Feasibility of dietary substitution of live microalgae with spray-dried Schizochytrium sp. or Spirulina in the hatchery culture of juveniles of the Pacific geoduck clam (Panopea generosa)

The production of live microalgae can be a critical constraint to economic bivalve culture due to high production cost, culture instability, and batch variability. The integration of alternatives to live algae is consequently a current trend in bivalve aquaculture, but dietary alternatives remain unexplored for the culture of the Pacific geoduck clam (Panopea generosa). To investigate the feasibility of substitution of live algae for juvenile geoduck feeding, the present study replaced a positive control microalgal diet [Chaetoceros muelleri (CM) and Tisochrysis lutea (TISO)] with various levels (0, 25, 50, 75, 100% substitution by ash-free dry weight) of the spray-dried heterotrophic thraustochytrid Schizochytrium sp. or the spray-dried cyanobacterium Spirulina (Arthrospira platensis). Feeding trials (14 days) were conducted on two sizes of juvenile geoducks [mean initial shell length: 2.47 mm (trial 1); 4.00 mm (trial 2)]. Geoduck growth and survival were measured to quantify treatment success and proximate (total protein, carbohydrate, lipid) and fatty acid analyses were conducted on diets and final geoduck tissues. Growth rate was significantly higher in individuals fed Spirulina than in those given Schizochytrium sp., but algal-replacement treatments resulted in significantly reduced growth in comparison to the control of live algae (i.e. 0% substitution) at substitution levels of 25% and higher or 50% and higher, depending on geoduck size and growth variable. The superior diet of live microalgae contained a polyunsaturated fatty acid Σn-3 / n-6 ratio of 2.9, a value which appears to be conserved in the bivalve literature (range: 2.0-3.3). Tissue analyses suggested limited digestion of spray-dried derived protein and carbohydrate, but indicated general lipid/fatty acid sequestration. Based on these findings, the dietary incorporation of spray-dried Schizochytrium or Spirulina in juvenile geoduck culture is not recommended. It is proposed that additional potential dietary alternatives attempt to mirror the nutritional composition of the combination of the live microalgae CM and TISO since this diet elicited the best growth rate in both the present study and previous research examining the effect of various microalgal species on juvenile geoduck growth.