The north-western Mediterranean Sea’s high biodiversity originates from a mixture oftemperate and subtropical species. Large-scale warming has been detected throughnorthward range shifts of ‘warm-water’ species, and mass-mortality events for ‘coldwater’invertebrates. Marine caves are affected in a more subtle way. By monitoringendemic species of cave mysids (Crustacea) and comparing our results with data in theliterature, we observed a strong decline in one species, coupled with the success ofanother. Near Marseille (France), this phenomenon began while two major thermalanomalies were reported. Different tolerances to temperature were demonstrated byboth the species distribution ranges and laboratory experimentation. We provide apossible physiological explanation for populations of cold stenothermal species ofendemic cave mysids being replaced by congeners of warmer affinities, with a high riskof extinction. We also found strong support for the view that Mediterranean marinebiodiversity is already under the threat of global warming.