Copepods avoid eating most toxic dinoflagellates and thus the toxin efficiently protects the algae against grazing
Many species of dinoflagellates produce toxic substances; when the algae are consumed by shellfish, the latter become toxic to humans and may cause serious shellfish poisoning. But what are the functional roles of these substances from point of view of the algae? Previous ‘black box’ incubation experiments have shown that copepods may consume the algae and subsequently become intoxicated and die. But this is of no use to the now dead algal cell, only potentially to its genetically identical siblings. We glued copepods to a hair straw, positioned the unaffected copepod in front of the lens of a high speed camera, and observed how copepods responded to individual cells of toxic and non-toxic dinoflagellates that arrived in the copepods feeding current. Algal cells that came within the dining sphere of the copepod were actively captured. Non-toxic cells were consumed, but toxic cells were examined for a few milliseconds and subsequently resolutely rejected. Thus, the expensive-to-produce toxins efficiently protect the cell from being eaten. It remains to be understood how the copepod knows whether a cell is toxic or not. Do the cells produce warning signals, similar to warning colours that bees and many other toxic animals produce?
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Xu J, Kiørboe T (2018) Toxic dinoflagellates produce true grazer deterrents. Ecology: in press, available online, doi.org/10.1002/ecy.2479