Abstract
1. Chain formation is common feature among non-motile marine
phytoplankton. Several theories exist around the potential benefits of
larger colonies. 2. Here we test the hypothesis that predation is one of
the evolutionary drivers behind chain formation and chain length
plasticity. We exposed cultures of Thalassiosira rotula, Chaetoceros
curvisetus and Chaetoceros affinis to copepodamides, a chemical alarm
signal from copepods. This was coupled with a grazing experiment which
compared grazing rates on different chain lengths. 3. Our results show
that T. rotula and C. curvisetus decreased their chain lengths by 79 %
and 49 %, respectively, in response to copepodamides. Single cells and
short chains were grazed at lower rates and the copepodamide driven size
shift led to 30 % and 12 % lower grazing in T. rotula and C.
curvisetus respectively. C. affinis did not respond to copepodamides and
did not show the same size dependent clearance rate. 4. We found that
more chain forming diatoms respond to predatory cues from copepods than
previously known, and that predation is likely an important driver in
the evolution of colony size and colony size plasticity.