Cellulase activity and stable isotope signature of benthic macroinvertebrates in estuarine habitats: potential assimilation of land-derived organic matter

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We investigated inter-species dietary variation and potential cellulose digestion of 12 macroinvertebrate taxa collected from two locations in the estuarine Idoura Lagoon, Sendai Bay. All taxa exhibited cellulase activity (CA), which was higher among surface-deposit feeders (bivalve Macoma contabulata, polychaete Tylorrhynchus osawai, and ocypodid crabs) and obligatory suspension feeders (bivalves Corbicula japonica and Nuttallia japonica) (0.108 to 0.764 µmol min⁻¹ mg-protein⁻¹). In contrast, CA was lower among facultative suspension-feeding and deep-deposit feeding polychaetes (Hediste spp. and Heteromastus sp., respectively), and was lowest in the deep-deposit feeding polychaete Notomastus sp. The stable isotope ratios of the macroinvertebrates differed among feeding groups. A δ¹³C-based isotope mixing model revealed that the major dietary component of the surface-deposit feeders was microphytobenthos (34–50%), regardless of their high CAs. Although CAs of obligatory and facultative suspension feeders were comparable to or lower than those of surface-deposit feeders, they were highly dependent on river-derived materials at the station near the freshwater input (38–59%). These results indicate that CA is a common physiological characteristic of macroinvertebrates in estuarine soft-bottom habitats, but the dietary contribution of riverine detritus is not correlated with enzymatic activity. Our findings indicate that several factors affect the realized dietary components of macroinvertebrates, including feeding mode, the selectivity of ingestion, and digestive enzyme activity.