decrease in the rate of fluid passing over or through feeding structures. To
counter this, many animals extend their bodies, or just their feeding structures,
up through the velocity gradient where they experience increased ambient
velocities. Depending on the velocities and turbulence of the fluid involved,
the actual distance an animal may need to negotiate can vary greatly (Denny,
1993 ; Vogel,1994). For instance, a barnacle on a wave-swept shore may experi-
ence velocity gradients of the scale of millimetres, while gorgonian corals on
reef walls may need to overcome gradients many centimetres thick. The costs of
maintaining position for such animals are increased somewhat, but are still
likely to be below those of maintaining position without contact with the
substratum. A further trick is utilized by larvae of the blackfly (Simulium), in
that the larva’s body and feeding structures are used to generate vortices that
travel back though the velocity gradient to dislodge food particles from the
substratum, which can then be captured from the water column by the larva
(Chance & Craig, 1986 ). Compensation also comes from the increase in gain
resulting from feeding in higher velocities. Examples include sea pens and
brittle stars, as well as corals and gorgonians, which also exhibit growth forms
that, by reducing the area exposed to the flow, decrease the forces acting on
their attachment to the substratum. Ideally, these animals want to increase the
area exposed to food particles, but without increasing their resistance to water
flow proportionately.
The development of a velocity gradient is not limited to the substratum on
which benthic suspension feeders live. Any solid–water interface will develop a
velocity gradient and, for benthic suspension feeders, the thickness of this zone
of reduced velocity on their body surface can be important for both feeding and
respiratory exchange. For a given surface, the thickness of the velocity gradient
that develops is related toRe, and hence to body size (the further downstream
one goes, the thicker the layer of reduced velocities; Vogel, 1994 ). In general,
larger animals experience thicker boundary layers that can result in decreased
rates of diffusion and lower concentrations of oxygen close to their bodies.Conclusions
Clearly, body size is an important factor determining the ecology of the majority
of organisms. With respect to suspension feeders, their direct connection to flow
regime, and the dependence of flow regime on scale, reveals powerful physical
control of many processes. The importance of suspension feeders to aquatic
ecosystems further reinforces the interaction between body size and flow.
The existence of non-linearities already investigated in some relationships
hints at unexpected functional shifts in other areas. Dependence of feeding rate
on body size presents both opportunities and challenges for suspension-feeding
animals. Correlations between feeding rates and body size suggest that freeing
feeding structures from the constraints of scaling with surface area can improve28 S. HUMPHRIES