Isotopic evidence of particle size-dependent food partitioning in cocultured sea squirt Halocynthia roretzi and Pacific oyster Crassostrea gigas

To examine the trophic relationship between the sea squirt Halocynthia roretzi and the Pacific oyster Crassostrea gigas cocultured in suspension, their delta C-13 and delta N-15 values were monitored monthly on the southern coast of Korea for a period of 2 yr. Suspended particulate organic matter was fractionated as coarse (>20 mu m, CPOM) and fine particles (<20 mu m, FPOM), and the seasonal variations in the delta C-13 and delta N-15 were determined. CPOM delta C-13 was slightly more variable than FPOM delta C-13 over the sampling period, whereas delta N-15 varied less for CPOM than for FPOM. Co- and monocultured sea squirts had a less variable VC, but a more variable delta N-15 than cocultured oysters over the sampling period, The delta C-13 and delta N-15 of cocultured sea squirts were consistently lower than those of cocultured oysters. The more pronounced difference in the delta C-13 between the cocultured suspension feeders was attributed to a striking isotopic change in oysters during their autumn to winter growing period. Differences in the VC were significant between co-, monocultured, and wild sea squirts, but not between co- and monocultured oysters. These suspension feeders can use different POM size fractions within the same habitat. The marked C-13-enrichment in oyster tissues, particularly during their fast growing period, may result from their strong selectivity of diatoms. In contrast, both the C-13- and N-15-depleted values in cocultured sea squirts indicate the importance of pico-/nano-size fractions as their dietary components. Size-related patterns in food resource exploitation between the cocultured suspension feeders may be due to different particle capture mechanisms.