Our field metal monitoring compares and highlights the accumulation potential of different fish helminths. Nine of eleven elements were found in significantly higher concentrations in the parasites in comparison to the host tissues. However, the range of elements in parasites differed between parasite taxa. The acanthocephalans accumulated primarily elements which are known for their toxicity to many organisms such as As, Cd and Pb . In contrast, the larval nematodes mainly bioconcentrated elements which are part of several enzymes and other macro-molecules and which are therefore considered as essential elements (Co, Cu, Fe, Se, Zn) for many organisms . Different authors reported high accumulation rates of heavy metals in acanthocephalans and demonstrated that they can be successfully used for monitoring purposes (summarized by [2, 20]). As expected, our study confirmed these results for the acanthocephalan P. laevi s, which is probably the most intensively investigated fish acanthocephalan regarding metal accumulation . Our data corresponded to field data published from the Danube River [17, 21, 22] and showed some parallels with the metal uptake experiments performed under controlled laboratory conditions [7, 13, 23]. Again, concentrations of As, Cd, Cu, Mn, Pb and Zn in P. laevis exceeded the levels in the fish host, which confirms the use of acanthocephalans as indicators for metal pollution.
Metal monitoring studies performed with the help of parasitic nematodes are comparatively scarce. Most of the available nematode papers focus on the accumulation potential of adult parasites, while larval nematodes are less intensively investigated and information about their accumulation capacity is missing. Our study focused on fourth stage larvae, which were able to accumulate a large number of elements, especially essential ones (Co, Cu, Fe, Se, Zn). This demonstrates that metal uptake already starts during an early stage of development. In contrast, the accumulation capacity of larval acanthocephalans (cystacanths) was found to be very low and the metal uptake starts in the intestinal lumen of their definitive host [24–26].
The higher levels of essential elements in the nematodes could be related to their biology and morphology. After the fish acquires the infection, the larva migrates through the intestinal wall into the body cavity, where it starts feeding on blood and tissues prior to encapsulation. Similar to adult nematodes, the fourth stage larvae have a completely developed digestive system , which suggests that they can accumulate metals by ingestion of food. Research on microstructure and properties of the nematode’s cuticle revealed that the cuticle of larvae is not as complex as that of adults [28, 29]. Therefore, larval nematodes are also able to adsorb nutrients and metals through their body surface. Taken together it appears that fourth stage nematodes exhibit an even better accumulation capacity than adult stages because of different uptake routes. This was probably one of the reasons why essential metals like Cu, Fe, Zn as well as Co and Se were predominantly accumulated. These macro and micro elements are important structural and functional factors, as they are involved in the architecture of many enzymes and other complex molecules . For example, elements like Fe were adsorbed or ingested most likely with host blood, as Fe is an essential part of the blood pigment hemoglobin. Therefore, its levels in host muscle and liver tissues correlated with the levels in the nematodes, (see Table 4). Similar uptake sources may also exist for the elements Co, Cu and Zn due to the fact that these elements are highly abundant in organisms as co-factors of various enzymes. Correlation analyses between Co levels in nematodes and host liver or intestine for example revealed positive associations, which again underlines that the parasites profit from high levels in the host and were not able to negatively affect the balance of microelements in their host.
Higher metal concentrations in the larval lung nematode Pseudalius inflexus were reported  not only for essential elements but also for toxic ones. The authors suggested that L4 larvae accumulated metals mainly from the food (blood and host tissues) via their digestive system. However, this nematode was not encapsulated, as was the case for Eustrongylides sp., therefore element uptake will not only occur via food, but also via the cuticle. This assumption is supported by the fact that levels of toxic elements such as As, Cd and Pb in the parasite were similar to those in the host tissues. More specifically, the concentrations of Cd and Pb in nematodes were positively correlated with those in muscle and liver, indicating that these metals were taken up from the tissues in which the parasites were located (for details see Table 4). Obviously, the specific microhabitat preferences of Eustrongylides sp. play a decisive role, as the availability of toxic elements within the fish host differs profoundly from those of the essential ones. With reference to this, high levels of Cd and Pb in larval nematodes of the genus Hysterothylacium sp. collected from the intestinal lumen and from mesenteries of the fish host have been reported recently . It seems that both metals were available to a high degree in the digestive tract which therefore results in metal accumulation rates similar to acanthocephalans.
Higher concentrations of various elements were also found in the adult nematodes Philometra ciprinirutili and P. ovata inhabiting the body cavity of fish [10, 12]. Interestingly, the authors reported higher levels of non-essential elements like Cd and Pb in the parasites in contrast to the results obtained in the present study. Mean ratios of Cd and Pb between P. ovata and host muscle ranged between 20 and 25 , which indicates a much higher accumulation capacity of Philometrids in comparison to Eustrongylides sp. On the other hand, the respective bioaccumulation rates for Cu (123) and Zn (12) in our study were much higher than those reported for P. ovata with only 22 and 3, respectively. These differences suggest that larval nematodes probably have a higher affinity to accumulate essential elements whereas the adult Philometrids demonstrated a higher accumulation capacity for toxic metals. Explanations for these differences could be the relative importance of different element uptake routes between the developmental stages of the nematodes or competition between Eustrongylides sp. and P. laevis in the double infected fish. The nematode larvae were encapsulated and thus were unable to feed actively in contrast to the adult stage. As the larval stages have to grow fast during their development they rely on the uptake of essential elements probably via uptake processes through their cuticle. Adult nematodes actively feed on host tissues and thereby take up and accumulate toxic metals like Cd and Pb.
An alternative explanation for the relatively low levels of toxic elements in Eustrongylides sp. could be competition between acanthocephalans and nematodes. It is suggested that acanthocephalans dwelling in the intestine compete for nutrients and metals with the host tissues  probably via interruption of the enterohepatic element cycle . Metals bound in bile complexes excreted in the small intestine are taken up by acanthocephalans and thus are not available for reabsorption by the host intestine. Therefore, toxic elements might become unavailable for the host target tissues and simultaneously occurring in parasites. In our study, the concentrations of Pb in Eustrongylides sp. were significantly higher in comparison to the muscle and liver tissues, but not higher than those in the intestine, which supports the assumption, that some heavy metals are predominantly available for intestinal parasites. In mass infection cases with P. laevis, which are common for barbel [15, 32], metal distribution in the host tissues might be significantly changed by the acanthocephalans.