Résumé
Aurelia coerulea and Rhizostoma pulmo are two bloom-forming scyphomedusae with a complex life cycle characterized by benthic scyphistoma and pelagic medusa stages. Understanding how environmental variability regulates developmental transitions and asexual reproductive pathways in medusozoans is central to evolutionary developmental biology, particularly under ongoing climate change. The Mediterranean Sea is a climate-change hotspot, where rising temperatures and increasingly frequent marine heatwaves provide a natural context to examine how seasonal and extreme thermal conditions shape life-cycle regulation. Here, we conducted a nine-months experiment rearing polyps of both species under controlled conditions that reproduced weekly in situ temperature variations from the Bages-Sigean lagoon, combined with two experimentally imposed heatwaves (+5 °C for 6 days in December and +3.5 °C for 5 days in April). A. coerulea polyps exhibited strong developmental plasticity, proliferating through budding at low temperatures and rapidly colonizing the substrate (0.12 cm² day -1 ), before a marked decline in asexual propagation when temperatures persistently exceeded 15 °C. In contrast, R. pulmo polyps followed a highly conservative developmental pathway, almost exclusively producing podocysts throughout the experiment, with minimal substrate colonization (<1.5% of a 100 cm² surface), and summer temperatures acting as a trigger for podocysts excystment. Strobilation timing and magnitude further differed between species: A. coerulea strobilated mainly from mid-November to February (>200 ephyrae per 100 cm²), whereas R. pulmo showed an extended strobilation period from January to July, peaking in late March (>30 ephyrae per 100 cm²). Temperature significantly regulated the production of buds, podocysts, and strobilae in both species, but with contrasting thermal optima: cold conditions favored A. coerulea strobilation, whereas warming enhanced R. pulmo strobilation. Heatwave exposure further modified these responses, indicating context-dependent developmental sensitivity to extreme events. Together, these results demonstrate that closely co-occurring medusozoans can express fundamentally different life-cycle regulation strategies under identical thermal regimes, highlighting how developmental pathways may mediate species-specific responses to climate warming and shape future jellyfish bloom dynamics.