A new method for encapsulating low-molecular weight, water-soluble substances in lipid-walled microcapsules (LWMs) for delivery to marine bivalves was developed, characterized, and tested. LWMs produced by a spray technique (spray microcapsules, SMs) were demonstrated to encapsulate both aqueous (polymeric dye or oxytetracycline hydrochloride) and particulate (riboflavin) core materials within a capsule wall composed of triacylglycerides. Core materials were most effectively delivered in a capsule wall composed of tripalmitin. Addition of lower melting point lipids to soften the capsule wall significantly reduced (up to a nine-fold reduction) delivery of aqueous core materials, but did not significantly affect particulate riboflavin delivery. The composition of the capsule wall was demonstrated to affect the digestibility of LWMs. Addition of 40% w/w fish oil to the tripalmitin wall was required for Manila clam spat (Tapes philippinarum) enzymatically digest LWMs (convert triaclyglycerides to free fatty acids). Delivery of aqueous and particulate core materials by microcapsules was assessed by feeding clams LWMs containing an aqueous core of polymeric dye or a particulate core of oxytetracycline hemicalcium salt (OTC.HEM). The physical appearance and absence of core material in capsules observed in clam fecal strands suggested release and delivery of core materials in the clam's digestive system. Optimal methods for encapsulating and storing oxytetracycline were assessed. Oxytetracycline hydrochloride was most efficiently encapsulated (3.2 mg core/ 100 mg lipid) and retained (30%) as an aqueous core in LWMs produced by a double-emulsion process (double-emulsion microcapsules, DEMs). OTC.HEM was most efficiently encapsulated (7.4 mg core/l00 mg lipid) and retained (66% of initial encapsulated core material remained after 24 hours suspension in seawater) as a particulate in SMs. SMs containing OTC.HEM were most stable in storage, retaining 86% of their core after 5 weeks of storage as a wet paste. Freeze-drying of SMs containing particulate OTC.HEM increased initial leakage losses of core material, but did not the affect the stability of capsules during long-term storage. Freeze-drying of DEMs containing aqueous OTC.HCl also increased initial leakage loses of core material, but improved the stability of DEMs during long-term storage. / Graduation date: 1995