The use of fish parasites as bioindicators of heavy metals in aquatic ecosystems
Bernd Sures
Aquatic Ecology 35: 245-255, 2001
Kluwer Academic Publishers (Netherlands)

Parasites are potential indicators of environmental quality due to the variety of ways in which they respond to anthropogenic pollution. They provide valuable information about the chemical state of their environment not only through their presence or absence but also through their ability to concentrate environmental toxins within their tissues.  Metal concentrations in adult acanthocephalans respond rapidly to changes in environmental exposure of their hosts. For example, Free living invertebrtates (ie. Bivalve molluscs) can be used to monitor the concentrations of bioavailable metals in aquatic ecosystems. One major example is the Intestinal acanthocephalans of fish.

 Knowledge of fish parasites is of particular interest in relation not only to fish health but also to understand ecological problems. This interest especially in fish parasites is related with the high number of parasites species commonly found in or on freshwater and marine fish (ie. 30,000 helminth species were assumed to be parasites of fish). Due to their wide abundance and distribution, several researchers started to focus on the use of parasites as indicators of environmental quality (esp. due to the variety of ways in which they respond to anthropogenic pollution).

Parasites are useful in two different ways.

First of all, they are "effect indicators" (effects of various forms of pollution on the abundance and distribution of parasites). We can monitor the changes of the whole population structure depending on the pollution of the environment.  However, there are also problems using parasites as effect indicators. This is because there is a wide variety of factors affect the population of parasites. Thus, although studies on diversity of fish parasites in different biotopes are important and extremely interesing, they do not allow any conclusions to be drawn concerning the concentration of specific toxins in the environment.

• ex. Monogenean Trematode are parasites which live on the gills of fish. Therefore, they are in direct contact with both the surrounding environment and the fish host. In addition, it has a short life, thus react immediately on changes in environmental factos.
• ex.. Zebra mussels Dreissena and Rainbow trout Salmo gairdneri are indicators for water treatment in sewage plants.
• ex. Effluent from a pulp and paper mill result in abundances within communities of Dactyloyrus and Paradiplozoon.

ie. Acanthocephalans a group of intestinal worms commonly found in fish. Adult worms live inside the intestine of the final host and absorb their nutrients across their tegument (no mouth/intestine). There are three major species: Pomphorhyndchus laevis, Acanthocephalus lucii, Paratenuisentis ambiguous. Among these, P. Laevis most rapidly reacts to changes in the environment. (ie. The mean concentrations of lead and cadmium in P. Laevis are respectively 2700, 400 times higher than in the muscle of the host and 11000, 27000 times higher than in water.) Acanthocephalans can accumulate toxic metals from the aquatic environment to concentrations even surpassing those in Dreissena polymorpha (=Mussel: one of the best established accumulation indicators in fresh and brackish waters in Eruope and USA).

ie. The lead burdens in the parasites is about 1000 times higher concentrations than that of host's muscle. This is because metal concentration in the parasite is likely to respond rapidly to changes in environmental exposure. The presence of acanthocephalans had a significant impact on lead accumulation in the intestinal wall. The fish infected with acanthocephalans only half of uninfected chub's lead concentration.
 

ie. European eels (Anguilla anguilla) infected with Paratenuisentis ambiguous were use to study the influence of water salinity on the bioavailability and the bioaccumulation of lead. However, there was no significant differences in the lead levels of the parasites and host tissues. This result allows us to use this method not only in freshwater but also in estuarine and marine ecosystems. But, lead concentration of different tissues of the fish host varied  depending on the mode of metal application. Thus, the distribution of the respective metal within the host may help to identify the main pollution source. Therefore, the combination of the results obtained from the host and the parasites would reveal a more reliable and detailed tool to ascertain the source of an environmental contamination than a study based on a single species.

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