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

摘要

ABSTRACT: To examine the trophic relationship between the sea squirt Halocynthia roretzi and the Pacific oyster Crassostrea gigas cocultured in suspension, their δ13C and δ15N 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 µm, CPOM) and fine particles (20 µm, FPOM), and the seasonal variations in the δ13C and δ15N were determined. CPOM δ13C was slightly more variable than FPOM δ13C over the sampling period, whereas δ15N varied less for CPOM than for FPOM. Co- and monocultured sea squirts had a less variable δ13C, but a more variable δ15N than cocultured oysters over the sampling period. The δ13C and δ15N of cocultured sea squirts were consistently lower than those of cocultured oysters. The more pronounced difference in the δ13C between the cocultured suspension feeders was attributed to a striking isotopic change in oysters during their autumn to winter growing period. Differences in the δ13C 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 13C-enrichment in oyster tissues, particularly during their fast growing period, may result from their strong selectivity of diatoms. In contrast, both the 13C- and 15N-depleted values in cocultured sea squirts indicate the importance of pico-ano-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.