Carbonate apatite formulation in cuticle structure adds resistance to microbial attack for American lobster

Various carbonate apatite formulae contribute to discrete cuticle structures participating in protective functions of the American lobster, Homarus americanus, integument. Canal walls use their lowest Calcium : Phosphate (Ca:P) ratios to protect exposed surfaces of gland and neuronal canals. The linings insulate more soluble calcium carbonate from attack by acid secreting micro-organisms. A trabecular bone-like structure, called here ‘trabeculae’ in analogy to vertebrates, utilizes high Ca:P in the inner exocuticle demonstrating an efficient use of environmentally scarce phosphate and provides the hardness layer of the cuticle that protects the superficial calcite layer from flexure. Strength is derived from phosphatic trabeculae being embedded in a phenolicly hardened inner exocuticle layer. A third location and use of carbonate apatite is at the interface of the calcite and inner exocuticle where it may cap calcite layer development. A fourth phosphatic localization is seen in cuticular nipples that accompany the site of organule canal entry at the epidermal face of the cuticle. This high Ca:P localization may be associated with accumulation of Ca and P by canal forming cells for use in the nascent canal wall construction. A schematic model of the cuticle emphasizes regional diversity of a composite cuticle suggesting mineral function. An outer calcite crystalline layer provides a dense barrier that dissolves slowly through an intact epicuticle providing an external alkaline unstirred layer inhibitory to bacterial physiology. Superficial injury to the epicuticle and calcite layer provides a stronger flush of alkalinity from bared calcite or deeper rapidly dissolving amorphous calcium carbonate, generating a concerted general immune response by increasing alkalinity of the unstirred