MEPS 255:303-309 (2003)  -  doi:10.3354/meps255303

Feeding of the bivalve Theora lubrica on benthic microalgae: isotopic evidence

H. Yokoyama*, Y. Ishihi

National Research Institute of Aquaculture, Fisheries Research Agency, Nansei, Mie 516-0193, Japan

ABSTRACT: A stable isotope study was conducted to identify the sources of nutrition for the semelid bivalve Theora lubrica living in estuarine subtidal sediments. Along an estuarine gradient in Gokasho Bay, central Japan, δ13C and δ15N were determined both in tissues of T. lubrica and its potential food sources, i.e. particulate organic matter (POM) from riverine and marine sites, sedimentary organic matter (SOM) and microalgae on an intertidal mudflat. Differences existed among δ13C and δ15N in riverine POM (yearly mean value: -26.5” and 0.5‰, respectively), marine POM (-20.4‰, 6.3‰) and benthic microalgae (-14.7‰, 4.9‰). SOM exhibited a gradual isotopic enrichment both in 13C and 15N from the estuarine station adjacent to the mudflats to the more seaward stations, indicating a progressive mixing of terrestrial and marine sources. However, the isotopic composition of T. lubrica did not follow this trend: as the sampling station was shifted seaward, δ15N increased from 7.5 to 9.4‰, whereas δ13C decreased from -16.0 to -17.8‰. These values cannot be due to the utilization of terrestrial organic matter carried by the river inflow. The observed spatial variation in the δ13C values suggest that T. lubrica has 2 dominant food sources, i.e. benthic microalgae (mainly diatoms) and marine POM (mainly coastal phytoplankton), and that in subtidal areas near the mudflats benthic microalgae were an important dietary component.

KEY WORDS: stable isotope · Benthic microalgae · Macrobenthos · Estuary · Theora lubrica

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