Thermal Tolerance of two ecosystem engineers of East African mangroves : forecasting the effects of climate change on ecosystem functionality

Rapid climate change has a strong impact on marine ecosystems, through warming, acidification, hypoxia and salinisation of sea water. Marine ectotherms are mostly affected by changes in temperature, which directly influence oxygen availability and their ability to utilise oxygen and so can have potential cascading effects on their overall fitness. As a consequence, understanding the thermal response of organisms, particularly ecosystem engineers, is crucial to forecasting the effects of climate warming on ecosystem functionality. Here we focus on the thermal tolerance of two mangrove ecosystem engineers inhabiting the eulittoral of East African and South African mangrove forests, the crabs Perisesarma guttatum (Sesarmidae) and Uca urvillei (Ocipodidae). In order to assess their sensitivity to acute temperature fluctuations across a wide latitudinal gradient, we studied the thermal window of a Kenyan and a South African population of each species using intermittent flow-throw respirometry. Metabolic rate was measured in the laboratory along a temperature ramp procedure of 1°C × h-1 between 17-27 °C in water and in air, as these species experience both conditions. The results showed a stenothermic response for both species in water, with a pronounced latitudinal effect, the South African populations being adapted to lower temperatures than the Kenyan ones. Both species showed a different response in air, where the thermal window was wider, highlighting a better tolerance to temperature change in air than in water. This suggests that oxygen limitation is the driving parameter in determining the limits of thermal tolerance. These 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 overall mangrove ecosystem functioning.