Impacts of marine heatwaves on disturbed mangrove ecosystems: A biomarker-based assessment using fiddler crabs as sentinel species

Marine heatwaves (MHWs) pose severe threats to coastal ecosystems. Fiddler crabs, key intertidal species in mangrove environments, are particularly vulnerable to MHWs in disturbed habitats, where vegetation degradation amplifies environmental stressors such as temperature and salinity. We conducted a biomarker-based assessment of the behavioral, physiological, and biochemical responses of fiddler crab species to MHWs in undisturbed and disturbed mangrove ecosystems. We also used a laboratory-based simulation of MHWs to investigate how environmental stressors (elevated temperature, salinity, and hypoxia) trigger behavioral responses of species in degraded habitats. Our findings reveal water escape and aggregation behaviors in crabs inhabiting disturbed mangroves during maximum high tide (MHW) events. These responses were triggered when burrow temperatures approached crabs' critical thermal maximum, suggesting a thermally induced escape mechanism. Crabs aggregating above the water level exhibited significant osmotic disruption, including a pronounced loss of osmoregulatory capacity, which heightened their vulnerability to desiccation. These physiological impairments were accompanied by signs of neuromuscular dysfunction, as indicated by altered acetylcholinesterase activity. In contrast, crabs from undisturbed mangrove habitats did not exhibit such behavioral or physiological disruptions. The laboratory experiments further demonstrated that the likelihood of escape and aggregation behaviors increased at lower temperatures under high-salinity and low-oxygen-concentration conditions. These results suggest that undisturbed mangrove habitats act as thermal refuges, mitigating the extreme effects of MHWs on the intertidal fauna.