Bivalve cultures support a host of epibionts, mainly suspension feeders, which can compete for food resources with the cultivated bivalves. However, the magnitude of interspecific competition for food in bivalve aquaculture settings remains inconclusive, especially in tropical areas. We investigated the interactions for food between the farmed pearl oyster Pinctada margaritifera and its epibionts, using stable isotope analysis and feeding experiments. Inter- and intraspecific variations of δ13C and δ15N stable isotope ratios (SIRs) were determined for oysters in the presence or absence of epibionts. The diet of the most abundant epibionts, Herdmania momus and Didemnum sp., was specified using isotope measurements and flow cytometry during feeding experiments, to determine the main phytoplankton groups consumed by these ascidians in natural conditions. The absence of intraspecific variation in SIRs among oysters with or without epibionts suggested that the diet of P. margaritifera was not affected by the presence of epibionts, indicating a reduced diet overlap and no food limitation. The δ13C signature of ascidians (−21‰) was lower than that of oysters (−18‰), indicating a difference in organic matter sources ingested by these filter feeders despite receiving the same food mixture. While the main carbon source of oysters came from large particulate organic matter (POM) >20 μm, our results showed that the diet of ascidians mainly came from smaller particles (POM <20 μm) and reflects the composition of ambient water (mainly picophytoplankton <2 μm), which confirmed their lack of food selectivity. In the studied conditions, food competition between oysters and epibionts, specifically ascidians, was not a limiting factor, in spite of a diet overlap for nanophytoplankton.