Cryptobenthic crab assemblages are more distinct across a 90 m depth gradient than 2500 km of shallow marine habitat in the Hawaiian archipelago.

Despite high biodiversity and the recognized importance of mesophotic habitats, most studies of coral reef community structure have focused on conspicuous taxa such as fishes and corals in shallow habitat <30 m. Here, we examined the variability of crab assemblages from Autonomous Reef Monitoring Structures deployed on shallow reefs across the Hawaiian Islands and a mesophotic depth gradient on O'ahu. We tested the effects of environmental, ecological, and anthropogenic factors on shallow (8-17 m) crab assemblages. These assemblages were significantly different between the densely inhabited high islands (Main Hawaiian Islands) and the relatively pristine uninhabited atolls of the Northwestern Hawaiian Islands. Drivers of these differences include sea-surface temperature, chlorophyll-A, depth, island slope, potential larval immigration, and human impacts. We then compared shallow assemblages to those sampled along a depth gradient (12-90 m) on O'ahu. Despite considerable variability among sites distributed across nearly 10 degrees of latitude, differences along the depth gradient on O'ahu alone were greater than among the shallow assemblages across the entire archipelago. This finding suggests that 90 m of depth is a stronger driver of brachyuran assemblage structure than the latitudinal, environmental, and anthropogenic gradients across the entire ~2500 km span of the Hawaiian Islands.