A siliceous arms race in pelagic plankton.

Coevolution between predator and prey plays a central role in shaping the pelagic realm and may have significant implications for marine ecosystems and nutrient cycling dynamics. The siliceous diatom frustule is often assumed to have coevolved with the silica-lined teeth of copepods, but empirical evidence of how this relationship drives natural selection and evolution is still lacking. Here, we show that feeding on diatoms causes significant wear and tear on copepod teeth and that this leads to copepods becoming selective feeders. Teeth from copepods feeding on thick-shelled diatoms were more likely to be broken or cracked than those feeding on a dinoflagellate. When fed a large diatom, all analyzed teeth had visible wear. Our results underscore the importance of the predator-prey arms race as a driving force in planktonic evolution and diversity.

Costs and benefits of predator-induced defence in a toxic diatom.

Phytoplankton employ a variety of defence mechanisms against predation, including production of toxins. Domoic acid (DA) production by the diatom Pseudo-nitzschia spp. is induced by the presence of predators and is considered to provide defence benefits, but the evidence is circumstantial. We exposed eight different strains of P. seriata to chemical cues from copepods and examined the costs and the benefits of toxin production. The magnitude of the induced toxin response was highly variable among strains, while the costs in terms of growth reduction per DA cell quota were similar and the trade-off thus consistent. We found two components of the defence in induced cells: (i) a 'private good' in terms of elevated rejection of captured cells and (ii) a 'public good' facilitated by a reduction in copepod feeding activity. Induced cells were more frequently rejected by copepods and rejections were directly correlated with DA cell quota and independent of access to other food items. By contrast, the public-good effect was diminished by the presence of alternative prey suggesting that it does not play a major role in bloom formation and that its evolution is closely associated with the grazing-deterrent private good.

Predator-induced defence in a dinoflagellate generates benefits without direct costs.

Inducible defences in phytoplankton are often assumed to come at a cost to the organism, but trade-offs have proven hard to establish experimentally. A reason for this may be that some trade-off costs only become evident under resource-limiting conditions. To explore the effect of nutrient limitation on trade-offs in toxin-producing dinoflagellates, we induced toxin production in Alexandrium minutum by chemical cues from copepods under different levels of nitrogen limitation. The effects were both nitrogen- and grazer-concentration dependent. Induced cells had higher cellular toxin content and a larger fraction of the cells was rejected by a copepod, demonstrating the clear benefits of toxin production. Induced cells also had a higher carbon and nitrogen content, despite up to 25% reduction in cell size. Unexpectedly, induced cells seemed to grow faster than controls, likely owing to a higher specific nutrient affinity due to reduced size. We thus found no clear trade-offs, rather the opposite. However, indirect ecological costs that do not manifest under laboratory conditions may be important. Inducing appropriate defence traits in response to threat-specific warning signals may also prevent larger cumulative costs from expressing several defensive traits simultaneously.