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This study was designed to investigate the impact of heat stress on the physiological changes and mortality rates of different life stages of the rhizostome jellyfish species Cassiopea xamachana, including planula larvae, scyphistomae (polyps), and medusae. Both larval and scyphistoma stages of C. xamachana are relatively tolerant to high temperatures, but both experience nearly 100% mortality at 36 °C. Increasing temperatures also induced stage-specific effects. Settlement rates of artificially induced larvae were near 100% at lower temperatures but decreased at 34–36 °C; larvae were dead at 36 °C. When scyphistomae of C. xamachana were subjected to a gradual increase in temperature from 28 to 38 °C, polyp size declined steadily in starved animals, with animals showing clear signs of temperature stress between 35 and 36 °C. Small medusae of C. xamachana pulsed more than larger medusae and tended to have peak pulse rates at higher temperatures (~35 °C) compared to larger medusae (~29–33 °C), though the latter was not significant. At a temperature of 39 °C, all the medusae exhibited signs of heat stress, including pulsing erratically (generally lower) rather than steady rhythmic pulsations, releasing copious amounts of mucus, and having withdrawn oral arms. Temperature data presented here, and in the literature, show that pulsing C. xamachana medusae exhibit a bell-shaped curve, with temperatures over 38 °C being detrimental and becoming lethal at 40 °C. Based on the findings of this study, it is proposed that the medusa stage of C. xamachana has a higher tolerance for elevated temperatures compared to both the larvae and the polyps. Predictions of global climate change indicate that populations of C. xamachana will likely face longer and hotter summer periods, leading to increased population sizes. However, higher temperatures pose a greater risk to the survival of the species as they increase mortality in the polyp and larval stages compared to the medusa stage. 

Abstract The existence of widespread species with the capacity to endure diverse, or variable, environments are of importance to ecological and genetic research, and conservation. Such “ecological generalists” are more likely to have key adaptations that allow them to better tolerate the physiological challenges of rapid climate change. Reef‐building corals are dependent on endosymbiotic dinoflagellates (Family: Symbiodiniaceae) for their survival and growth. While these symbionts are biologically diverse, certain genetic types appear to have broad geographic distributions and are mutualistic with various host species from multiple genera and families in the order Scleractinia that must acquire their symbionts through horizontal transmission. Despite the considerable ecological importance of putative host‐generalist symbionts, they lack formal species descriptions. In this study, we used molecular, ecological, and morphological evidence to verify the existence of five new host‐generalist species in the symbiodiniacean genusCladocopium. Their geographic distribution and prevalence among host communities corresponds to prevailing environmental conditions at both regional and local scales. The influence that each species has on host physiology may partially explain regional differences in thermal sensitivities among coral communities. The potential increased prevalence of a generalist species that endures environmental instability is a consequential ecological response to warming oceans. Large‐scale shifts in symbiont dominance could ensure reef coral persistence and productivity in the near term. Ultimately, these formal designations should advance scientific communication and generate informed research questions on the physiology and ecology of coral‐dinoflagellate mutualisms.