Fiddler crab spatial distributions are influenced by physiological stressors independent of sympatric interactions

- Crabs experience stress when placed in new environments.
- The presence of other crabs did not affect the activities of any crab species.
- Removing predators did not affect the activities of any crabs.

Understanding how and why organisms such as intertidal invertebrates are distributed spatially helps ecologists to determine ecosystem functioning and make predictions in the face of changing scenarios. Tropical Australian fiddler crabs (Uca species) are differentially distributed in mangrove habitats according to levels of canopy cover. Here we conducted experiments to test three hypotheses explaining fiddler crab distributions in a tropical mangrove. Firstly we recorded the time that it took fiddler crabs from different habitats to reorientate themselves upon being placed on their backs. Secondly we transplanted forest inhabiting U. flammula and clearing inhabiting U. elegans into enclosures set up in clearing and forest sites and measured their activities. We then excluded predators from enclosures containing U. elegans in the forest and monitored crab activities over 10 weeks. We found that righting-response times were longer for crabs from low compared to high intertidal zones and longer when in full sun for all crabs living in or near forests compared to those from clearings, suggesting that forest-dwelling crabs experience physiological stress in open canopy habitats. After 10 weeks, transplanted crab activities varied according to species and habitat. In the clearing, crab activities remained high with burrow-enclosure distance varying between species. Neither transplanted crab activities nor burrow-enclosure distance varied with the presence or absence of other species. Our predator exclusion experiments also found no predator effects on the activities of transplanted crabs. Our results suggest that fiddler crab spatial distributions across the mangrove ecosystem are influenced by physiological stressors independent of any sympatric interactions.