Thermal tolerance under climate warming in the mangrove fiddler crab Paraleptuca chlorophthalmus: an integrative metabolic and behavioral approach

Thermal tolerance is a key determinant factor of species persistence and distribution under climate change, particularly in ectotherms inhabiting highly variable environments such as mangroves, where fiddler crabs must rely on both physiological and behavioral mechanisms to cope with environmental stress such as temperature. Although the thermal limits of the mangrove fiddler crab Paraleptuca chlorophthalmus were first reported over six decades ago, no study has since jointly characterized its metabolic and behavioral thermal responses, despite the species being recognized as a sentinel of mangrove ecosystem health in a region facing rapid climate warming. We investigated oxygen consumption and locomotor activity in 96 adult males across a broad thermal gradient (5–44°C) under controlled acclimation, and fitted thermal performance curves (TPC) and thermal behavioral curves (TBC) to identify optimal temperatures and model-derived thermal performance limits under current and projected climate conditions defined by the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5). Metabolic rate was measured using closed-chamber respirometry, and behavioral responses were quantified through video tracking across the distance covered. Respiration showed a typical tropical-stenotherm, unimodal thermal response, with an optimum near 33°C and model-derived lower and upper performance limits near 11°C and 42°C, whereas locomotor activity remained high and stable across a substantially broader intermediate range (22–38°C), a markedly wider thermal window than metabolic performance. Climate projections showed strong seasonal variability in respiration-based thermal suitability, whereas behavior-based suitability remained consistently high across western Indian Ocean regions under both current and future scenarios. These results suggest that locomotor thermal performance may buffer P. chlorophthalmus against the direct metabolic costs of warming, and that thermal conditions alone are unlikely to constrain its persistence in the region. P. chlorophthalmus therefore appears thermally equipped to expand into lower-latitude mangroves under future warming; whether it does so will depend on habitat availability, salinity, biotic interactions, and larval dispersal, factors beyond the reach of thermal models alone.