Long-term changes in a trochid gastropod population affected by biogenic sediment stability on an intertidal sandflat in regional metapopulation context

Although destabilization and stabilization of soft sediments by macro-infauna are regarded as key to understanding benthic community dynamics, how component populations are affected concurrently by both agents was poorly investigated. On an intertidal sandflat, Kyushu, Japan during 1979 − 2014 (previous study) and 2015 − 2019, monitoring was made of the populations of the filter-feeding gastropod, Umbonium moniliferum , the burrow-dwelling ghost shrimp, Neotrypaea harmandi (destabilizer), and the tube-building polychaete, Mesochaetopterus minitus (stabilizer). Results revealed that gastropod population changes were driven by an interplay of shrimp, polychaete, and the stingray, Hemitrygon akajei , foraging for shrimp by sediment excavation. The gastropod went through high abundance (1100 m −2 ) in 1979, extinction during 1986 − 1997, two marked recoveries with peaks in 2001 and 2009, a slight recovery in 2016, and near extinction in 2019. These changes largely followed the fluctuation in shrimp density across a threshold of 160 m −2 inhibiting gastropod recruitment. The polychaete exhibited intermittent outbreaks with peaks in 2000, 2007, and 2016, with maximum densities of 15,000 − 24,000 m −2 . Sandflat topography and sedimentary variables were measured during 2015 − 2017. Sediment stabilization by polychaete aggregations at the mid-tidal zone is suggested to have boosted gastropod recruitment. Release at sea and retrieval on shore of drift cards mimicking gastropod larvae with 3- to 9-day planktonic duration was conducted in 2008 − 2009 to specify source populations sending larvae to the present population. Potential source populations were censused in 1998 and 2017 − 2018. Their recent virtual extinction appears responsible for the present population’s decline from 2011. This raises the need for metapopulation perspective to understand local dynamics.