Leveraging differences in multiple prey traits allows selective copepods
to meet their threshold elemental ratios
Abstract
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). 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, and ingested
more nitrogen-deficient cells. When offered diatoms that differed in
both size and stoichiometry, copepods mixed their diets to achieve a C:N
close to their independently estimated optimal TER. 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.