Ebook Influence of diet on pre-ingestive particle processing in bivalves I: Transport velocities on the ctenidium
Dense populations of suspension-feeding bivalve molluscs, such as oysters and mussels, play dominant roles in the ecology of coastal ecosystems. Bivalves not only convert heterogenous mixtures of suspended material into animal flesh that can be used by higher trophic levels, but also are one of the strongest interactors in shallow water benthic–pelagic coupling (Dame, 1993a,b, 1996). Through suspension-feeding activities, bivalves can strongly affect pelagic and benthic processes by removal of phytoplankton, deposition of feces and pseudofeces, and cycling of dissolved nutrients to the water column. Tight coupling may lead to stimulation of phytoplankton blooms (Lewin et al., 1975), depletion of phytoplankton (Cloern, 1982; Officer et al., 1982), changes in phytoplankton composition (Baker et al., 1998), or more complex ecosystem level effects (Dame et al., 1984; Newell, 1988; McKee, 2002).
Bivalve molluscs are exposed to a suspended food supply that varies in size, concentration, and quality, over both spatial and temporal scales. The food source includes material supplied directly from the water column or by resuspension events (Frechette et al., 1989; Judge et al., 1993). The way in which bivalves mediate benthic–pelagic processes is complicated by the fact that very few particle feeders are so simple that they merely encounter particles, engulf them, and excrete metabolic end products. Rather, in bivalves there is strong evidence that changes in the content of non-nutritive particles elicit compensatory feeding responses (Bayne et al., 1987, 1993; Iglesias et al., 1992; Barille et al., 1993; Bacon et al., 1998; Hawkins et al., 1998; Cranford and Hill, 1999; Levinton et al., 2002). Studies have also indicated differential handling of particles by bivalve pallial organs (Ward et al., 1997, 1998a) and qualitative selection (Newell and Jordan, 1983; Prins et al., 1991; MacDonald and Ward, 1994; Bougrier et al., 1997; Defossez and Hawkins, 1997; Ward et al., 1998a; Brillant and MacDonald, 2000), even of similar-sized particles (Shumway et al., 1985, 1997; Ward and Targett, 1989; Ward et al., 1997). Therefore, the way in which bivalves deal with the heterogenous mixture of suspended particles could affect the quantity and quality of material that is cycled back to the water column or delivered to the benthos via pseudofeces and feces.
In order to understand the issue of particle processing in relation to water column supply, it is crucial to have a mechanistic understanding of the feeding activities of bivalves, including clearance rates, organic and inorganic particle sorting, ingestion capacities, digestion efficiencies, and the potential feedback mechanisms that integrate and coordinate these feeding functions. Equally important is an understanding of how changes in ncentration and quality of suspended material affects the coordination of suspension-feeding mechanisms and the ability of bivalves to respond to changing particle regimes to optimize energy intake. Although a large body of data exists concerning the behavioral and physiological responses of bivalves to changing environmental conditions (e.g., Hawkins and Bayne, 1992; and see papers in Dame, 1993a; Bayne and Warwick, 1998), the underlying mechanisms accountable for these responses are largely unknown. In particular, there is a paucity of information on how the major feeding organs, ctenidia and labial palps, respond to changing particle regimes or gut fullness. This includes little information on the factors that mediate the velocity at which material is transported along the ctenidia. From the few studies that do exist (Beninger et al., 1992; Levinton et al., 1996; Richoux and Thompson, 2001), there is compelling evidence that changes in particle collection and transport may serve as a mechanism to regulate the amount of material delivered to the labial palps and ultimately to the mouth for ingestion.
Studies using tools such as endoscopic examination and confocal microscopy have provided unique insights into the fine-scale feeding processes of bivalves (Ward et al., 1993, 1994, 1998b, 2000; Tankersley, 1996; Beninger et al., 1997; Baker et al., 2000), and conceptual models of particle capture and processing have been developed (Levinton et al., 1996, 2001; Ward, 1996; Ward et al., 1998b). In this study, we used video-endoscopy to examine how changes in the quantity and quality of suspended material affect particle transport velocities within the ventral grooves and dorsal tracts of the ctenidia of four species of suspension-feeding bivalves. The goal of our research was to begin quantifying the rate functions associated with particle handling and feeding to obtaining data on feeding processes at the scale of the pallial organs. A knowledge of the mechanisms of particle processing at the level of the individual could contribute to a better understanding of processes at the population level (Dame, 1993b).
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