Two infaunal species, the purple clam Amiantis purpurata and the razor clam Solen tehuelchus, are common species in the 15–20 m sandy bottom sediments between southern Brazil and central Argentina. Both species are food sources for the coastal food web, but the extent of their contribution to this food web is still unknown. Based on stomach content analysis and δ¹⁵N and δ¹⁵C stable isotope signatures we explored the position of these clams in the food web. Stomach content shows that the ray Sympterigia bonapartei and the white croaker Micropogonias furnieri prey on entire A. purpurata and S. tehuelchus. The stable isotopic analysis confirms this result. The elephant fish Callorynchus callorynchus and the Brazilian codling Urophycis brasiliensis also show values of C and N consistent with those expected from species that are preying on A. purpurata. Based on the prediction from the isotopic analysis the clam A. purpurata is a food source for the shrimps Artemesia longinaris, Pleoticus muelleri, the anemone Antholoba achates and the gastropod Buccinanops monilifer. The gastropods Adelomelon brasiliana, Olivancillaria deshayesiana, Olivancillaria urceus and Zidona dufresnei also show evidence of consuming A. purpurata but with contribution from other species with heavier C content. According to complementary results the razor clam Solen tehuelchus may be this other species. The stable isotopic analysis shows that both clam species are at the base of the consumers in the food web. A. pupurata showed lighter C mark than S. tehuelchus, but the N isotopic mark showed higher value but still being within the same trophic level.