Marine heat waves drive bleaching and necrosis of temperate sponges

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Marine heat waves (MHWs) are extended periods of excessively warm water that are increasing in frequency, duration, intensity, and impact, and they likely represent a greater threat to marine ecosystems than the more gradual increases in sea surface temperature. Sponges are major and important components of global benthic marine communities, with earlier studies identifying tropical sponges as potential climate change "winners." In contrast, cold-water sponges may be less tolerant to predicted ocean warming and concurrent MHWs. Here, we report how a series of unprecedented MHWs in New Zealand have impacted millions of sponges at a spatial scale far greater than previously reported anywhere in the world. We reported sponge tissue necrosis and bleaching (symbiont loss/dysfunction), which have been previously associated with temperature stress, for three common sponge species across multiple biogeographical regions, with the severity of impact being correlated with MHW intensity. Given the ecological importance of sponges, their loss from these rocky temperate reefs will likely have important ecosystem-level consequences.