An important element in most approaches to the subject of parasitism is the consideration of environment. Parasites are set apart within animal ecology because they experience two environments, one the ‘external’ conditions and the other created by the living body of the host. As in any ecological system, external environmental conditions have a major influence on life history parameters: these conditions may be experienced directly by ‘off-host’ stages of a parasite or, to a greater or lesser extent, indirectly through the body of the host. However, uniquely in parasitic associations, the internal (host) environment has a dual influence on the physiological conditions encountered by parasites: first, the host buffers the external conditions (by homeostatic mechanisms moderating environmental fluctuations, by behavioural responses selecting appropriate habitat conditions, etc.), but second, the host creates a suite of hostile factors associated with immune defence. This living, reactive environment has no parallels elsewhere in free-living animal ecology: it has the characteristic of reacting specifically to kill the organisms within its boundaries.

Low temperatures affect the rate of growth, development and metabolism of parasites and when temperatures fall below 0°C may expose the parasite to the potentially lethal risk of freezing. Some parasites have mechanisms, such as diapause, which synchronise their life cycle with favourable seasons and the availability of hosts. Parasites of endothermic hosts are protected from low temperatures by the thermoregulatory abilities of their host. Free-living and off-host stages, however, may be exposed to subzero temperatures and both freezing-tolerant and freeze-avoiding strategies of cold hardiness are found. Parasites of ectothermic hosts may be exposed to subzero temperatures within their hosts. They can rely on the cold tolerance adaptations of their host or they may develop their own mechanisms. Exposure to low temperatures may occur within the carcass of the host and this may be of epidemiological significance if the parasite can be transmitted via the consumption of the carcass.

The ability of certain species of parasitic nematodes to survive desiccation for considerable periods is a fascinating example of adaptation to the demands of fluctuating environments that occasionally can become extreme and life threatening. Behavioural and morphological adaptations associated with desiccation survival serve primarily to reduce the rate of drying, either to prolong the time taken for the nematode's water content to reach lethal low levels or, in true anhydrobiotes, to enable the structural and biochemical changes required for long-term survival to take place. Examples of these adaptations are reviewed, together with information on the factors involved in rehydration that ensure successful exit from the dormant state. Information on desiccation survival is central to effective management and control options for parasitic nematodes. It is also required to assess the feasibility of enhancing the longevity of commercial formulations of entomopathogenic nematodes, both before and after application; current research and future prospects for enhancing survival of these bio-insecticides are discussed.

Deserts represent universally recognized extreme environments for animal life. This paper documents the highly specialized adaptations of Pseudodiplorchis americanus, a monogenean parasite of the desert toad, Scaphiopus couchii. Building on a long-term record of parasite population ecology (continuing since the early 1980s), field studies focus on the effects of severe drought in the Sonoran Desert, Arizona, in the mid 1990s. This provides a test of the ability of the host-parasite system to tolerate exceptional perturbation. The analysis provides new insight into parasite infection dynamics in a natural wildlife system through integration of host and parasite population age structure. The environmental check interrupted host recruitment in 1993–95 and parasite recruitment in 1995–97. This produced an imprint in age structure and infection levels recognizable over several years: parasite recruitment failure reduced transmission 2–3 years later. The host (maximum life span 17 years) tolerated the disruption but the impact was more serious for the parasite (life span 3 years) leading to extinction of some previously stable populations. Despite this demonstration of a rare event exacerbating external environmental constraints, experimental studies suggest that the internal (host) environment normally creates the most severe conditions affecting P. americanus. Only about 3 % of parasites survive from invasion until first reproduction. Post-invasion factors including host immunity, characteristic of most parasite life cycles, constitute a greater constraint upon survival than external conditions, even in a desert environment.

This review deals with the problems faced by those monogenean (platyhelminth) parasites that attach themselves to fish skin. The structure of the skin and the ways in which the posterior hook-bearing haptor achieves virtually permanent attachment to the skin are considered. Small marginal hooklets are specialized for attachment to superficial host epidermal cells, finding anchorage in the terminal web of keratinous tonofilaments, while large hooks (hamuli) may penetrate into and lodge in the collagenous dermis. The complementary roles of suction and sticky secretions in haptor attachment and the role of the pharynx in temporary attachment during feeding are also considered. During leech-like locomotion the haptor is briefly detached and, at this critical time, the anterior end is strongly fixed to the wet, current-swept and possibly slimy skin by a sticky secretion. This secretion is deployed on paired pads or discs, the latter sometimes backed up by suction. After attachment by the haptor is re-established, the special tegument covering the anterior adhesive areas may be instrumental in their instant release. The role of fish skin in the phenomenon of host specificity and in the generation of a defensive response against monogeneans is considered and site-specificity of parasites on the host's body is discussed. Possible selection pressures exerted by predatory ‘cleaner’ organisms are briefly evaluated.

