engineers. In addition, many benthic species are known to mediate competitive
interactions between species, to influence larval settlement, to control sediment
resuspension, and to impact the feeding rates of neighbours through effects on
the local flow environment arising from their morphology (Okamura,1984 , 1985 ,
1988 ;Johnson,1990 ; Wildish & Kristmanson,1997 ; Gili & LaBarbera,1998 ;
Friedrichset al., 2000 ;Cardinaleet al., 2002 ; Lindegarth, Jonsson & Andre ́, 2002 ;
Wotton, Malmqvist & Leonardsson,2003 ; Pratt,2004 ).
Benthic suspension feeders can also take advantage of some of the hydro-
dynamic peculiarities of the fluid–solid interfaces at which they live. Being
attached to a solid surface reduces the energy required to maintain position in
the flow, and thus the animals can take greater energetic advantage of the water
passing over or through feeding structures. However, the presence of velocity
gradients produced as fluid moves over a solid surface, means that average flows
near the interface are lower than in the bulk of the fluid some distance away
(Vogel,1994). These velocity gradients are generated because water passing over
a solid surface obeys what is known as the no-slip condition, where water in
contact with the surface does not move, propagating reduced velocities out into
the water column through friction between fluid ‘particles’ (Vogel,1994). While
a velocity gradient can be seen as an advantage in some cases, by allowing
animals to live in higher ambient velocities, in general the net result is a
Figure 2.4(a) Deposition and erosion of
sediment around a sedentary scallop resulting
from the ‘horseshoe vortex’ that develops
around a projecting structure in shear flow
(viewed from above), and modulation of
vortex formation (b) behind a protruding
structure (at characteristic wavelengthl,
viewed from above), and (c) behind an
accretive structure (promoting deposition at
characteristic distances 2l behind the object,
viewed from side). In (a) dark shading
indicates erosion of sediment, lighter shading
indicates deposition. Flow is from left to right
in all cases. (a) redrawn from Grant, Emerson &
Shumway (1993 ), (b) and (c) redrawn from
Turner (2000 ).BODY SIZE AND SUSPENSION FEEDING 27