The influence of an intermittent food supply on the feeding behaviour of Yoldia hyperborea (Bivalvia: Nuculanidae)

Yoldia hyperborea (Lovén) is a protobranch bivalve of circumboreal distribution and an important bioturbator of muddy sediments of cold-water embayments in Newfoundland, Canada, where it is exposed to a strong seasonal input of sinking phytoplankton during spring, sporadic events of sediment resuspension, and sediment of low nutritional value during winter. To explain field-observed patterns of population dynamics, we quantified the behavioural response of Y. hyperborea to pulses of settling algae and sediment resuspension events, and hypothesised that Y. hyperborea behaviour is modified during settling of nutrient-rich organic matter. Yoldia hyperborea responded rapidly to the arrival of settling microalgae by extending its siphons into the water column. Once the pulse of algal material reached the sediment the animals partially emerged, extended the palp proboscides over the sediment surface and maintained close contact with the area of highest algal concentration. In contrast, the activity of animals not exposed to settling algae or to resuspended sediment was primarily restricted to strata below the sediment surface. Thus the predominant subsurface feeding behaviour of Yoldia hyperborea is switched to surface deposit-feeding as a response to cues contained in microalgae. Results suggest active suspension-feeding during algal sedimentation events, although deposit-feeding resumes once the algae are no longer in suspension, and suspension-feeding is probably of little nutritional significance. The rapid behavioural response of Y. hyperborea to the influx of high quality food, such as fresh microalgae, is probably an adaptive foraging strategy to a food-limited environment and suggests higher bioturbation rates of surface sediments during spring, a key role being played by the protobranch in redistribution of labile phytodetritus within the benthos.