Rising seawater temperatures affect the fitness of Rhopilema nomadica polyps and podocysts and the expansion of this medusa into the western Mediterranean

Abstract

Since first reported in the eastern Mediterranean in the 1970s, the swarm-forming scyphomedusa, Rhopilema nomadica, has been continuously expanding westward. We examined the effect of temperature on the benthic stages of this jellyfish (polyps, podocysts, and strobilae) to assess whether it can survive and reproduce in the western Mediterranean temperature regime (12.2-26.2°C). In the first experiment, polyps and podocysts were maintained at 4 constant temperatures (12, 18, 24, and 30°C) for 55 d. In the second experiment, temperatures were gradually reduced over 14 d, kept constant for 18 d, and then gradually brought back to 25°C. High temperatures proved beneficial to polyp survival and asexual reproduction, yet in some cases, polyps were able to survive temperatures as low as 12°C. Strobilation occurred at temperatures between 22 and 30°C. Podocyst production, excystment, and strobilation all take place following a temperature increase, and excystment of podocysts occurs mainly during the first 2 wk following podocyst production. We propose that the role of podocysts in R. nomadica is mainly to increase the current season’s polyp population, contributing to swarm formation rather than as a means to maintain polyp populations during harsh environmental conditions. Since polyps are able to survive winter temperatures in the western basin (12°C) and reproduce by podocysts and strobilation during summer temperatures (24°C), thermal constraints will probably not restrict the expansion of R. nomadica into the western Mediterranean. Expected future climate change conditions will increase the performance and range expansion of this species.