Parasitism and environmental pollution : parasites and hosts as indicators of water quality

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Symposia of the British Society for Parasitology V O L U M E 39

Parasitism and environmental pollution : parasites and hosts as indicators of water quality







Parasitic organisms are found in almost every environment ranging from the extremes of cold conditions in the polarregions to hot climates in the tropics. Natural abiotic factors such as temperature, oxygen, salinity and hydrogen ion concentration are known to influence the temporal and spatial occurrence of parasites and in particular the helminths of fish (Chubb, 1979, 1980). Several scientists through their efforts to use fish parasites as biological tags to assess fish populations have appreciated theimportance of parasites as sensitive probes to monitor a range of environmental factors including stress due to pollution (Mackenzie et al. 1995 ; Lafferty, 1997). Fish parasites are also efficient monitors of the physiological–immunological state of their fish hosts. The main pathways used by assemblages of aquatic parasites in response to pollutants were reviewed by Poulin (1992) who showed that parasitecommunities are influenced indirectly by pollutants which are toxic both to fish and invertebrate hosts and directly to the parasites and their free living stages. Whilst fish and invertebrates have been used extensively in standard toxicity testing, Kennedy (1997) has argued that the relationships between the environment and the fish host and its parasites will not provide an easy option formonitoring environmental change in pollution incidents. Kennedy also suggested that parasites of aquatic hosts and, in particular, fish did not offer clear advantages over the use of free living organisms as indicators of pollution other than providing additional, or confirmatory, sources of information. However much

progress has been made over the past five years and papers presented at the 2002 AutumnSymposium demonstrate how aquatic hosts and their parasites are likely to indicate changes in water quality and play a significant role in our understanding of natural aquatic ecosystems. The topics in this volume consider the impact of pollution on host–parasite systems in the marine and freshwater environment, commencing with Morley & Irwin who, in their presentation on the effects of pollutionon the transmission of larval digeneans through molluscan hosts, show that complex interactions influence parasite populations under extreme environmental conditions. To date, laboratory experiments, especially with heavy metals, have to a large extent demonstrated those toxicity effects which occur as episodic pollution events. Apart from standardising experimental procedures, more combinedlaboratory and field studies are needed to clarify the complexity of abiotic and biotic factors involved in the transmission of digeneans under natural conditions. In the marine environment, Williams & Mackenzie provide an update on criteria used for selecting marine parasites as indicator/ monitor species and data on the monogenean species Diclidophora merlangi and Dictyocotyle coeliaca confirm theirvalue as potential indicators of hydrocarbon pollution in the North Sea. In their study on pollution and parasites in Finnish Lakes, Valtonen et al. report that some parasite communities in fish show evidence of recovery following a reduction in chemical and nutrient loading from a pulp mill in

Parasitology (2003), 126, S1–S3. f 2003 Cambridge University Press DOI: 10.1017/S0031182003003962Printed in the United Kingdom



central Finland. Apart from an increase in the population of anadontid clams, enhanced immune responses in the fish also reflect much improvement in water quality. The paper by Sures summarises our present knowledge on parasites as bioindicators and demonstrates from field and laboratory studies that acanthocephalans in particular have the capacity for...
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