Differing temperature regimes have no impact on the heat stress response of shallow and upper-mesophotic populations of a temperate calcareous sponge

Marine heatwaves (MHWs) are increasing in intensity and frequency globally owing to anthropogenic climate change. Temperate mesophotic ecosystems (TMEs; 30-150 m) are light-dependent habitats that generally experience cooler and more stable temperatures than adjacent shallow zones, potentially leading to intraspecific differences in thermal tolerance between shallow and mesophotic populations. Sponges, often the dominant benthic organisms in TMEs, show varied responses to thermal stress. We hypothesized that shallow-water sponges may be more resilient to heat stress than conspecifics from upper-mesophotic depths. To test this, we exposed Rowella lancifera sponges from one shallow and one upper-mesophotic population to a simulated MHW and measured respiration, buoyant weight, mortality and microbial community composition. Temperature records from the preceding year confirmed cooler, more stable conditions at upper-mesophotic depths. Sponges displayed strong stress responses under MHW exposure, resulting in high mortality, but responses did not differ between depths. Elevated respiration rates also coincided with microbial community shifts. Overall, our results indicate that a 1°C increase above previous MHW peaks could cause widespread mortality in R. lancifera.