fundamentally determine the food-capture rate of suspension feeding animals.
In this context body size can be seen as synonymous with flow regime, withRe
relating as it does, a single measure of size to environmental conditions.
In biological systems, small (and hence slow) organisms tend to operate under
conditions where the fluid they are immersed in is dominated by viscous forces
(lowRe), while larger, faster organisms operate at higherRe, where inertial
forces prevail (Vogel,1994). While highRecharacterizes the flows at a human
scale, the behaviour of lowRe flows are frequently counterintuitive. For
instance, if a bacterium were suddenly to cease swimming, it would come to a
halt in a distance much less that the diameter of a hydrogen atom (Berg, 1983 ).
Lengths of suspension feeders vary over five orders of magnitude, from single-
celled protists to baleen whales, while the Reynolds number of such organisms
in general ranges betweenRe 10 ^6 (bacteria) andRe 108 (large whales), or 14
orders of magnitude (Nachtigall,2001). This provides a dramatic range of con-
ditions in which suspension feeders operate, and leads to an array of adapta-
tions to this feeding mode.
Despite such large variations in body size, aerosol theory co-opted from engi-
neering (Rubenstein & Koehl,1977 ) suggests that there are only five inclusive
mechanisms by which particles can encounter collecting elements: (i) direct
interception, (ii) inertial impaction, (iii) gravitational deposition, (iv) diffusional
deposition and (v) electrostatic attraction. Simple sieving (where particles are
caught because they are larger than the gap between collecting elements) is
generally added to this group, although in the process of sieving particles encoun-
ter the feeding structure via one of the five classic mechanisms. Direct intercep-
tion involves streamline kinematics: particles travelling on a streamline that
passes around the collecting element will come into contact with that element
if they come within one particle radius. Inertial impaction relies on particles that
are denser than the surrounding fluid failing to follow rapidly turning stream-
lines due to their inertia, while gravitational deposition relies on a particle’s mass
causing it tocross streamlinesand thusimpact the collector. For smaller particles,
diffusional deposition may be important, whereby Brownian motion of tiny
(< 1 mm) particles causes them to cross streamlines and impact the collecting
element. Finally, electrostatic attraction may act to draw some particles towards
collecting elements in non-electrolyte fluids.
The obvious restrictions imposed by only five inclusive mechanisms for par-
ticle capture suggest that evolutionary convergence in feeding methods may
be inevitable. Differences do exist, however, in the importance of the different
mechanisms to particular organisms, a factor itself determined by morphology,
particle type and availability, and to some extent by body size. What is now clear
is that these particle-encounter mechanisms comprise an ‘envelope of the pos-
sible’, and are common to almost all suspension feeders (Rubenstein & Koehl,
1977 ; LaBarbera,1984 ; Shimeta & Jumars,1991 ; Vanderploeg,1994 ).18 S. HUMPHRIES