To provide a method for delivering at least one drug to an egg (s) of an egg-aquatic animal, wherein the presence of a guanidine-contai compound capable of enhancing the permeability of the chorion of the egg (s) , Comprising contacting the fertilized or unfertilized egg (s) of the ovipositing aquatic animal with at least one agent. A method for drug scree and compound toxicity assay is provided. Methods for the production of breeding fertile fish and aquatic animals for aquaculture, aquarium industry are also provided and control of invading species is explained. The method involves interfering with the development of the gonads through administration of a drug which leads to poor development of reproductive gonads. The drug can be delivered directly to the egg before or after fertilization by bringing the egg into contact with an immersion medium contai the agent of interest. BACKGROUND OF THE INVENTION
