Foraging based on prey elemental content could be more pronounced if consumers leveraged differences in physical traits among stoichiometrically distinct prey to ingest more favorable food, allowing consumers to meet their threshold elemental ratios (TER), the ratio of nutrients in the food where neither carbon nor nitrogen is limiting. We offered the marine copepod, Acartia tonsa, different combinations of large and small diatom congeners, grown to be either stoichiometrically replete or nitrogen-deficient. When offered each diatom alone, A. tonsa exhibited compensatory grazing on large diatoms; although ingestion was low overall, the copepods ingested more nitrogen-deficient cells relative to replete cells, resulting in A. tonsa obtaining similar amounts of N, yet differing amounts of C. When offered diatoms that differed in both size and stoichiometry, copepods mixed their diets to achieve a C:N indistinguishable from the literature-obtained value of A. tonsa's optimal TER. This was achieved by grazing relatively large amounts of the smaller diatom species, regardless of whether it was replete or N-deficient. When offered pairs of different-sized diatoms of similar stoichiometry, A. tonsa preferred larger cells, suggesting evolutionary constraints drive choice when stoichiometric differences are minimal. Experiments describing selectivity on single traits may seriously underestimate how precisely copepods can optimize their dietary stoichiometry when feeding on a diverse natural prey assemblage.