SEASONAL VARIATION IN DIEL AND TIDAL EFFECTS AMONG BENTHIC AMPHIPODS WITH DIFFERENT LIFESTYLES IN A SANDY SURF ZONE OF KOREA

Although endogenous rhythms such as diel and tidal effects can influence the distribution of sandy beach amphipods, the consequences of these for the seasonal endogenous rhythms of amphipods within the subtidal surf zone of sandy beaches are little understood. We examined the seasonal variation in diel and tidal effects among benthic amphipods with different lifestyles in a sandy shore surf zone in southern Korea. Using a sledge net, quantitative samples of amphipods were taken for 24 h at three stations: at the bottom and surface at a water depth of 1 m, and at the water's edge. The epifaunal, nestling amphipods, Pontogeneia rostrata Gurjanova, 1938 and Allorchestes angusta Dana, 1856 and the burrower, Haustorioides koreanus Jo, 1988 were significantly more abundant at the surface at night than during the day, suggesting nocturnal vertical migration. However, the abundance of the burrower, Synchelidium lenorostralum Hirayama, 1986 did not change between day and night, suggesting that this species did not perform vertical migrations. Most of the amphipods, particularly P. rostrata, showed a seasonally different response to tidal effects, appearing at higher densities during ebbing than flooding tides in one or more seasons, and depending on the species. Amphipods increased in abundance at night regardless of seasonal tidal variation, suggesting that the diel effect may be more important than the tidal effect. Seasonal variation to diel and tidal effects by these amphipods varied with temperature and different lifestyles.