Our results for element concentrations in the host-parasite system are in line with other studies and confirm accumulation patterns described earlier for acanthocephalans and their fish hosts. For example Schludermann et al. [17] found considerably higher concentrations of Cd, Pb and Zn in P. laevis compared to the levels in barbel tissues in fish caught in the Austrian part of the River Danube. Similar results were also published by Thielen et al. [18] for fish from the River Danube near Budapest after analysing a wide number of elements. Apart from barbel, P. laevis showed an enormous accumulation capacity (e.g. for lead) also in other fish species such as chub (Squalius cephalus) [19]. According to the data available, there is no doubt that the acanthocephalans are very useful in terms of metal indication, due to their excellent response to ambient element levels.
The observed seasonal pattern of metal concentrations in the parasites was suggested not to be a result of an incidental contamination (hotspot pollution) in the respective part of the river. Aqueous concentrations of the investigated elements at two sites (upstream and downstream from our sample location) showed no considerable variation over the months in the period 2005–2006 (see Table 2). Therefore, different body burdens during the year can probably be explained by the seasonality of acanthocephalan transmission, i.e. the degree of development (maturation) in the gut of the final host [10] and/or changes in the activity, feeding behaviour and physiology of the latter [20]. Seasonal aspects in the transmission were reported for various aquatic parasites, with climate conditions playing the decisive role. Moravec & Scholz [11] observed seasonality in the occurrence and maturation of the acanthocephalan Neoechinorhynchus rutili in barbel. Interestingly, the seasonal pattern was in accordance with that observed for P. laevis in the present study. The variation in prevalence of acanthocephalans in intermediate and final hosts was assumed to be a consequence of seasonal fluctuation in the temperature of the habitat [9–11]. For instance barbel’s activity differs strongly in terms of water temperature, as it decreases progressively with the decrease of water temperature until reaching its thermal limit for activity at 4 ° C (dormancy phase) [20]. The geographical area where the present barbel sampling was conducted is characterised by a typical continental climate and therefore the water temperature in winter months (December to March) is mostly below or around the barbel’s thermal limit of activity [16]. As a result, the reduced fish activity and altered feeding behaviour probably leads to a lack of infection during winter months, as the final host stops feeding on intermediate hosts, gammarids. With rising temperatures, the barbel starts feeding again, but an increase in new parasite infection occurs in late summer, when more infected intermediate hosts are available. The low temperatures may affect feeding behaviour and reproductive activity of the amphipods as well [21]. Therefore, a pronounced seasonality of infection with cystacanths could be expected, as has been already observed by many authors (e.g. Molloy et al. [22]; summarised by Kennedy [8]).
Finally, the combination of the available literature data [8, 10, 23] and the chemical data acquired in the present study suggests that the seasonal pattern of metal accumulation corresponds most likely to the seasonal dynamics of acanthocephalan transmission (Fig. 3). Due to the seasonality of infection in the final host, it could be expected that the age composition of acanthocephalan infrapopulations changed during the seasons. The differences obtained for the worms’ mean weight in the course of the year (see Fig. 1) could be taken as evidence for this assumption. Simultaneously, the concentration patterns for all elements were similar throughout the year, which was an additional sign that metal uptake (independent of the type of metal) was probably related to the stage of development. Thus, following the development of acanthocephalans with respect to the accumulation process it seems that in autumn P. laevis infrapopulations consisted mainly of young worms, which occurred in their growth phase (prepatent period). Therefore, due to accelerated metabolic processes the mean concentrations of the accumulated elements were the highest. The negative relationship between the concentrations of the elements Cd and Pb and the mean individual weight additionally explained the relationship between the accumulation process and the degree of development (characterised by the mean individual weight). A similar tendency was described for other organisms established in metal monitoring like shellfish, for which smaller individuals are usually characterised by faster uptake than the larger ones [24]. Increased surface-volume ratios of younger (smaller) specimens lead to a higher uptake from the water and causes negative relationships between size and concentration. The same could be true for fish acanthocephalans, as the assimilation of nutrients and metals occurs mainly through the worm’s tegument, which supports the importance of a surface to volume ratio also for acanthocephalans.
For winter and early spring, it could be expected that the down-regulated metabolism of barbel during the cold months affects the metabolism of acanthocephalans, including their metal uptake capacity. This leads to a decrease of metal concentrations in spring compared to autumn. In the period of dormancy, a process of metal elimination might appear as well. These aspects combined with the growth factor during the autumn, led possibly to a decrease in element concentrations. If the tissue grows faster than metal uptake occurs, concentrations in parasite tissue may be diluted as described by Strong & Luoma [24] for free living sentinels. This is most likely one of the major reasons for the deviation of metal concentration in the autumn-spring period.
During late spring and early summer the mean individual weight slightly increased, which suggests that almost all individuals reached the adult stage at which they reproduce and subsequently die. Accordingly, the mean metal concentrations in the parasites continue to decrease, which indicates that the accumulation process mainly takes place during the prepatent period (in autumn). Our accumulation data suggest that the acanthocephalans have reached the element equilibrium concentration at the beginning of the patent period when metal accumulation is (at least) compensated by elimination processes [19]. Active excretion of elements with the eggs released by females during the reproduction period might be an additional factor during the metal elimination process, as Sures et al. [25] reported that acanthocephalans are able to discharge metals via the shells of their eggs. This assumption was proven by higher Pb concentrations in eggs in comparison to the worm’s body wall and host tissues [25]. This kind of detoxification mechanism probably appears not only to be true for the archiacanthocephalan Moniliformis moniliforms, for which it was first described, but also for palaeacanthocephalans, to which P. laevis belongs. Due to the similarity in the concentration patterns of the elements investigated here, it appears likely that other metals are also excreted via egg shells.
Based on the present data and laboratory data from chub infected with P. laevis [5, 19] a model can be suggested. It visualises the accumulation kinetics of metals and As under natural conditions (Fig. 4). This model considers a generally accepted metal uptake kinetics [26], and similarly combines laboratory uptake data of lead [5,19] as well as the specific annual reproduction and infection cycle of P. laevis under the local climate conditions. Slight deviations between field and laboratory accumulation kinetics can be attributed to the time of infection, which can last over a range of some weeks during the warm months, whereas in laboratory studies uninfected fish were experimentally infected at the same time (for details see [19]). The natural infection process reduces the homogeneity of acanthocephalan infrapopulations as not all individuals are in the same developmental stage and are therefore not exposed to metals for the same period of time. Accordingly, there is a shift in the accumulation process when comparing the initial metal concentrations (see Fig. 4). Overshooting metal concentrations between October and December are caused by an accelerated metabolism of young acanthocephalans during their growth phase immediately after establishing in the definitive host’s intestine. Subsequently, the level decreases to a steady state concentration during winter and spring. This slightly lower steady state level results from reduced host activity during winter month. After the acanthocephalans matured in early spring they start reproducing during April to June. Therefore, our model considers a decrease in element concentrations, which was related to the metal elimination via egg release. A synthesis of the annual course of element concentrations and the natural infection process allows predicting the lifespan of P. laevis to be approximately 7–8 months.
