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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erratically (generally lower) rather than steady rhythmic pulsations, releasing copious amounts of mucus, and having withdrawn oral arms.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 Ephyrae, the early stages of scyphozoan jellyfish, possess a conserved morphology among species. However, ontogenetic transitions lead to morphologically different shapes among scyphozoan lineages, with important consequences for swimming biomechanics, bioenergetics and ecology. We used high-speed imaging to analyse biomechanical and kinematic variables of swimming in 17 species of Scyphozoa (1 Coronatae, 8 “Semaeostomeae” and 8 Rhizostomeae) at different developmental stages. Swimming kinematics of early ephyrae were similar, in general, but differences related to major lineages emerged through development. Rhizostomeae medusae have more prolate bells, shorter pulse cycles and higher swimming performances. Medusae of “Semaeostomeae”, in turn, have more variable bell shapes and most species had lower swimming performances. Despite these differences, both groups travelled the same distance per pulse suggesting that each pulse is hydrodynamically similar. Therefore, higher swimming velocities are achieved in species with higher pulsation frequencies. Our results suggest that medusae of Rhizostomeae and “Semaeostomeae” have evolved bell kinematics with different optimized traits, rhizostomes optimize rapid fluid processing, through faster pulsations, while “semaeostomes” optimize swimming efficiency, through longer interpulse intervals that enhance mechanisms of passive energy recapture.