High thermotolerance to extreme marine heatwaves for four New Caledonian sponges

Marine heatwaves (MHWs) are increasingly recognised as a more immediate threat to the persistence of coral reefs than the longer-term impacts of climate change. However, major gaps still exist in our understanding of MHW impacts on many reef organisms. Sponges are functionally important on coral reefs globally. Despite earlier research showing that many sponges have high thermal tolerances, there are currently no data on how New Caledonian reef sponges will respond to MHWs. We conducted a simulated MHW aquarium experiment on four ecologically important New Caledonian sponges from two abiotically contrasting sites. We hypothesised that sponges from the highly fluctuating and thermally extreme Bouraké site would show thermal tolerance, and those from the more stable Nouméa lagoon (NL) would be sensitive to short-term temperature increase. Significant sublethal effects on respiration and photosynthetic efficiency were found in Cliona orientalis (both sites) and Spheciospongia vagabunda (NL), while Rhabdastrella globostellata (Bouraké) and Cliona jullieni (NL) exhibited only minor signs of stress up to +4 °C and +6 °C, respectively, above their site-specific monthly mean temperature (26 °C at NL; 28 °C at Bouraké). Above ∼33.5 °C, both Bouraké species presented sudden and acute stress (bleaching, necrosis and mortality). All four species showed tolerance up to at least 32 °C, suggesting relatively high thermal thresholds, with specimens of C. orientalis displaying similar thermal tolerance to 32 °C independent of their collection site. These findings support the notion that tropical sponges can persist under future warming and MHW scenarios.