physical support than in terrestrial organisms. However, physical restrictions
on the size of feeding structures will start to appear as body size increases, in
common with many other mechanical devices.
Energetic optimality models suggest that growth should stop at some ener-
getically optimal body size in both passive (Sebens, 1982 , 1987) and active
suspension feeders (Acun ̃a, 2001; Humphries, unpublished). Nonetheless, it
should be remembered that the size limit set by energy balance may never be
met in animals with determinate growth (Sebens,1987).
The most obvious way to increase energetic intake from suspension feeding is
simply to increase the size of the feeding structure. As long as (i) positive net
energy gain is possible, and (ii) costs and gains increase at a similar rate with
increasing size of the capture apparatus, suspension feeding will be viable given
feeding structure(s) sufficiently large to collect enough food for reproduction
and/or growth. However, this method of increasing energy gain will produce
diminishing returns if the costs of producing and maintaining a larger feeding
structure increase more rapidly than the gain provided by that structure. This is
more than likely when body size is increased, as the metabolic costs of greater
tissue volume tend to increase much faster than the gain from an essentially
two-dimensional feeding structure. For instance, passive suspension feeders
should theoretically have capture rates directly proportional to the surface
area of their capture apparatus (i.e. increasing as body mass0.67) if their growth
is isometric (Sebens,1982, 1987). In active suspension feeders, gain rate is
proportional to the volume of water processed per unit time, thus leading to a
predicted proportionality to cross-sectional area, ciliated surface, or other appo-
site surface. Concurrently, basic metabolic costs will increase with any increase
in body size, although not necessarily linearly (Fig.2.2). As far as it is possible to
generalize, metabolic costs are likely to scale as somewhere between body
mass0.67and body mass0.75for most animals (Peters, 1983 ; Schmidt-Nielsen,
1984 ; White & Seymour, 2005 ). In addition to any possible metabolic drawbacks,
an increase in size simply may lead to movement restrictions, or compromise
function in some other way as for any other organism.
One method of circumventing, or at least offsetting, the costs of constructing
a larger feeding apparatus and a body to support it is to find a way of increasing
the size of the feeding apparatus at a rate disproportionately more than that of
the increase in body size (allometric scaling). For instance, colonial sea ane-
mones, corals and gorgonians have a growth form that allows the surface area
of their feeding apparatus to increase at a rate proportional to their mass rather
than their surface area as might be expected of animals whose body surface is
where their feeding structures are situated (Sebens, 1982 , 1987). The flat-sheet
or multiply-branched body form of these colonial or clonal animals allows their
feeding surfaces to increase linearly with body mass as colony growth appears to
occur through the production of new polyps (of equal size to the originals),22 S. HUMPHRIES