Separating the grain from the chaff: particle selection in suspension- and deposit-feeding bivalves

Particle feeding on suspended and deposited material is a common mode of food collection among many groups of the Metazoa. In particular, many members of the Mollusca, most notably the bivalves, have specialized in particle feeding. Because suspension- and deposit-feeding activities often play significant roles in ecosystem processes, particular attention has been given to the mechanisms of particle feeding by these molluscs, and to the impacts of environmental factors upon these mechanisms. Early studies of the feeding organs of bivalve molluscs were descriptive and emphasis soon was placed upon the actual uptake of particulate material from the environment and identification of this material. Introduction of electronic particle counters led to a better understanding of the rates and efficiencies at which particle removal occurred, and the interface between food availability in the ocean and responses of these animals to temporal and spatial variability. While some early workers inferred the capabilities of these animals to select particles, it was the introduction of more advanced technologies (e.g., flow cytometry, video endoscopy, confocal microscopy) that allowed more detailed studies of the mechanisms associated with particle uptake and selection by these animals. These techniques have provided a more comprehensive analysis which clearly demonstrate that the mechanisms associated with particle feeding and selection are complex, with species-specific processes based upon both physical and chemical characteristics of the particles. In this review, we summarize prior research on particle selection in suspension- and deposit-feeding bivalve molluscs, discuss current theory and controversy regarding these processes, and propose areas for further study. In particular, more research is needed to elucidate the mechanisms responsible for particle selection, particulate characteristics that impact selection, and the intrinsic and extrinsic factors that mediate these processes.