Spatial and temporal recruitment variability of sandy subtidal megabenthic populations on an open coastline influenced by a tsunami caused by the 2011 Tohoku earthquake

The spatial distribution and size structure of two infaunal filter-feeding bivalves, that is, Sakhalin surf clam Spisula sachalinensis (Schrenck, 1862) and dwarf sunray surf clam Mactra crossei (Dunker, 1877), and an epibenthic deposit feeder, namely the sand dollar Scaphechinus mirabilis (A. Agassiz, 1864), were investigated on an open sandy beach in northern Japan after the tsunami caused by the 2011 Tohoku earthquake. Settlement of S. sachalinensis was relatively abundant in the 2011- and 2012-year classes, but then substantially decreased until the early months of 2013. The 0+-year-old distribution was associated mainly with a depth gradient and weakly with the median grain size of the sediment. Mactra crossei had (1) an inverse trend in long-shore variation in abundance at 2+ years of age and (2) later-post-settlement processes that regulated its population dynamics, compared to those of S. sachalinensis. The 2010-year classes for both Mactridae species survived until 2+ years of age, although their abundance showed a high level of spatial variation. Scaphechinus mirabilis showed fine-scale spatial variation in their recruitment dynamics. The cross-shore distribution of S. mirabilis showed a clear ontogenetic shift from shallow to deep water. The cross-shore distribution of small S. mirabilis (<40 mm in test diameter) overlapped that of the juvenile S. sachalinensis, but their interaction was unclear. Overall, the level of spatial and temporal recruitment variation was relatively high in each species. However, the tsunami has scarcely influenced the cross-shore distribution and population structure of each species.