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The last 2 decades have seen much controversy concerning the negative impacts that consumption of diatoms may have upon copepods. Because diatoms have traditionally been consid- ered a major food item for copepods, this controversy has raised fundamental questions concerning our understanding of the marine food chain. The mode of operation of this negative impact is via so- called insidious effects on the survival of early-stage copepod progeny, caused for example by polyunsaturated aldehydes (PUA), with the mechanism proposed as a defence evolved by diatoms. Here the effects of diatoms on copepod reproduction rates are evaluated using a stoichiometric weight- or age-stage copepod model coupled with variable stoichiometric descriptions of prey. We show that, considering different combinations of copepod population age structure and diet compo- sition (diatoms, non-diatoms, microzooplankton and copepods), the concept of an 'insidious' effect of diatom consumption upon copepods cannot be sustained as a defence mechanism, as it does not con- fer any advantage to the diatoms. While metabolites that deter grazing can be advantageous, slowly killing a predator (via the death of juvenile copepods) responsible for consuming competitor primary producers, and also consuming another predator (microzooplankton), is not advantageous to the diatoms. Indeed it is shown to be counterproductive on occasion. While PUA (or compounds that act in a similar fashion) have the potential to adversely affect copepod production, alternative explana- tions for their production need consideration. Such alternatives include the potential impact of PUA against microzooplankton, or simply that PUA represent secondary metabolites of no evolutionary selective advantage.

Aldehyde-induced insidious effects cannot be considered as a diatom defence mechanism against copepods