Pentastomids comprise a highly specialized taxon of arthropod-like parasites that probably became adapted to the lungs of amphibians and reptiles early in their long evolutionary history. Few other macroparasites exploit this particular niche. Pentastomids are often large, long-lived and yet they cause little observable pathology in lungs, despite being haematophagous. The lungs of all tetrapods are lined with pulmonary surfactant, a remarkable biological material consisting of a complex mixture of phospholipids, neutral lipids and proteins that has the unique ability to disperse over the air-liquid lining of the lung. In the lower tetrapods it acts as an anti-glue preventing adhesion of respiratory surfaces when lungs collapse during swallowing prey or upon expiration. In mammals, pulmonary surfactant also plays a critical role regulating the activity of alveolar macrophages, the predominant phagocytes of the lower airways and alveoli. This review outlines the evidence suggesting that lung-dwelling pentastomids, and also nymphs encysted in the tissues of mammalian intermediate hosts, evade immune surveillance and reduce inflammation by coating the chitinous cuticle with a their own stage-specific surfactant. The lipid composition of surfactant derived from lung instars of the pentastomid Porocephalus crotali cultured in vitro is very similar to that recovered from the lung of its snake host. Pentastomid surfactant, visualised as lamellate droplets within sub-parietal cells, is delivered to the cuticle via chitin-lined efferent ducts that erupt at a surface density of < 400 mm-2. The fidelity of the system, which ensures that every part of the cuticle surface is membrane-coated, testifies to its strategic importance. Two other extensive glands discharge membrane-associated (hydrophobic ?) proteins onto the hooks and head; some have been purified and partly characterized but their role in minimising inflammatory responses is, as yet, undetermined.

We develop the hypothesis that parasites do not invade extreme environments, i.e. hostile hosts, but rather ‘create’ them. We argue that parasites may have driven the evolution of the constitutive and adaptive immune system. This leads to several implications. First, parasites respond to ‘genes to kill’ by ‘genes to survive’ and this triggers an indefinite selection of measures and counter-measures. Second, these revolutionary arms races may lead to local adaptation, in which parasite populations perform better on local hosts. Third, the evolution of the immune system, whose responses are predictable, may allow parasites to specialize, to evade and even to manipulate. Finally we show that the correlations between the increase in the antibody repertoire, the expansion of MHC loci and parasite pressures support our hypothesis that both host complexity and parasite pressures can be invoked to explain the diversity of antibodies, T-receptors and MHC molecules.

Even the most generalist parasites usually occur in only a subset of potential host species, a tendency which reflects overriding environmental constraints on their distributions in nature. The periodic shifting of these limitations represented by host-switches may have been an important process in the evolution of many host-parasite assemblages. To study such events, however, it must first be established where and when they have occurred. Past host-switches within a group of parasites are usually inferred from a comparison of the parasite phylogeny with that of the hosts. Congruence between the phylogenies is often attributed to a history of association by descent with cospeciation, and incongruence to host-switching or extinction in ‘duplicated’ parasite lineages (which diverged without a corresponding branching of the host tree). The inference of host-switching from incongrucnt patterns is discussed. Difficulties arise because incongruence can frequently be explained by different combinations of biologically distinct events whose relative probabilities are uncertain. Also, the models of host parasite relationships implicit in historical reconstructions may often not allow for plausible sources of incongruence other than host-switching or duplication/extinction, or for the possibility that colonization could, in some circumstances, be disguised by ‘false’ congruence.

Studies on the digenean parasites of deep-sea (> 200 m depth) teleosts are reviewed and two case study generic phylogenies are presented based on LSU rDNA and ND1 mtDNA sequences. The phylogeny of the lepocreadiid genus Lepidapedon, the most common deep-sea digenean genus, is not clearly resolved as the two gene trees are not compatible. It can be inferred, however, that the genus has radiated in the deeper waters off the continental shelf, mainly in fishes of the gadiform family Macrouridae. Steringophorus, a fellodistomid genus, is better resolved. In this case a deep-sea radiation is also indicated, but the pattern of host-specificity is not clear, with evidence of much host-switching. Results of studies of the parasites of the macrourid fish Coryphaenoides (Nematomirus) armatus from various depths have reinforced recent views on the lack of zoned depth-related communities in the deep-sea. The diversity of deep-sea digeneans is relatively low with only 18 families (of about 60) reported. Little, or nothing, is known from most deep-sea areas and nothing from trenches and mid-ocean ridge systems.