pallipes has resulted from crayfish plague (Kemp et al. 2003). The invasive crayfish, P. leniusculus, commonly acts as a reservoir for Aphanomyces astaci (the causative agent of crayfish plague), which is fatal to the native species (Holdich 2003). Also of interest are two further parasites. The microsporidian parasite Thelohania contejeani (Henneguy), infects Austropotamobius pallipes causing porcelain disease and is the most widely recorded parasitic infection of this species (Alderman and Polglase 1988). Whilst the pathology of T. contejeani is not as severe as that of crayfish plague it can be a serious threat within crayfish aquaculture (Edgerton et al. 2002) and may cause changes in the ecology of its host through changes in diet (Chartier and Chaisemartin 1983); however the consequences of infection by many pathogen groups in European freshwater crayfish are largely poorly understood (Edgerton et al. 2004). Microsporidia are widespread in crustacean hosts (Edgerton et al. 2002; Terry et al. 2004) and can cause significant mortality (Alderman and Polglase 1988). A second parasite, the protist Psorospermium haeckeli (Haeckel) infects crayfish and has recently been isolated from A. pallipes (Rogers et al. 2003) and Pacifastacus leniusculus (Dieguez-Uribeondo et al. 1993). The influence of these parasites upon native/invasive interactions in crayfish is unknown. In the UK, Yorkshire is a stronghold for A. pallipes: although P. leniusculus is present within the county in substantial numbers, it has not yet displaced many native populations and mixed populations do exist (Peay and Rogers 1999). Here we investigate possible competitive interactions by comparing the sizes of native and invading individuals in single species versus mixed species populations. Secondly we use PCR screening and sequence analysis to compare parasite diversity in the native and invasive crayfish, focusing on microsporidian parasites.
(Söderbäck 1992) may underpin the observed reduction in juvenile P. leniusculus in mixed populations. Parasitism in native and invasive crayfish Four species of microsporidia were detected in the invasive crayfish P. leniusculus. In contrast, only one microsporidian parasite was detected from A. pallipes although the sample size was The overall prevalence of microsporidian infection in P. leniusculus was 38 % (Table 2). This prevalence is higher than previous reports of visible microsporidiosis in A. pallipes in Britain (9 %, (Brown and Bowler 1977); 26 % (Rogers et al. 2003); 30 % (Evans and Edgerton 2002)), France (0-8%, Chartier and Chaisemartin 1983) and Spain (1%, Dieguez-Uribeondo et al. 1993), probably reflecting a higher detection efficiency by PCR. The T. contejeani sequences we obtained were identical to those previously isolated from Astacus fluviatilis (Genbank accession numbers AF492593 and AF492594, Lom et al. 2001). This is, to our knowledge, the first molecular confirmation of T. contejeani infecting P. leniusculus, as well as the first report of the parasite in an invasive species in Europe. Whilst T. contejeani has previously been reported from Austropotamobius pallipes in the UK (Brown and Bowler 1977; Edgerton et al. 2002; Rogers et al. 2003), these reports were based on light microscopy and lack the ultrastructural or molecular information to confirm species identity (Dunn and Smith 2001). This is the first molecular confirmation of the presence of T. contejeani infecting A. pallipes. The presence of T. contejeani in the invasive P. leniusculus leads to the question of how the parasite has come to infect this species. Firstly, P. leniusculus may have
brought the parasite with it from its native range. T. contejeani has been reported from a number of crayfish hosts (Graham and France 1986; Quilter 1976), and there is a single report of T. contejeani from P. leniusculus in its native range in California (McGriff and Modin 1983); but identification is based on spore size, and molecular or ultrastructural confirmation is lacking. The pattern of infection in the current study leads us to suggest that it is more likely that P. leniusculus in the UK has acquired T. contejeani from the native host. T. contejeani was detected in A. pallipes only sites, in mixed sites and in sites where only P. leniusculus occurred. Furthermore, identical sequences were found in the native and invading species (Table 4). These data fit a pattern of transmission from the native A. pallipes to the invading species in mixed sites. Detailed studies of the fitness effects of T. contejeani and its mode of transmission within and between crayfish species are required. In addition, our T. contejeani sequences had 98-100 % sequence similarity to the unclassified microsporidium, Microsporidium sp. JES2002H, which was detected in three species of amphipod in France (Terry et al. 2004). We suggest that Microsporidium sp. JES2002H and T. contejeani may be the same species, although confirmation awaits ultrastructural analysis of Microsporidium sp. JES2002H. In one P. leniusculus we found a microsporidium sequence with 97 % sequence similarity to Bacillidium vesiculoformis, a parasite previously described from the oligochaete worm Nais simplex in Scotland, UK (Morris et al. 2005). The sequence similarity indicates that the parasite is likely to be in the same genus as B. vesiculoformis; however further molecular and morphological analysis would be required to confirm this. This is the first record of a Bacillidium spp. in crayfish and
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