We compared gut length, delta13C and delta15N signatures, and expressed genes in the pyloric ceca and liver of wild-caught individuals of four closely-related prickleback species (family Stichaeidae) with different diets: Xiphister mucosus (herbivore), its sister taxon X. atropurpureus (omnivore), Phytichthys chirus (omnivore) and the carnivorous Anoplarchus purpurescens. We also measured the same parameters after feeding them carnivore (all four species) or omnivore (X. mucosus and A. purpurescens) diets in the laboratory for four weeks.