Mucus functions in many invertebrate physiological processes and also influences structuring of the community and the ecosystem . Molluscan mucus is mostly water . The remaining components are protein. carbohydrate and lipid . The detailed structure of the protein-polysaccharide acidic glycosaminoglycan component is not yet known . Mucus is probably released in dehydrated form in distinct. membrane-bound packages. which then absorb water . A functioning mucus is probably formed by mixing of mucins from different types of gland . Under small deformities. hydrated mucus is a viscoelastic solid, able to function as a rope. As stress increases, it yields to become a liquid which can return to the solid state once the stress is released. It is these properties that allow locomotion by molluscs on what is seemingly an adhesive. On dehydration, the strength and stiffness of mucus increase such that molluscs can suspend their body by a thread of it. Mucus production has been studied quantitatively by various methods, some gravimetric and some colorimentric using pedal, faecal, pseudofaecal and hypobranchial mucus: there is much spatial and temporal variation.
In locomotion mucus is a coupling agent between foot and substratum; a medium in which propulsive cilia beat; and a drogue. Mucus deposited as a trail by gastropods is an important facet of their environment. Many species follow mucus trails possibly contributing to the observed patchy distributions of gastropods. The methods by which the presence and polarity of mucus trails are detected is poorly understood.
Mucus plays a vital role in feeding. In filter-feeding bivalves, mucus aids the transport of food from gill to mouth and is employed to cleanse the mantle cavity of particles rejected by the labial palps. In gastropods mucus nets and bags are used to trap food prior to ingestion and some groups roll their prey in mucus to prevent its escape. Pedal mucus may be ingested after it has become studded with organic material and perhaps act as a fertilizer for microbial growth.
A copious secretion of epithelial mucus is used to isolate molluscs from their environment and mucus may also serve as an ionoregulator. Mucus may also contain specific products to render the animal poisonous, distasteful or irritating. Agglutinin and lysozyme have been found in mucus from marine molluscs. Mucus secretion can present a considerable drain of energy (up to 70% of consumed energy). The fate of molluscan mucus is largely unknown and probably makes a considerable contribution to POM in inshore waters, although its is readily degradable by marine microbes. Given the persistence of mucus, densities of benthic gastropods and their motility patterns, much of the gastropod-inhabited benthos is likely to be covered for most of the time with a layer of pedal mucus.