The distribution of the European shore crab, Carcinus maenas, with respect to mangrove forests in Southeastern Australia

There is growing interest in associations between non-native species and native foundation species. Along the southeast coast of Australia, the European shore crab, Carcinus maenas, overlaps with the distribution of the grey mangrove, that provides refuge to many native invertebrates from predators and physiological stress. We tested the hypothesis that C. maenas would be more abundant under the canopy of mangrove forests than in adjacent unvegetated intertidal habitat. Trapping surveys within three estuaries found greater abundances of C. maenas in mangrove forests than in adjacent unvegetated habitat and, within mangrove forests, under the canopy than in the pneumatophores zone. Average temperatures under the mangrove canopy were up to 2°C lower than in unshaded habitats and maximum temperatures up to 5.7°C less. The results of tethering studies did not support the hypothesis that predatory mortality of C. maenas was reduced in mangroves. To the contrary, survivorship of tethered crabs was much lower under the mangrove canopy than in adjacent unvegetated habitat. Habitat choice experiments, however, indicated that crabs chose shaded and protected over unshaded and exposed habitat. Hence, along this coastline where summertime maximum air temperatures may approach the known LD₅₀ of C. maenas (40min at 40°C), mangroves may help to facilitate persistence in intertidal habitats. Our study adds to a growing number indicating that foundation species may not only facilitate native species, but non-native species too. Additional studies are now needed to confirm the mechanism of the association between C. maenas and mangroves in eastern Australia.