Nutritional Contribution Of Seaweed Ulva Lactuca Single-Cell Detritus And Microalgae Chaetoceros Calcitrans To The Growth Of The Pacific Oyster Crassostrea Gigas

The Pacific oyster Crassostrea gigas (Thunberg, 1793) is the most cultivated bivalve in the world. Nonetheless, the massive production of microalgae as feed represents a substantial cost during laboratory production stages. The use of single cell detritus (SCD) from seaweed Ulva lactuca, cultivated in fish farm effluents, has been proposed as an alternative to microalgae Chaetoceros calcitrans in oyster culture, with the aim of reducing microalgae production costs. Seaweed meal was subjected to an acid and enzymatic digestion process to obtain SCD particles smaller than 20 mu m to feed the oysters. Five levels of SCD (w:w) replacing microalgae were evaluated: 0, 25, 50, 75, and 100% in a feeding assay of 5 weeks. At the end of the experiment, growth parameters, condition index, enzymatic activity in the digestive gland (amylase, protease, lipase, and aminopeptidase) were analysed. In addition, during the course of the experiment, the stable isotope ratio of nitrogen (815N) was analysed at natural abundance levels in both feed sources and in the mantle tissue of oysters reared on different feeding regimes. Contribution to growth was estimated using an isotope mixing model. Better growth (74 to 94% dry weight gain) and condition indexes (81-75) were observed in oysters fed on experimental regimes having from 0 to 50% substitution of microalgae with SCD, showing no significant differences among them. Oysters under the latter treatments also showed similar enzymatic activity for amylase, alkaline-protease, and lipase. At higher substitution levels of microalgae (75-100%), oysters presented lower growth (13 to 34% dry weight gain) and poor condition indexes (<60); 100% of SCD also elicited higher amylase, alkaline-protease, and lipase activities, whereas (Leu) aminopeptidase N activity was lower. The digestive capacity of lipase was improved in oysters fed with 50 and 75% levels of SCD. Isotopic equilibrium for nitrogen (815N) was reached by day 14 in the 50% substitution treatment. Metabolic turnover rates of nitrogen decreased (0.15 to 0.08 day-1) whereas elemental half times in tissue increased (4.1 to 8.2 days) with higher microalgae substitution. Oysters under treatment with 50% microalgae substituted by SCD incorporated similar amounts of dietary nitrogen and dry matter from the microalgal biomass as from SCD to meet the nitrogen requirement for oyster growth. In conclusion, results suggest that SCD from U. lactuca can substitute up to 50% of microalgae C. calcitrans without modifying C. gigas productivity.