CONVENCIÓN DE VIENA SOBRE EL DERECHO DE LOS TRATADOS, DE 23 DE MAYO DE

Prior to dissection, a number of the European eel specimens showed a clear mucosal secretion from the gill opening (Figure 2.10).

Figure 2.10 A representative image of the mucosal secretion observed from the gill opening (eel specimen R3 which was subsequently shown upon dissection to harbour 22

pseudodactylids). E: external gill opening with exudate.

Upon examination of the gill surface of each European eel it was difficult to ascertain the tissue damage and pathology caused directly by pseudodactylids since the hosts were subject to long-term storage and repeated freeze-thaw cycles which had contributed to some tissue deterioration. Consequently, the presence of exudate was utilised as the clear pathological indicator of parasite infection. Excessive exudate was clearly present on European eel examined from the Rivers Cadoxton, Rhymney and Clwyd-Elwy in Wales and the Rivers Leven and Petteril in England. However, in a small number of eel specimens from the Rivers Cadoxton and Rhymney exudate was present without subsequent detection of pseudodactylids. When present, the exudate covered the external surface of gills and also, it was present between the gill filaments (Figure 2.11). The exudate contained most of the pseudodactylids and hence it was not possible to associate any specific area of the gill with parasite preference.

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Figure 2.11 An image of the gross pathology of a pseudodactylid infected gill (eel RL2 subsequently shown to be infected with 125 pseudodactylids). E: excessive amounts of exudate covering the gill surface.

2.6 Discussion

Prior to initiation of this study, the only reports of pseudodactylid infection in UK eel showed that P. anguillae was present in specimens of A. anguilla sampled from three locations in Devon (Nie & Kennedy, 1991d; Kennedy, Nie, Kaspers, et al., 1992). In the intervening years, there has been no further scientific study published that addresses the extent of pseudodactylid infection in UK eel. As such, the work presented in the chapter was aimed at establishing a more coherent and robust set of data that reports current levels of pseudodactylid infection in UK eel that were sampled from different regions of the United Kingdom. To this end, pseudodactylid infection was observed in eel specimens sampled from the majority of catchment sites in England and Wales.

The mode of acquisition of the eel specimens unfortunately did not permit a clear morphological characterisation of the two common pseudodactylid species reported in A. anguilla. Indeed, even when fresh parasite specimens are obtained, morphological distinction between P. anguillae and P. bini is often difficult; moreover, differences have even been reported for the same species at different geographical regions (Zolovs et al., 2016). As such, a molecular-based diagnostic approach was developed that involved amplification of the 18S rRNA gene of pseudodactylids and then restriction of the resulting PCR product with the Xag I endonuclease. This novel approach allowed confirmation, as reported elsewhere (Copley & McCarthy, 2001; Kennedy, 2007b), that P. bini is indeed present in UK eel populations. Indeed, by screening 28% of the recovered pseudodactylids, it was apparent that P. bini was more prevalent that P. anguillae. There also appeared to be some catchment sites that contained eel harbouring single species infections whereas at other sites the hosts carried mixed infections. However, some of the single species infection data should be treated with caution; for example, only P. bini was characterised from eel present in the rivers Petteril and Taff but the analysis was based upon low numbers of parasites.

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Overall, pseudodactylids were more prevalent in English rivers than Welsh rivers. However, given that the actual eel sampling dates are unknown, a degree of caution is necessary when interpreting the infection data. A similar pattern of the higher prevalence of the swim bladder nematode infection in the European eels in English rivers than Welsh rivers was reported (Ab-Aziz, 2012). Nonetheless, assuming that the sampling dates were within the same season, the data within this thesis shows that there were statistically significant differences in prevalence between most of the geographic regions. A similar profile of statistically significant differences was also observed for the regional parasite abundance data. As such, it is reasonable to conclude that pseudodactylids may have adapted to the different environmental conditions within these regions with varying levels of success. Interestingly, eel from the two most northern regions examined had the lowest prevalences of infection whereas eel from the most southern region had the greatest prevalence, and also abundance, of pseudodactylid infection. One possible explanation for this might be that the parasites are less well adapted to surviving in rivers that have a lower water temperature. Indeed, pseudodactylid eggs are reported as not able to hatch when the water temperature is below 10oC (Buchmann, 1987). Interestingly, the most comprehensive data on water temperatures in UK rivers discusses the reasons for water temperature variation and notes that there are on average 153 days a year when water temperatures are <10oC in the South of England whereas in the North West this increases to 181 days (Orr et al., 2010). To this end, the parasite dynamics are likely to vary between the regions with the more southern rivers supporting enhanced parasite development and hence greater overall infection rates.

In terms of host factors, the data presented in this chapter highlights that when eel are less than 20cm in length the prevalence of infection with pseudodactylids is significantly less than occurs with larger eel. Moreover, the intensity of infection is also significantly less in the eel that are less than 20cm in length. Since body length is a reasonably good pseudomeasure of

age then this data most probably indicates that the smallest and hence youngest eel have incurred less exposure to the parasite (Buchmann, 1989b). A similar statistically significant infection profile is also observed when the data is analysed with respect to body weight. As the larger bodied fish will have an increased surface area of gill compared to the smaller eel then it is perhaps unsurprising that the bigger animals generally have a larger intensity of infection. Indeed, the intensity range increased as the overall size of the eel increased (Buchmann, 1988a; Mayo-Hernandez, Serrano, Penaver, et al., 2015).

The condition factor is a useful measure of fish health and hence the infection data was also analysed with respect to the eel condition factor. No statistically significant data emerged from the analysis and hence it can be concluded that the overall health of the eel in this study was not directly impacted by pseudodactylid infection. In contrast to this data, a recent study documented that pseudodactylid infection decreased eel body condition in relation when the intensity of parasite infection increased (Gérard et al., 2013). One reason for this anomaly might be that the Gerard et al., (2013) study was based upon analysis of migrating silver eel; none of the specimens in this thesis were migrating and only a few eel from the river Crane were likely to be at the silvering stage of development.

At the level of the individual host, it was evident that eel with a higher parasite burden often displayed a pathology that was characterised by the presence of exudate. This is indicative of the parasite causing damage to the gill as it penetrates the gill tissue with the hook in order to attach and feed. Indeed, as reported elsewhere, the parasite causes erosion and ulceration of the gill and the host defence is characterised by secretion of excessive amounts of exudate (Abdelmonem et al., 2010). In this thesis, exudate was observed on the eel sampled from rivers shown to have hosts that were infected with only P. bini (rivers Clwyd-Elwy and Petteril), only P. anguillae (river Rhymney) and mixed species infections (rivers Cadoxton and Leven). As such, it is not possible to comment upon the relative pathological impact of

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one species of pseudodactylid relative to the other. Moreover, this study was not focussed upon pathology and given the nature of specimen acquisition, it would be difficult to speculate further. However, other reports on pseudodactylid infections in A. anguilla conclude that P. bini is more pathogenic to eel than P. anguillae and this is a consequence of differences in the anatomy of their respective hooks (Arafa & Reda, 2012).

In summary, this chapter highlights that P. anguillae and also, P. bini, are present in European eel populations in the UK. Indeed, these parasites are present in the majority of rivers (10/14) from which eel were examined across England and Wales. Based upon analysis of the 140 eel in this thesis, the overall prevalence of pseudodactylids was 36% and the mean abundance and mean intensity data was 4.42 ±13.94 and 12.38 ±21.12 respectively. The data also show that infection differences occur between the geographic regions of the UK and that host factors may influence the infection.