Keywords:Corbicula, isotopic ratios, food sources, terrestrial organic matter, Harai River
In order to eventually reveal factors controlling distribution and abundance of the bivalve Unionidae group in the Harai River, a branch of the Kushida River, Mie Prefecture, central Japan, the bivalve Corbicula as alternatives were analyzed for carbon and nitrogen isotope ratios. The Harai River is withdrawn from a dam constructed at the Kushida River, and has rich riparian forests. Samples collection was performed 4 times in 2016 and 2017, at 4 localities; C. leana from the two upstream and midstream sites (Sites 1 and 11) and C. japonica from the two estuary sites (Sites 15 and 16). Particulate organic matter (POM) in river water was also collected from 5 sites including those for Corbicula collections. We obtained 54 and 58 isotopic data for C. leana and for C. japonica, respectively, and 20 for POM. While carbon and nitrogen isotope ratios of Corbicula range widely from -26.3 to -22.2 per mil and from 5.6 to 10.8 per mil, respectively, samples from each site cluster closely (Figure). Seasonal variation is negligible, except for the C. leana of Site 11, compared differences between sites. Two populations of C. leana and one population of C. japonica comprise an array of negative correlation between carbon and nitrogen isotope ratios, which is parallel to the distribution of POM. Carbon and isotopic ratios of these three Corbicula populations are likely constrained by averaged available food sources (POM) at each site. Population of C. japonica from the lowermost site shows a positive correlation between carbon and nitrogen isotope ratios, implying their utilization of two end-members of POMs with distinct carbon and nitrogen isotopic ratios. C. japonica samples in this study was significantly lower in carbon and nitrogen isotope ratios than those reported by Kasai and Nakata (2005) (Fish. Sci. 71, 151-158), who demonstrated that terrestrial organic matter was significantly important even for C. japonica diet. The distinct result of this study may be explained by richness of riparian forests at Harai River, which have made C. japonica there further depend on terrestrial food sources. On the other hand, C. leana population from the uppermost site of the Harai River has heaviest carbon isotopic ratios, which could be attributed to an increase in utilization of planktons flourished in the dam.
Kasai, A. and Nakata, A. (2005) Utilization of terrestrial organic matter by the bivalve Corbicura japonica estimated from stable isotope analysis. Fisheries Science 71, 151-158.