Abstract. Intertidal species are subject to both terrestrial and aquatic predation, and face the challenge of shifting vision due to changing water level. Overhead light intensity information is important for sensing terrestrial avian predators, which are a major source of mortality among intertidal crustaceans. In this study, we examined the differences in overhead light intensity preference of Metapograpsus frontalis, a mangrove crab common on the sheltered rocky shores in Stradbroke Island, in two experimental treatments ­––– exposed on dry sand and submerged under water. The collective of M. frontalis individuals in our study established a population preference for the low light intensity (340-390 lux) in wet treatment, but not in dry treatment (wet: p=0.0024, dry: p=0.69, population difference: p=0.034). This corresponded to the natural tidal behavior of M. frontalis, which hide under rocks during high tide and forage during low tide. Given this tidal pattern, it is likely that intertidal crustaceans tune their eye structure to mainly detect either terrestrial or aquatic predators, but not both. In the environment where their vision is compromised, they rely on shelters for predator avoidance. It is thus essential to maintain proper shelters for intertidal crustaceans and avoid clustering of human activity that might over-stress their vision in coastal area.

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Additional Keywords: intertidal adaptation, tidal rhythm, predator-prey, crab behavior, compound eyes