Size-related and habitat-dependent trophic dynamics of Manila clam, Ruditapes philippinarum, via bulk and amino acid isotopic signatures

초록

Individual body size plays a fundamental role in shaping the trophic dynamics of large marine predators that feed on prey at different trophic levels. However, this has rarely been explored in filter feeding animals that rely on temporally and spatially variable diet sources of similar trophic level. We tested whether the trophic dynamics of the filter-feeder Manila clam ( Ruditapes philippinarum ) are significantly associated with size gradients in different environments, a less sandy site (Pado) and a more sandy site (Hwangdo), via analyzing bulk and amino acid (AA)-specific carbon and nitrogen isotope ratios. Our results showed that there were significant site variations in clam abundance in addition to bulk isotope values, phenylalanine (Phe)-δ15N values, and AA-based trophic positions (TP<inf>AA</inf>), suggesting site-specific trophic baselines. Consistently, in both sites, larger individuals exhibited higher isotope values and elevated even TP<inf>AA</inf> (adjusted R2 = 0.21–0.44), except for Phe-δ15N values, implying size-dependent trophic shifts in the filter-feeder clam, determined as herbivore. Based on δ13C patterns of essential AAs, clams along with microphytobenthos co-sampled in situ were close to a microalgal group (e.g., dinoflagellates), regarded as nutritionally dominant resources. Our data suggest that the individual size spectrum, equivalent to environmental differences, influences the trophic variation in herbivorous clams that feed on benthic algae. These findings underscore the need for ecosystem-based fishery management approaches that incorporate the size-dependent trophic dynamics of benthic consumers, rather than relying on uniformly sized consumers. © © 2026. Published by Elsevier Ltd.

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