Reaching a breaking point: upper thermal limit of the limpets, Scutellastra granularis and Siphonaria capensis, along the southern and west coasts of South Africa

Understanding how thermal tolerance varies in intertidal species is essential for predicting their responses to climate change, including future distribution patterns. This study investigates the upper thermal limits of the patellogastropod, Scutellastra granularis and pulmonate gastropod Siphonaria capensis, two sympatric limpet species from the west and southern coasts of South Africa. Cardiac performance (heart rate) was measured to determine sublethal (Arrhenius Breakpoint Temperature, ABT) and lethal (Flatline Temperature, FLT) thresholds under controlled temperature ramping. Results showed that Siphonaria exhibited higher sublethal (ABT: 40.23 ± 1.96 °C) and lethal (FLT: 45.15 ± 2.13 °C) thermal limits than Scutellastra (ABT: 35.78 ± 1.58 °C, FLT: 40.71 ± 2.08 °C), highlighting Siphonaria's greater thermal tolerance. Thermal Safety Margins (TSMs) also varied, with Scutellastra demonstrating site-dependent sensitivity, particularly lower at West Coast National Park (TSM: 3.19 ± 1.41 °C). The differences observed are attributed to phylogenetic and habitat adaptations: with Siphonaria's dual respiration system (lungs and gills) supporting a higher thermal tolerance, while Scutellastra's reliance on anaerobic pathways during low tides possibly results in limited thermal tolerance. These findings highlight the greater resilience of Scutellastra to warming, suggesting a greater ability to withstand future climatic conditions, unlike Siphonaria, which faces increasing vulnerability. The study provides a better understanding of the adaptive capacity of intertidal limpets and provides insight into species-specific risks under climate change.