Thus in the case of constant energy-specific force, the larger limit AF or ARF determines the
minimum feasible filter area. Our model places most observed filter feeders 168 in the force-limited
regime in typical oceanic environments with large energy-specific areas A > ARF > AF > Amin
170 (figure 3).
If we assume a smaller maximum investment Rmax < Rb in flow creation, the feasible range
172 of trait combinations would shrink and ARF as well as Amin would become larger and thus more
strict. A larger Rmax, on the other hand, would not increase the feasible range significantly, due to
174 the force limitation.
176 We used an energy-budget model to determine optima and limits to body composition and kinematics
in active planktonic filter feeders. The available strategies are found to be limited mainly by
178 a universal maximum energy-specific motor force which leads to constant energy-specific clearance
rates across species groups. This is confirmed by empirical findings on clearance as well as filtra180
tion rates (Alldredge and Madin 1982; Kiørboe and Hirst 2014). The limits to motor performance
restrict the access to optimum strategies.
When representing the observed energy densities for small plankton as a function of their energy
content, we observe three distinct groups (figure 4) (Kiørboe 2013). Unicellular organisms with a
‘natural’ dense body composition and a small energy content, pelagic tunicates and jelly fish with
a large energy content but low energy density, and other zooplankton with a ‘natural’ dense body
composition and high energy content. We here argue that distinct body plans are a consequence
of foraging strategy. The argument for filter feeders is that there exists a minimum energy-specific
filter area, below which the filter feeding strategy becomes unfeasible. From this minimum, A, we
can determine a maximum energy density as a function of body energy content E as
182 We have determined different characteristic values for the minimum A, the transition ARF (15),