Special Issue: Wildlife Parasitology The butterfly effect: parasite diversity, environment, and emerging disease in aquatic wildlife Robert D. Adlard 1 , Terrence L. Miller 2 , and Nico J. Smit 3 1 Natural Environments Program, Queensland Museum, South Brisbane, QLD 4101, Australia 2 School of Marine and Tropical Biology, James Cook University, Cairns, QLD 4870, Australia 3 Unit for Environmental Sciences and Management, North-West University, Potchefstroom 2520, South Africa Aquatic wildlife is increasingly subjected to emerging diseases often due to perturbations of the existing dy- namic balance between hosts and their parasites. Accel- erating changes in environmental factors, together with anthropogenic translocation of hosts and parasites, act synergistically to produce hard-to-predict disease out- comes in freshwater and marine systems. These out- comes are further complicated by the intimate links between diseases in wildlife and diseases in humans and domestic animals. Here, we explore the interactions of parasites in aquatic wildlife in terms of their biodiver- sity, their response to environmental change, their emerging diseases, and the contribution of humans and domestic animals to parasitic disease outcomes. This work highlights the clear need for interdisciplinary approaches to ameliorate disease impacts in aquatic wildlife systems. Connectivity of aquatic wildlife parasites It is intuitive that aquatic wildlife systems do not exist as discrete units but rather they interact seamlessly with neighbouring environments through a variety of inputs and outputs that together create a dynamic ecosystem. Although this overarching statement is undeniable and holds true for all sectors of the environment, it is only recently that the intimate links between diseases in wildlife and diseases in humans and domestic animals have been emphasised [1,2]. These observations have prompted transdisciplinary approaches in surveillance, health assess- ment, and monitoring to provide predictive models allowing timely response to the emergence of disease [3]. Such approaches should ideally also provide early indicators of environmental changes that may impact ecosystem health. A review on parasitic zoonoses and wildlife [4] focussed further attention on the complex interplay between human and wildlife diseases through the One Health interaction triad and proposed that research into parasite biodiversity in wildlife should not only provide an inventory of parasites, but also include an assessment of the impacts that such pathogens may have on nonwildlife hosts. Equally, similar consideration should be given to the contraflow of human or domestic animal parasites and how they may affect wildlife health. The aquatic ecosystem, central to this review, is broadly divided in freshwater and marine components. Although the former accounts for only 0.8% of global surface area, it is critical to the survival of many organisms, and is subject to some of the most ecosystem-altering perturbations [1]. By contrast, the marine component covers 71% of the globe, but it too shows recent and relatively rapid environmental changes and acts as the final sink for land-based inputs that impact initially on biodiverse coastal ecosystems. A survey of the literature shows that parasitologists have focussed their studies on aquatic wildlife through several drivers that include: biodiversity stocktakes; identification of causative agents in events of aquatic wildlife mortality and morbidity; focussed prevention and remediation attempts for aquatic parasites pathogenic in both aquacul- ture and harvest fisheries; multidirectional aquatic parasite flow between wildlife and aquaculture; and current and projected impacts of often anthropogenic environmental change on the incidence, prevalence, and pathogenicity of aquatic parasites. Given these drivers, we have framed this review to cover aquatic parasites in terms of their biodiver- sity, response to environmental change, and emerging dis- eases and disease interactions with nonwildlife species. Biodiversity of aquatic parasites Parasites are extraordinarily diverse in aquatic ecosys- tems, where parasitism likely first arose, long before ter- restrial life came into existence. Evidence for such an ancient association is increasingly recognised in the fossil record through distinct pathology or morphologically in- duced change that parasites cause to their long-dead hosts [5]. The vast diversity and richness of parasites we observe today in aquatic systems reflects this long evolutionary history, with representatives of nearly all known parasitic lineages of life found in marine and freshwater environ- ments [6]. Among the most biodiverse parasites known from aquatic ecosystems are the cestodes, monogeneans, trematodes, and myxozoans, with thousands of species Review 1471-4922/ ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pt.2014.11.001 Corresponding author: Adlard, R.D. (robert.adlard@qm.qld.gov.au). Keywords: aquatic parasites; wildlife; biodiversity; environment; emerging disease. 160 Trends in Parasitology, April 2015, Vol. 31, No. 4