ADHESION IN BYSSALLY ATTACHED BIVALVES

The byssus is a structure produced by marine bivalve molluscs to adhere, usually permanently, to substrata under water. As the adhesion of synthetic polymers to surfaces is predictably compromised by the presence of water, particularly bulk water, it is of particular interest to discover the mechanism of byssal adhesion. In most species, the byssus consists of at least four essential components: acid mucopolysaccharides, adhesive protein, fibrous proteins, and an oxidative enzyme, polyphenoloxidase. The function of the mucopolysaccharide component is still uncertain, but it can conceivably be used by the animal as a temporary adhesive, a surface modifying agent, and/or a stabilizing filler for the permanent adhesive. The adhesive protein known as the polyphenolic protein in Mytilus is but a thin plaque applied to the substrate surface by the foot of the animal. The molecular and physical properties of this adhesive protein conform remarkably well to what one expects of an ideal synthetic polymer, i.e. high molecular weight, abundance of large and polar side chains, near-zero surface contact angle, and total water-insolubility after setting. The fibrous proteins constitute the major portion of the thread or ribbon-like material connecting the animal to the adhesive plaque on the substrate surface. These proteins are packed in ordered crystalline arrays, e.g. β-pleated sheet and collagen helix (in mytilids) as is to be expected from structural tensile elements of Nature. The enzyme polyphenoloxidase is presumed to induce intermolecular cross-linking of proteins in the fibrous and adhesive portions of the byssus. In Mytilus the natural substrates of the enzymc may be the dopa-containing polyphenolic protein and accessory gland protein.