Marine heatwave conditions drive carryover effects in a temperate sponge microbiome and developmental performance.

Marine heatwaves are increasingly subjecting organisms to unprecedented stressful conditions, but the biological consequences of these events are still poorly understood. Here we experimentally tested the presence of carryover effects of heatwave conditions on the larval microbiome, settlers growth rate and metamorphosis duration of the temperate sponge . The microbial community of adult sponges changed significantly after ten days at 21°C. There was a relative decrease in symbiotic bacteria, and an increase in stress-associated bacteria. Sponge larvae derived from control sponges were mainly characterised by a few bacterial taxa also abundant in adults, confirming the occurrence of vertical transmission. The microbial community of sponge larvae derived from heatwave-exposed sponges showed significant increase in the endosymbiotic bacteria Settlers derived from heatwave-exposed sponges had a greater growth rate under prolonged heatwave conditions (20 days at 21°C) compared to settlers derived from control sponges exposed to the same conditions. Moreover, settler metamorphosis was significantly delayed at 21°C. These results show, for the first time, the occurrence of heatwave-induced carryover effects across life-stages in sponges and highlight the potential role of selective vertical transmission of microbes in sponge resilience to extreme thermal events.