Utilization of deposited and suspended particulate matter by benthic “interface” feeders

The polychaetes Boccardia pugettensis and Pseudopolydora kempi japonica, like many invertebrates, switch from deposit feeding to passive suspension feeding as the flux of suspended particles increases. We tested the hypothesis that this behavior is energetically profitable because suspended particles have greater food value. In laboratory experiments, animals were restricted to deposit feeding, restricted to suspension feeding, or allowed to do both. Suspended particles had greater mass-specific concentrations of total organic matter, organic C, labile protein, N, and Chl a. Both species fed at significantly reduced volumetric rates when suspension feeding, but the suspension feeding rate of Pseudopolydora (only 19% of the deposit-feeding rate) was more depressed than that of Boccurdia (45-75% of deposit-feeding rate). Pseudopolydora’s growth rate was lower when suspension feeding (0.3-0.8% AFDW d-l) relative to deposit feeding (2.8-3.8% AFDW d-l). In contrast, Boccardia grew as well or better when suspension feeding (2.7-7.6% AFDW d-l) relative to deposit feeding (2.2-5.0% AFDW d-l). Despite similar morphologies and behaviors, we found unexpected differences in the abilities of these species to utilize particulate organic matter under varying boundary-layer conditions. Most benthic environments are characterized by strong vertical gradients in fluid velocity. These steep velocity gradients, and the resultant shearing forces, have important consequences for chemical exchanges between sediments and the overlying water (Aller 1984), for geophysical processes involving particle transport in the benthic boundary layer (Jumars and Nowell 1984), and for various biological processes, such as growth of autotrophic and heterotrophic microbial populations and the recruitment and feeding behavior of benthic invertebrates (Eckman 1983; Butman 1987; Frechette et al. 1989). In this paper we focus on how conditions in the benthic boundary layer affect the feeding behavior and growth of infaunal invertebrates. Invertebrates living in soft sediments are typically characterized as carnivores, deposit feeders, or suspension feeders (Hunt 1925). While convenient for classification, such divisions are often misleading and may not apply to the same species in different locations or under different environmental 1 Present address: Institute of Marine and Coastal Sciences, Cook College, Rutgers, The State University of New Jersey, New Brunswick 08903-023 1. Acknowledgments This research was supported by U.S. National Science Foundation grant OCE 88-00642. conditions. A particularly dramatic example is the ability of many infaunal species to switch between deposit feeding and suspension feeding depending on the flow regime (Brafield and Newell 196 1; Taghon et al. 1980; Dauer et al. 198 1). At low flow speeds, feeding is almost exclusively on particles on the sediment surface. At higher flow speeds, distinctive shifts in feeding behavior are now known to occur in several taxa. These behavioral changes are often characterized by reorientation of feeding appendages to feed on material in the overlying water. Even in the absence of behavioral shifts, many invertebrates that feed at the sediment-water interface may make little distinction between recently deposited particles and particles in suspension. Dauer et al. (198 1) used the term “interface” feeders to refer to species that were not obligatory deposit or suspension feeders. It may be advantageous to feed on suspended particulate matter, compared to deposited sediment. The debate continues over the relative importance of bacteria, algae, detrital particles, and the more refractory remnants of dead marine and terrestrial plants in the nutrition of deposit feeders (Tenore and Rice 1980; Lopez and Levinton 1987), although there seems to be general agreement that, at least on a gross level, particles of low specific gravity will be high-