the current climate change predictions. Finally, at this life stage, the symbiotic relationship seems less important for growth than heterotrophy.