deposit-feeding (picking particles from the substratum) at low velocities, and
suspension-feeding when deposited particles are resuspended by higher velocities.
Body size is somewhat nebulous and difficult to specify when it comes to
suspension feeders, as it is for other ecological aspects of body size in this
volume. Whether to use volume, length, body mass or dry weight becomes
problematic when members of the suspension-feeding functional group include
gelatinous pelagic animals that have large physical size, but low carbon content
as an adaptation to low food concentrations (Acun ̃a, 2001). Analogously, sus-
pension feeders with dense and massive shells, such as barnacles and bivalve
molluscs, have a non-trivial proportion of metabolically inactive body mass.
Similar arguments can be made for those suspension feeders that use external
structures to capture food; examples include silk nets (Trichoptera), as well as
mucus nets (polychaetes, appendicularians) and sheets (pelagic molluscs).
Many benthic suspension feeders also exhibit a predominantly two-dimensional
body plan.
The majority of issues concerning body size, for instance in relation to scaling
in aquatic versus terrestrial ecosystems (Schmidt-Nielsen,1984 ; Denny,1993 ;
Alexander,1998 ; Brown, Allen & Gillooly, this volume), clearly apply to suspen-
sion feeders. However, this chapter will focus on the organismal- and habitat-level
implications of body size that are directly relevant to the process of suspension
feeding.
The hydrodynamic implications of body size
The effects of water velocity, viscosity and scale (size of the object of interest)
in any biological system can be understood most easily by using a parameter, the
Reynolds number (Re), that describes the flow regime within or around that
system. The Reynolds number is a scaling parameter that provides a measure
(ratio) of the relative importance of inertial and viscous forces within a fluid, and
describes the way in which fluids will behave at difference scales. The Reynolds
number is given byRe¼ul/, whereuis velocity,lis the linear length scale of
interest, andis the kinematic viscosity of the fluid. Crudely, for biological
systems where size and speed are positively correlated (Vogel,1994), low values
(Re 1) indicate slow uniform (laminar) flow, while high values (Re 1000)
indicate faster, more turbulent flow. More importantly, any combination of
velocity, viscosity and scale that results in the sameRewill result in a geometric-
ally similar flow regime, as characterized by the ratio of inertial to viscous
forces. Thus, doubling the length scale will result in a flow regime that can
also be realized by doubling velocity or by halving kinematic viscosity.
As a tool,Reprovides us with a way to conceptualize the link between the
size of an object and the flow characteristics it experiences. Few organisms are
as directly dependent on these flow characteristics as suspension feeders:
although modulated by behaviour and morphology, purely physical processes
BODY SIZE AND SUSPENSION FEEDING 17