Diatoms–copepods: An evolutionary arms race

Abstract Diatoms and copepods are dominating and diverse groups of phytoplankton and zooplankton, respectively. Diatoms account for 25–40% of ocean primary production, and their main predators, copepods, are arguably the most abundant group of metazoans in the ocean, typically accounting for ~ 80% of zooplankton biomass. The two groups have co‐existed in the pelagic for maybe > 200 million years, where they have engaged in what appears to be a never‐ending evolutionary arms race. The arms race includes the evolution in diatoms of a thick silicious shell, the frustule, with a delicate nanostructure that makes it hard to break, the production of toxins that may deter copepod grazing, the ability to change colony size to get outside the prey size spectrum of copepod grazers, and the potential to form rapidly sinking aggregates that allow the diatoms to rescue from predators in the ocean depths or sediments. These defenses are all inducible, that is, they are harnessed or strengthened in response to diffusible cues from copepod grazers. In all cases, copepods have evolved anti‐defense mechanisms that allow them to at least partly overcome the defenses and/or responses that protect the copepods against poisoning and mechanical damage. We argue that the evolutionary arms race between diatoms and copepods is key to their evolutionary success in terms of abundance and diversity, and that it has partly shaped the pivotal role of diatoms and copepods in the biological carbon pump and carbon sequestration in the ocean.