Effects of oyster age on selective suspension-feeding and the chemical composition of biodeposits: insights from fatty acid analysis

Abstract

The study objective was to clarify how the growth stages of the Pacific oyster Crassostrea gigas affect selective suspension-feeding of particulate organic matter (POM) and the composition of biodeposits. A day-long (22 h), continuous-flow mesocosm experiment was conducted with 3, 15, and 27 mo old oysters. The suspended particulate matter (PM), settled PM (mostly biodeposits in the oyster mesocosms), and oyster soft tissues were analysed to determine the content of fatty acids, organic carbon, and nitrogen, as well as the carbon and nitrogen stable isotope ratios to trace compositional changes in POM through oyster biodeposition. Regardless of oyster age, the stable isotope ratios of biodeposits were similar to those of the body tissues but not to those of the suspended PM, indicating that oysters selectively fed on assimilable fractions of POM. Compared with the suspended PM, a higher concentration of long-chain polyunsaturated fatty acids was found in the body tissues and, consequently, in the biodeposits; in contrast, the concentrations of shorter-chain fatty acids were generally lower in the biodeposits. Furthermore, the biodeposits produced by the older oysters had higher carbon, nitrogen, and fatty acid contents compared with the biodeposits produced by the 3 mo old oysters. The oxygen consumption rate of biodeposits was positively related to organic carbon content, but less so to fatty acid composition. Our findings demonstrate that older oysters not only produce larger amounts of biodeposits, but that these biodeposits have higher organic and fatty acid contents, potentially exhibiting greater effects on biogeochemical and ecological processes in nearby benthic habitats.

The Negative Relationship between Fouling Organisms and the Content of Eicosapentaenoic Acid and Docosahexaenoic Acid in Cultivated Pacific Oysters, Crassostrea gigas

“…There are two possible reasons for this. First, more than 97% of the EPA was distributed in particles of >2 μm near farm A in Shizugawa Bay [ 50 ], and dietary segregation by utilising small particles (<2 μm) by M. galloprovincialis was not valid in our study fields. The second reason was the vertical distribution of C. gigas and M. galloprovincialis in the cluster.…”

“…A portion of the diatoms ingested by the bivalves can survive [ 51 ], indicating that the faecal material of M. galloprovincialis , including diatoms, may supply the C. gigas , which are located inside the cluster. However, faecal material generally contains less EPA than suspended matter [ 50 ]. Therefore, M. galloprovincialis fouling on C. gigas could have substantial negative effects on the C. gigas in oyster aquaculture farms, in terms of EPA acquisition.…”

“…The oyster body condition index (CI1), which was developed by Lawrence and Scott [ 50 ], was calculated by CI1 = (Dfw (g))/(Ww (g) − Shw (g)) × 100 where Dfw is the dry weight of soft tissue, which was measured after freeze drying, Ww is the total wet weight of the shell and soft tissue without any fouling organisms, and Shw is the dried shell weight. CI1 expresses the ratio of the dry weight of soft tissue to the internal shell volume, with the assumption that the density of soft tissue is almost the same as that of seawater.…”