THE POTENTIAL USE OF LIPID MICROSPHERES AS NUTRITIONAL SUPPLEMENTS FOR ADULT OSTREA-EDULIS

A method of preparing lipid microspheres is described. These have show n potential as a diet alternative suitable for supplementing algae as sources of essential fatty acids and other lipids in the culture of ma rine suspension-feeders such as oysters. The preparation technique is fast, not labour intensive and uses inexpensive raw materials. The met hod is by sonication of a mixture of fish oil, soy lecithin, vegetable oil and vitamin E in the ratio 50: 20:29: 1 (w/w/w/w). Fish oil was a lso successfully replaced with a concentrate of n-3 fatty acid ethyl e sters, thereby increasing the amount of the essential fatty acids eico sapentaenoic (EPA) and docosahexaenoic (DHA). The microspheres have a log-normal particle size distribution of approximately 1 to 20 mum, wh ich is in the range accepted by adult bivalves. With minor modificatio ns, the size of the particles could be adjusted to match the food of o ther developmental stages of bivalves. Actual analysis for lipid class composition revealed that more than 75% (w/w) of the mass was energy- rich fish oil triacylglycerides or ethyl esters of essential fatty aci ds with a polyunsaturated fatty acid content of up to 64% (w/w). Micro spheres were stable in recirculating seawater with temperatures of up to 21-degrees-C. The oxidation stability of stock emulsions was assess ed over a storage period of 8 days and oxidation products measured as anisidine values did not increase. Bacterial growth was also not a pro blem over 8 days of storage. The water quality of a recirculating syst em was not degraded by the microspheres. Particle concentrations were in the range of 1. 8-1.9 x 10(8) ml-1 for stock emulsions, which could be diluted to any desired dispersion in the culture media. To test th e ingestion and digestion processes under laboratory conditions, fluor escent beads were encapsulated into the microspheres and fed to adult oysters together with algal cultures. Oysters were able to ingest and digest the microspheres in a concentration of 50% of the total particl es supplied as an algal-microsphere mixed diet.