Feeding and digestion periodicity of Manila clam Ruditapes philippinarum in natural intertidal and subtidal zones estimated from the morphological condition of the crystalline style

ABSTRACT Feeding periodicities and rhythms of suspension-feeding bivalves in coastal waters are closely related to diel and tidal cyclic environmental changes, such as food availability and predation risk. Although such feeding periodicities of bivalves are important for understanding how their feeding strategies adapt to localized habitats, the feeding periodicity in bivalves in the field remains to be clarified, especially in infaunal species. The present study focused on the morphological condition of the crystalline style as an indicator of the feeding activity of the infaunal bivalve Manila clam, Ruditapes philippinarum, and estimated their feeding periodicity in the field. First, the effects of feeding (siphon-extending) behaviour, food and the light/dark cycle on the condition of the crystalline style (defined based on its morphology and hardness) were investigated in the laboratory. Results of the ordered logistic models and model selection showed that clams extending their siphons kept their crystalline styles hard. Furthermore, the effects of the light/dark cycle and tidal phase on the condition of the crystalline style were investigated in natural intertidal and subtidal zones and assessed using the results of the ordered logistic models. In the intertidal zone, the effect of the tidal phase was significant, whereas the effect of the light/dark cycle was not, and feeding was estimated to be active at ebb and flood tides, when the density of food particles was likely to be high. On the other hand, in the subtidal zone, the results of the model selection were not clear because of significant effects of interactions between the light/dark cycle and tidal phase. However, when the interactions were removed from the models, the effect of the light/dark cycle was significant, and feeding was estimated to be active at night when the predation risk from visual predators would be relatively low. These results confirm that R. philippinarum possesses behavioural plasticity in feeding periodicity, which is adaptable to both intertidal and subtidal zones.

Storm Surges in the Bohai Sea: The Role of Waves and Tides

A storm surge is a complex phenomenon in which waves, tide and current interact. Even though wind is the predominant force driving the surge, waves and tidal phase are also important factors that influence the mass and momentum transport during the surge. Devastating storm surges often occur in the Bohai Sea, a semi-enclosed shallow sea in North China, due to extreme storms. However, the effects of waves on storm surges in the Bohai Sea have not been quantified and the mechanisms responsible for the higher surges that affect part of the Bohai Sea have not been thoroughly studied. In this study, we set up a storm surge model, considering coupled effects of tides and waves on the surges. Validation against measured data shows that the coupled model is capable of simulating storm surges in the Bohai Sea. The simulation results indicate that the longshore currents, which are induced by the large gradient of radiation stress due to wave deformation, are one of the main contributors to the higher surges occurring in some coastal regions. The gently varying bathymetry is another factor contributing to these surges. With such bathymetry, the wave force direction is nearly uniform, and pushes a large amount of water in that direction. Under these conditions, the water accumulates in some parts of the coast, leading to higher surges in nearby coastal regions such as the south coast of the Bohai Bay and the west and south coasts of the Laizhou Bay. Results analysis also shows that the tidal phase at which the surge occurs influences the wave–current interactions, and these interactions are more evident in shallow waters. Neglecting these interactions can lead to inaccurate predictions of the storm surges due to overestimation or underestimation of wave-induced set-up.

Impact of Tidal Phase on Inundation and Thrust Force Due to Storm Surge

Impact of storm surge largely varies depending on the tidal phase during the landfall of a tropical cyclone. This study investigates comparative variance in inundation condition and thrust force for an identical cyclone during low tide and high tide by applying a numerical model (Delft3D) and a semi-analytical model (DFM). A moderate strength cyclone, Mora, which made landfall on Bangladesh coast in May 2017 is selected to study its impact on land during low tide and high tide. Actual landfall time of Mora was during low tide. To study the impact of storm surge during high tide, a synthetic cyclone is created which has similar strength and track to that of Mora but makes landfall during high tide. The results show that inundation depth, inundation extent, and thrust force increase when a cyclone makes landfall during high tide compared to the condition when the cyclone makes landfall during low tide. But the relation between storm surge impact and tidal phase is not linear. It depends on the land topography of the location, direction of cyclone movement, direction and magnitude of water velocity and wind velocity, gradients of water surface and wind velocity, and proximity and position of the location with respect to cyclone track.