Host-specificity of monogenean (platyhelminth) parasites: a role for anterior adhesive areas?

Monogeneans (flatworms) are among the most host-specific of parasites in ge neral and may be the most host-specific of all fish parasites. Specificity, in terms of a restricted spatial distribution within an environment, is no t unique to parasites and is displayed by some fungi, insects, birds, symbi onts and pelagic larvae of free-living marine invertebrates. The nature of cues, how "habitats" are recognised and how interactions between partners a re mediated and maintained is of interest across these diverse "association s". We review some experiments that demonstrate important factors that cont ribute to host-specificity at the level of infective stages (larvae of ovip arous monogeneans; juveniles of viviparous gyrodactylids) and adult parasit es. Recent research on immune responses by fish to monogenean infections is considered. We emphasise the critical importance of host epidermis to the Monogenea. Monogeneans live on host epidermis, they live in its products (e .g. mucus), monopisthocotyleans feed on it, some of its products are "attra ctants" and it may be an inhospitable surface because of its immunological activity. We focus attention on fish but reference is made to amphibian hos ts. We develop the concept for a potential role in host-speciality by the a nterior adhesive areas, either the specialised tegument and/or anterior sec retions produced by monogeneans for temporary but firm attachment during lo comotion on host epithelial surfaces. Initial contact between the anterior adhesive areas of infective stages and host epidermis may serve two importa nt purposes. (1) Appropriate sense organs or receptors on the parasite inte ract with a specific chemical or chemicals or with surface structures on ho st epidermis. (2) A specific but instant recognition or reaction occurs bet ween component(s) of host mucus and the adhesive(s) secreted by monogeneans . The chemical composition of fish skin is known to be species-specific and our preliminary analysis of the chemistry of some monogenean adhesives ind icates they are novel proteins that display some differences between parasi te families and species. (C) 2000 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.