Tissue Adhesion Temporary Adhesion to Living Surfaces

All of the above examples relate to the importance of adhesion for free-living organisms, mostly but not exclusively in a marine environment, to maintain secure attachment to inert, abiotic substrates. Walker (1987) states that any surface placed in sea water becomes coated rapidly by a monolayer of polymeric material comprising deposited or adsorbed macromolecules, most of which are proteins. Subsequently, bacteria attach to the 'conditioning film' and form the 'primary film' (Walker, 1987). The primary film is a moderately negatively-charged layer which coats all marine surfaces (Characklis, 1981; Walker, 1987; Flammang, 1996), but it is not a living layer. A large proportion of organisms, however, has adopted a parasitic lifestyle (Price, 1977; Windsor, 1998) and live in or on another living organism, the host. Unlike free-living organisms, parasites of invertebrates and vertebrates must contend with a concerted effort by the host's immune system to control or reduce their populations or to remove them completely. There are many examples of the subterfuges evolved by parasites that enable them to live in sites that are particularly immunologically active, such as the blood system (e.g. molecular mimicry; Behnke, 1990; Kearn, 1998). Symbiotic and parasitic flatworms attach to a variety of cuticular, exoskeletal, epithelial and membranous surfaces in or on their hosts and their mechanical methods of attachment were referred to in the Introduction. The significance of adhesion to living tissues by parasites, however, is an area that has received no attention. Some barnacles are known to 'infect' whales, but there has been a surprising lack of study of this phenomenon although there are reports of tissue reactions against these invaders (Ridgway et al., 1997). Here, we propose the term tissue adhesion as a fourth type of bioadhesion, the importance of which has not been recognized to date.

Relatively hard, rigid and non-secretory surfaces such as the carapace of crustaceans probably provide a relatively stable and non-threatening surface for attachment by platyhelminths known to use adhesives such as the temno-cephalans (Sewell and Whittington, 1995). Attachment to cuticular surfaces probably falls between temporary adhesion and tissue adhesion. The biggest likely threat to temnocephalans is moulting when the external cuticle of the crustacean host is shed. Epithelial tissues that line the intestine of vertebrates (e.g. Castro and Harari, 1982; Ishikawa et al., 1994) and which cover the body surfaces, scales, fins and gills of fishes (e.g. Buchmann, 1998a, 1999; Whittington et al., 2000a) are active secretory layers that grow, regenerate and produce mucus containing peptides and carbohydrates that may have immunological activity. The mucus of fish, produced by the goblet or mucous cells in the epithelium, is known to be highly variable chemically between species and there are accumulating data for the presence of a variety of non-specific immune responses in the mucus of fish infected by monogenean parasites (Buchmann, 1998b, 1999; Whittington et al., 2000a). Furthermore, specific antibodies and lymphocytes may also be involved (Buchmann, 1999). Therefore, epithelial surfaces of fishes and also of amphibians (Whittington et al., 2000a) must be considered highly inhospitable and their physical properties (i.e. slimy and wet) appear to be a deterrent to colonization by organisms (Ebran et al., 1999) and 'designed' to prevent adhesion. Nevertheless, adhesion to such surfaces, especially by many species of monogeneans, is achieved with considerable success. That there may be chemical differences between the mucus of different species of fish (Buchmann, 1998a) or even between mucus from different sites on the same species offish (Buchmann and Bresciani, 1998) may have implications for the host- and site-specificity of monogeneans. This is considered further by Whittington et al. (2000a) and in Section 5.7. If mucus or perhaps some specific chemical signal in mucus or from epithelial tissue of fishes is involved in monogenean host- and/or site-specificity, it has interesting parallels with the apparent favourability or unfavourability of inert substrates in the marine environment that can be detected, identified and 'chosen' before settlement by the pelagic larvae of many free-living, benthic marine invertebrates (Hadfield, 1998).

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