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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 μm 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 (δ15N) 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 ( |
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