4 Summary of the results
One strategy is to maximise the specic scope for growth at a certain (characteristic)
prey concentration (Hmax) and another is to minimise the prey concentration that the
organism is able to survive at and each strategy leads to a dierent optimum lter
ow speed. Both fast growth and prevention of starvation at low prey abundance are
essential for successful strategies in marine pelagic organisms, but the measured ow
speeds in choanoagellates as well as salps are lower than what is expected from the
described optimum strategies.
Thus we consider the motor performance limits that depend on the motor size (energy
content) and which determine maximum feasible lter ow speeds (gure 4.7 B).
Both a maximum motor power and a maximum motor force are considered and we
assume that either can ultimately limit the lter-feeding performance. We nd the
force limit dominating in most of the feasible range, where the scope for growth is
positive at realistic prey concentrations. This leads to constant specic clearance rates
across size classes as optimum within the force constraint and we nd that this trend
is conrmed by species-overarching data on clearance rates.
The specic scope for growth decreases linearly with the energy-specic lter area
and becomes negative below a certain minimum energy-specic area at characteristic
prey concentrations. With a typical body energy density the surface area and thus the
lter area for small organisms is naturally large compared to their volume and energy
content and thus they will as lter feeders have large enough scopes for growth to survive.
This is what we call the dense dwarf strategy. However, with increasing energy
content the area to volume ratio and thus the energy-specic lter area decrease and
can only be increased by changing the body composition, leading to large gelatinous
bodies (gelatinous giant strategy). We can generally predict the existence of a maximum
energy density that decreases with body energy content (gure 4.8). Organisms
that rely on actively created ows through internal, ne-meshed lters are not able to
survive with higher densities. Thus, with a high energy content those low Reynolds
number lter feeders, that cannot nd dense patches of prey or switch to raptorial
feeding, can only capture enough food in dilute environments, if they are gelatinous.