Increasing Temperature Facilitates Polyp Spreading and Medusa Appearance of the Invasive Hydrozoan Craspedacusta sowerbii

SIMPLE SUMMARY: The jellyfish Craspedacusta sowerbii is one of the most widespread freshwater invasive species. The aim of this study is to analyze polyp and medusa responses to different temperatures using experimental studies on polyp colony growth and medusae to develop a model capable of predicting the Thermal Habitat Suitability (THS) for the polyp and medusa stages. Temperature had a significant effect on the total number of polyps and colonies over time and on the structure of polyp colonies. Polyp and colony numbers were greater at high temperature while colonies were composed of more polyps per colony at 19 °C compared to 29 °C. The thermal tolerance of the medusa stage showed that temperature will favor the expansion of the species in the future to higher latitudes. ABSTRACT: The freshwater jellyfish Craspedacusta sowerbii is among the most widespread invasive species, observed across a wide temperature range. The aim of this study is to analyze the polyp and medusa stages response to different temperatures by using (i) an experimental study on the polyp colony growth at 19 and 29 °C, and (ii) prediction of the Thermal Habitat Suitability (THS) based on the thermal tolerance of the medusa stage. The total number of polyps and colonies was greater at high temperature. At 19 °C, colonies with 1 to 5 polyps were present, with colonies of 1 to 3 polyps numerically dominating. At 29 °C, colonies were 80% composed of 1- to 2-polyps. Based on the published medusa pulsation rhythm data, a Thermal Performance Curve (TPC) regression was performed and used to monthly predict the THS for current and future (2050 and 2100) scenarios. The southern hemisphere offered optimal conditions (THS > 0.6) year-round. In the northern hemisphere, the optimum period was predicted to be between June and September. The future THS were considerably larger than at present with an increase in optimal THS at higher latitudes (up to 60° N). The combination of experimental and modeling approaches allows to identify the optimal thermal conditions of the polyp and medusa stages and to predict their invasive capacities.