1.3 Framework and focus of this thesis
animals is that the prey size is microscopic and often nearly independent of predator
size Fenchel, 1986; Lombard et al., 2011.
An important group of unicellular lter feeders are choanoagellates that possess
one agellum which drives a feeding current through a collar lter Fenchel, 1982;
Leadbeater, 2015. In paper III we use ow measurements on one choanoagellate
species as well as computational and analytical modelling to connect prey encounter
rates with morphological characteristics of agellum, lter, and additional supporting
structures in individual organisms.
Beside their dierent body size and motor design, large planktonic lter feeders are
often characterized by more watery, gelatinous bodies than their microscopic opponents.
In paper IV we develop a model of the energetics of general planktonic lter
feeders with dierent body compositions that we apply to unicellular choanoagellates
as well as gelatinous tunicates. Using the model we demonstrate that large lter feeders
need to become gelatinous in order to support large enough lters, while small
organisms have no need to change their body composition.
In paper V we investigate the physical selection of prey sizes by agellates. The
predator-prey size ratio and prey size range can vary signicantly in small feedingcurrent
feeders, which encounter everything that arrives in the feeding ow Gonzalez
and Suttle, 1993; Hansen et al., 1997a; Lombard et al., 2011. We investigate how
physical prey encounter and contact are aected by prey size. We numerically and
analytically explore the advective-diusive transport and capture of particulate prey
on a towed sphere as a simple model of a direct-interception feeder. For sloppy feeders,
which lose a certain fraction of their prey through nite foraging rates, we investigate
the inuence of contact times by exploring how the prey capture rates for dierent prey
sizes increase towards their asymptotic maximum that represents a perfect absorber.