Pallial cavity residence time in two species of bivalved molluscs: Mytilus edulis and Crassostrea virginica

Populations of bivalved molluscs can play a large ecological role by linking benthic and pelagic systems. Previous studies have shown that bivalves can compensate for changing food conditions through processes such as preferential selection and ingestion of particulate matter. Although the gross response of bivalves to changes in the quantity and quality of food have been examined, the underlying mechanisms responsible for these compensations are largely unknown. To address this issue, the fine scale feeding processes at the level of the ctenidia and labial palps were examined under conditions of differing particle quality. Pallial cavity residence time, or amount of time it takes a tracer particle to travel from the inhalent aperture to the stomach, was determined for M. edulis and C. virginica, and residence times on the labial palps were calculated. Bivalves were offered one of three food types: Rhodomonas lens cells, particles prepared from ground Spartina sp. detritus, or a 50/50 mixture of both. Once actively feeding, bivalves were delivered 10 mu m fluorescent polystyrene beads as a tracer. Bivalves were then removed at intervals from 30s up to 20 min and placed in liquid nitrogen, ensuring the cessation of particle transport. Digestive systems were isolated and examined for the presence of tracer beads, and pallial cavity residence times calculated. For mussels, it appears that food quality has little affect on pallial cavity residence time, as the residence time was less than or equal to 90s regardless of particle type. In oysters, tracer beads were initially detected at 30s when exposed to R. lens but not until 90s when feeding on the 50/50 mix. Pallial cavity residence time in oysters, when feeding on R. lens and the 50/50 mix, was twice as long as in mussels, perhaps due to extensive particle processing on the ctenidia. Furthermore, oysters given Spartina sp. particles were still lacking the presence of beads in their gut even after 20 minutes of feeding. These results suggest that food quality and degree of particle processing on the ctenidia and labial palps may affect feeding rate.