Thicker shells reduce copepod grazing on diatoms

The diatom frustule provides partial protection against copepods, yet we do not know the mechanism behind the reduced grazing. We used direct observations of copepod-diatom interactions to compare rejection frequency and prey-handling times of diatoms with different degrees of silica content in three differently sized species. 

We compared rejection frequency and prey-handling times of thick- and thin-shelled cells of the diatoms Cyclotella cryptica, Thalassiosira weissflogii, and Ditylum brightwellii. We found that for all three species, that thick-shelled cells were more frequently rejected by the copepods subsequent to being captured. 

The copepod prey-handling times, i.e., the time from the copepod reacted to the prey until it had been ingested or rejected, generally increased with prey size. We further found that thicker-shelled cells in the two larger species, T. weissflogii and D. brightwellii, were handled for so long that the copepods lose time for foraging. Both of these mechanisms have implications for carbon sequestration and for securing a large seed population at depth, and may be why many diatoms are able to form blooms even in the presence of dense grazer populations. 

Methods
Please see Methods in accompanying manuscript for details.