Background <p>Assessment of the dietary ecology of organisms is crucial for understanding community ecology. Delineation of food webs requires in-depth understanding of how predatory fish diet varies from species to species, as well as how diet varies across the life history of any single species. The cunner wrasse (<i>Tautogolabrus adspersus</i>) is a common neritic demersal temperate marine fish. Yet, little is known about how its diet changes as a function of ontogeny, size and sex. This study investigated the effect of size (weight, total length, body condition) and sex on dietary composition of cunner across a 1755-km stretch of the northwestern Atlantic coastline using traditional morphological and dietary DNA metabarcoding methods of gut content analysis.</p> Results <p>Analysis of diet using ordination (PCA/PCoA) and statistical tests (PERMANOVA) revealed that size metrics like weight and length significantly influence the diet of this species. Small cunner contained more soft-bodied organisms (ascidians, anemones) whereas larger cunner guts contained more hard-shelled organisms (bivalves, crustaceans). This increase in consumption of hard-shelled organisms with size could be potentially driven by jaw gape limitations of smaller cunners. Sex was rarely found to have a significant effect on diet except in regions wherein only large cunner were present. Limited size ranges within many locales may have reduced our ability to detect size differences in diet.</p> Conclusions <p>These results demonstrate that size-based differences in cunner diets do exist, even when regional differences in size are accounted for. These results highlight the significant change in prey consumption with increasing adult size in this species; with a shift in general diet from small soft-bodied macroinvertebrates to larger invertebrates with hard body parts. This work demonstrates how secondary consumers can link different parts of marine food webs even within a single individual’s lifetime.</p>