2 Ecological and biological background
Figure 2.10: Dorsal and lateral view of typical copepods. Both displayed species are
rather large with a body length of several millimetres. (A) In this dorsal
view of Paraeuchaeta norvegica one can see the antennules with owsensing
setae. (B) Lateral view of Metridia longa with feeding appendages.
Images by courtesy of Erik Selander, University of Gothenburg.
Ambush feeding copepods such as Oithona davisae rely solely on the movement of
their prey (by swimming, diusion, or external ows), both for the transport of prey
towards their encounter zone and for the detection through the created ows. When
noticing a disturbance in its environment, both due either to predators or to prey, O.
davisae is able to jump in a very ecient way with high speed towards its prey or away
from its predator Kirboe et al., 2009, 2010.
While copepods are important grazers of agellates, bacteria serve as their main prey.
Bacteria are prokaryotic organisms, which are abundant everywhere on earth and in all
kinds of aquatic environments, but also on surfaces as biolms, often making them a
threat to sterile environments. There are non-motile, but also motile bacteria that can
slide along surfaces or freely swim. As prey for pelagic organisms we mainly consider
bacteria which are suspended in the water column as plankton. Due to their small
size, typically of the order of 2 µm or less, bacteria are never truly still in water, even
if they do not swim. Molecular collisions make bacteria wiggle around randomly in
water. This random walk of small particles in a uid is what we call Brownian motion.
However, motile bacteria such as Escherichia coli and Vibrio harveyi can move much
faster and swim freely with the use of bacterial agella, which are ten times thinner
than the eukaryotic agella and xed in a rather rigid helical structure Berg, 2004.