Role of suction in sandy beach habitats and the distributions of three amphipod and isopod species

Sandy beach ecology has progressed rapidly with the emergence of several theories developed on the basis of understanding of hydrodynamic and morphodynamic processes on sandy beach habitats. However, the possible role of dynamic geophysical processes in the sediments remains poorly understood. The present study aims to explore the role of such geophysical processes in the sediments in forming the habitat environment and how they influence the species distributions. We conducted sets of integrated observations and surveys on intertidal and supratidal geophysical environments and the distributions of three amphipod and isopod species at four exposed sandy beaches located on the Japan Sea coast of Niigata Prefecture, Japan. The field results combined with a series of laboratory soil tests demonstrate that suction governed the variability of habitat environments observed, involving the degree of saturation, density, and hardness of the cross-shore intertidal and supratidal sediments, depending on the severity of the suction-dynamics-induced sediment compaction. While the observed species abundances were consistent with existing theories relating to intertidal and supratidal species, the observed magnitudes of suction were responsible for the distribution limits of the amphipods Haustorioides japonicus and Talorchestia brito and the isopod Excirolana chiltoni manifested consistently throughout the different beaches. The results of controlled laboratory experiments and field tests further revealed three distinctive suction-induced mechanisms, associated with their burrowing and physiology and the stability of the burrows. The novel role of such suction-induced geophysical processes in forming the habitat environment and influencing the species distributions may advance our understanding of sandy beach ecology in intertidal and supratidal zones.