Collaborative Research: Effects of Flow on Feeding Behavior and Growth Rate of Interface-Feeding Benthos: Size-Dependent Changes and Recruitment Bottlenecks

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Hentschel 0000951Juveniles of species that, as adults, feed on deposited or suspended particles face a transition from a rich larval diet (plankton or yolk) to a poor adult one (sediment-associated organics). Small body size further imposes digestive and foraging limitations that probably constrain juveniles to specialize on relatively labile foods in the sediment or benthic boundary layer (e.g. benthic diatoms, phytodetritus, organic-rich flocs, animal prey). Recent research has revealed size-dependent changes in the diets of several interface-feeding polychaetes. Although evidence of changing diets is clear, the ways in which worms' feeding activities shift as juveniles grow to adults is unresolved. For species foraging at the sediment-water interface and known to facultatively switch between deposit feeding and suspension feeding in response to hydrodynamic conditions, more frequent suspension feeding during the juvenile stage is a likely means by which small worms might increase the quality of their diet and their rate of growth. Such a size-dependent shift in feeding behavior and niche could have dramatic implications for population and community dynamics. If suspension feeding were the dominant means by which juveniles overcome size-specific food limitation, a habitat's hydrodynamic characteristics would strongly affect rates of juvenile growth and recruitment to the reproductive, adult stage. While flow's influence on dispersal and larval settlement has received considerable attention, flow-related effects on juvenile growth are a relatively unstudied organism-flow interaction that might impact the structure and dynamics of benthic populations and communities as significantly as flow's effects on dispersal and settlement.The study focuses on four species of spionid polychaetes because 1) although spionids are the best studied interface feeders, size-dependent feeding behavior and growth rates have not been measured in relation to flow; 2) existing data point to size-dependent diet changes and food-related recruitment bottlenecks in several spionids; and 3) spionids are abundant members of many benthic communities, playing central roles in food webs and geochemical processes. To test the hypothesis that juveniles tend to suspension feed more often than adults, the size-dependent feeding behavior of individuals will be observed in a laboratory flume under a variety of flow and food conditions. To quantify the nutritional consequences of size-dependent shifts in feeding behavior, growth rates of different size classes will be measured under selected flow and food treatments in four identical, counter-rotating annular flumes that will be set up as hydrodynamically distinct mesocosms. To reveal physical constraints on feeding mechanics and behavior in relation to intraspecific body-size variations, rates and efficiencies of particle capture will be quantified in a small-volume flume. To better extrapolate results from lab experiments on individuals to dense field populations, density--dependent effects on juvenile growth rate and particle ingestion will be tested. Insight into linkages among feeding behaviors and mechanics, growth rates, particle characteristics, and flow speed will provide a multifaceted and mechanistic understanding of the relationships between a habitat's hydrodynamic regime and the rates of growth and recruitment to adulthood in spionid populations.In terms of educational and human resources, the collaborative effort combines and broadens the expertise of a postdoctoral associate, a junior faculty member, and a tenured faculty mentor. In addition, integrating behavioral observations and manipulative experiments involving ecological, physical and biochemical parameters provides an ideal format to introduce undergraduates to interdisciplinary research at both a major research university and a small liberal-arts college.