This study experimentally investigated behavioral and physiological responses of the invasive sea spider Ammothea hilgendorfi to thermal stress, following observations of the highest local densities (average 3.7 individuals/oyster). Locomotor performance, including notable locomotor capability (median >63m/24h at 15°C), vertical/substrate habitat selection, and upper thermal tolerance (TDT) were assessed at 15°C and 25°C. Elevated temperature (25°C) generally reduced locomotor activity in unstructured environments, though diurnal activity persisted. In inclined arenas, A. hilgendorfi preferred shallow water regardless of temperature, shifting to increased nocturnal activity at 25°C. Substrates dramatically decreased locomotion; oyster shells were preferred at 15°C, with increased mussel shell preference at 25°C. TDT experiments revealed high physiological thermal tolerance: ~37°C, showing limited plasticity post-acclimation and consistency across demographics. The extrapolated for prolonged survival (~7 days) was ~33.5°C. Findings suggest A. hilgendorfi invasive success, underscored by its high local abundance and dispersal potential, is supported by significant behavioral plasticity (modulating activity, temporal niche shifts, adaptive microhabitat selection) compensating for its relatively fixed but high physiological thermal tolerance, enabling it to cope with thermally variable coastal environments.
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