Shellfish ... crustaceans and mollusks ... use mainly calcium carbonate as a structural material in their shells. Usually thought of as serving a mechanical protective purpose I suggest that it is used in a natural anti-fouling mechanism. A shell covering in both cases allows the CaCO3 to dissolve slowly creating an un-stirred layer in which the dissolved carbonate combines with H2O to produce HCO3 and OH which raises the pH of the un-stirred layer. This proceeds fast enough to maintain a high pH 100 micron layer and slow enough to allow the shell to survive until replaced by molting or thickened by growth on the inner surface. Such a high pH layer is inhibitory to bacterial films forming because bacteria (micron sized organisms) depend upon protons to drive their locomotion and co-transport of nutrients into their cytoplasm. They can not function properly in the 100 micron unstirred layer. This natural protective mechanism can be mimicked by an industrial antifouling paint that does not use the usual poisonous metals (usually copper) of ordinary antifouling paints. Huge amounts of these antifouling paints are used on military, commercial travel, transport and fisheries vessels and navigational buoys plus recreational boats. A natural based product would used encapsulation of Calcium and/or Magnesium Carbonate by natural sea products such as high density agar that would emulate the slow release exhibited by natural shell surfaces. Magnesium carbonate is inherantly more soluble than calcium carbonate and is useful in adjusting the solubility profile of the product. Those surfaces with their high pH un-stirred layer would inhibit microbial films which are recognized to be the substrate upon which more advanced fouling such as barnacles are built. Inhibit the initial microbial films and you interrupt the fouling process. By using combinations of calcium and magnesium carbonate as calcite or amorphous CaCO3 one provides levels of solubility mimicking the natural defense system. By providing naturally gradual effacement of the coating by natural degradation of the agar surface matrix one allows fresh surface to serve as protection over the service period of the coating. In nature the shells of lobsters, despite being thin, last several years in the older lobsters. Small injuries to the surface cuticle composed mainly of calcite (crystaline CaCO3), reveal the underlying amorphous CaCO3 which is more soluble producing an enhanced unstirred layer. Thus a properly designed coating would be effectively self-healing, extending the life of the antifouling coating. The Mg/CaCO3 and agar are all natural occurring compounds in the ocean and provide a reversal of the usual acidification that humans impose on the oceans.