Gastropod Secretory Glands and Adhesive Gels
Gastropod molluscs are known for slime, yet the complexity and variety of their slimes is not always appreciated. These snails and slugs secrete visco-elastic mucous gels with functions that include feeding, protection, reproduction, locomotion, lubricati
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Gastropod Secretory Glands and Adhesive Gels Andrew M. Smith
Contents 4.1 4.2 4.3
Introduction Background Limpets and Limpet-Like Molluscs 4.3.1 True Limpets 4.3.2 Abalone 4.3.3 Slipper Shells 4.4 Periwinkle Snails 4.5 Land Snails 4.6 Terrestrial Slugs 4.7 Summary References
4.1 Introduction 41 42 43 43 44 45 45 46 47 49 50
Gastropod molluscs are known for slime, yet the complexity and variety of their slimes is not always appreciated. These snails and slugs secrete visco-elastic mucous gels with functions that include feeding, protection, reproduction, locomotion, lubrication, defense, and adhesion (Denny, 1983). While the functional demands of such disparate tasks obviously vary widely, there has been little work on the biochemical variations and different secretory structures that give rise to these functional differences. The general structure and mechanics of Molluscan mucus have been reviewed (Denny, 1983; Smith, 2002), and the biochemical structure of some adhesive gels has been analyzed (Smith, 2006). Nevertheless, we still have a long way to go in characterizing the diversity of these gels and linking differences in structure to differences in their functional properties. This is too bad, as gels are unusual materials with many important practical applications. We have much to learn about the diversity and potential of gels from molluscs; they are sophisticated gel architects, manufacturing custom materials with properties tailored to different functions. Detailed investigation of these gels could lead to significant advances in the development of novel materials. While these gels are all named mucus, that term does not denote a common biochemical structure. In fact, among gastropods and probably most other animals, the term mucus is used for any viscous secretion from an epithelium (Davies and Hawkins, 1998). Such secretions can be created by any polymers that form giant complexes. Mammalian mucus is based on glycoproteins from the mucin family (Perez-Vilar and Hill, 1999; Silberberg and Meyer, 1982). These heavily glycosylated proteins form huge complexes that entangle at low concentrations to create a loose, slippery gel. Gastropod mucus can form gels by tangling of similarly huge polymers,
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though the polymers may be quite different from mucin with different amino acid compositions and different carbohydrates. Gastropod mucus often consists of protein-polysaccharide complexes with heavily charged carbohydrates that may or may not be firmly linked to proteins (Denny, 1983), as opposed to the small, numerous oligosaccharides that are covalently bound to mucin. Denny (1983) emphasizes the range of different proteinpolysaccharide complexes that may be involved in mucus formation. The polysaccharides in gastropod mucus may consist of repeating units (Shashoua and Kwart, 1959), as in a glycosaminoglycan. In mucus, such polysaccharides are typically referred to as mucopolysaccharides. Mucopolysaccharides can be neutral or acidic, with varying degrees of charge. Because of the high charge density of
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