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.

Aspects of the feeding biology of the pectinacean Bathypecten vulcani, a peri-hydrothermal vent bivalve

The feeding biology of Bathypecten vulcani, a peri-hydrothermal vent pectinacean, was investigated using histological and scanning electron microscope studies of the gills and stomach contents of specimens sampled from the 9° and 13°N sites of the East Pacific Rise. Salient characteristics were compared with those of Bathymodiolus thermophilus from the same and similar habitats. The gills of Bathypecten vulcani displayed heterogeneous organic and mineral particles on their frontal surface. The digestive tract possessed well-developed structures (whose anatomical and cytological characteristics indicated full functionality), typically found in bivalves from littoral aerobic environments, as well as in Bathymodiolus thermophilus, which is capable of suspension-feeding: oesophagus, stomach, crystalline style, digestive gland, and intestine. Observations of stomach contents revealed diverse particles from the photic zone, including debris from diatoms and coccolithophorans. In contrast to Bathymodiolus thermophilus, bacteria were rarely observed in the digestive tract, indicating a more complete reliance on surface-originating particles.

Seasonal variation in the feeding physiology and scope for growth of green mussels, Perna viridis in estuarine Ma Wan, Hong Kong

Seasonal variations in feeding physiology and scope for growth of green mussels Perna viridis (Bivalvia: Mytilidae) were investigated in Ma Wan, an estuarine mariculture site located in the southern waters of Hong Kong. Total particulate matter, particulate organic matter, particulate inorganic matter and organic fraction of seston (f), and food availability were monitored monthly from January to December 1998. Feeding rates, including clearance rate, filtration, ingestion and absorption, and absorption efficiency were higher in summer and autumn and lower in winter. Pseudofaeces were produced but the preferential selection of organic materials by P. viridis was not concluded. Scope for growth in July and October were higher than in February. Activities of digestive enzymes including amylase and cellulase in the digestive diverticula and crystalline style were high in May and July and lower in February and October and correlated with f. Faster growth rates obtained for P. viridis in Hong Kong's summer are considered a result of enhanced feeding and absorption at high temperatures and f values.