dual-function organs, these animals might profit by increasing the size of their
feeding apparatus for a given body size. However, the above scaling arguments
still hold for further increases in the size of feeding apparatus, and thus body size.
A particularly interesting solution to the scaling problem is the use of external
capture apparatus, usually silk or mucus nets, by animals such as the larvae of
caseless caddis flies (Trichoptera) and many polychaete worms, respectively.
Here, the solid substratum provides support and anchor points for a net that
the animal constructs, and the animal is thus able to increase the size of its
feeding structure without an increase in body size. The increase in filter area in
relation to body size can be very large: one species of caseless caddis fly with a
body mass of less than 6mg regularly builds nets up to 20cm long (Petersen,
Petersen & Wallace,1984 ). Although the costs of net construction are relatively
high (inPolycentropus flavomaculata, the process of silk spinning and net construc-
tion raises metabolic rates by about 17% (Dudgeon,1987 )), they are likely to be
considerably less than the metabolic costs of a body large enough to hold an
internal feeding structure of similar size. Furthermore, the costs of building an
external structure are ‘capital’ costs, in that they do not require sustained invest-
ment at a high level, as would equivalent volumes of metabolically active tissue.
Use of an external feeding structure is also common in oceanic environments.
Pelagic pteropod molluscs use large mucus nets suspended from their body to
capture planktonic food. The low-turbulence environment of the open ocean
allows them to construct fragile nets that are not constantly torn apart by forces
generated by turbulent water movement (Harbison,1992). Many pelagic sus-
pension feeders probably also utilize another mechanism for increasing gain
and reducing costs that is facilitated by their low turbulence environment: the
development of gelatinous bodies. Pelagic tunicates appear to use metabolically
inert gelatinous tissue to increase their body size so that they can support a
larger filter without the energetic costs of increasing the volume of metabol-
ically active tissue (Acun ̃a, 2001). Indeed, some deep-sea species of append-
icularians with bodies 75 mm long build external gelatinous ‘houses’ with
diameters of up to 1 m (Ruppert & Barnes,1994), presumably in response to
the very low seston concentrations in their environment. Acun ̃ a’s (2001) argu-
ment is based on the scaling of gain to costs in a similar way to that discussed
above, but invokes a quadratic increase in pumping costs with filter size, in
comparison to a linear increase in gain as the driving force for the development
of gelatinous bodies. As seen above, however, the same principle of costs
increasing more rapidly than gains is equally applicable to passive suspension
feeders (Sebens, 1987 ).Particle capture as a predator–prey relationship
The existence of a positive relationship between the body size of a predator to
that of its prey (Peters,1983) is currently only an assumption for suspension24 S. HUMPHRIES