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

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, delta(13)C and delta(15)N 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 delta(13)C and delta(15)N in riverine POM (yearly mean value: -26.5parts per thousand and 0.5parts per thousand, respectively), marine POM (-20.4parts per thousand, 6.3parts per thousand) and benthic microalgae (-14.7parts per thousand, 4.9parts per thousand). SOM exhibited a gradual isotopic enrichment both in C-13 and N-15 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, delta(15)N increased from 7.5 to 9.4parts per thousand., whereas delta(13)C decreased from -16.0 to -17.8parts per thousand. These values cannot be due to the utilization of terrestrial organic matter carried by the river inflow. The observed spatial variation in the delta(13)C 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.