Thermal Response of Mangrove Macrobenthos: Measurement of Population Fitness in Endangered Coastal Systems

Climate change has a strong impact on marine ecosystems, through warming, acidification and hypoxia of seas. Marine ectotherms are mostly affected by changes in temperature, which directly influences their overall fitness. Understanding the thermal response of organisms is therefore crucial to forecast the effects of climate change on ecosystem functionality. Here we focus on the thermal tolerance of adult males, females and gravid females, of two mangrove ecosystem engineers inhabiting the East African mangroves, the crabs Perisesarma guttatum and Uca urvillei. To assess their sensitivity to acute temperature fluctuations across a wide latitudinal gradient, we studied the thermal window of Kenyan and South African populations of both species. The metabolic rate, haemolymph oxygen saturation were measured in the laboratory along a temperature ramp (17-37 °C) in water and in air. Additionally, we characterised the environmental temperature range which the animals are subjected to and compared this to their relative body temperatures measuring their spatio-temporal use in the field. To evaluate the species sensitivity to temperature, we fit these data with the thermal model constructed by our experiments. The different thermal regimes found show different life strategies in the use of time and space. A stenothermic response in water compared to air, with a pronounced latitudinal effect was established, with the South African populations better adapted to lower temperatures than the Kenyan ones. Females with eggs show a higher metabolic rate than males and females with no eggs, representing the most vulnerable adult life stage. The results suggest that these subtropical mangrove populations are vulnerable to long-term increases in temperature, particularly because of reduced oxygen content in water as it warms. This is likely to lead to a loss of fitness with serious consequences for the persistence of such populations and the overall mangrove ecosystem functioning.