Anguillicola crassus

Dangel, K. C., Keppel, M., Le, T. T. Y., Grabner, D. & Sures, B., 2015, Competing invaders: Performance of two Anguillicola species in Lake Bracciano, International Journal for Parasitology: Parasites and Wildlife 4 (1), pp. 119-124 : 120

3.1. Anguillicola crassus

After its introduction to Europe and its spread throughout the European eel population, A. crassus became of significant interest. Compared to the Japanese eel, the European eel has no effective immune response against the parasite ( Knopf and Mahnke, 2004; Knopf, 2006; Knopf and Lucius, 2008). This leads to a high prevalence of up to 100 % as well as a high infection intensity in European eel populations compared to findings in the original habitat in Asia, where recordings indicate a prevalence ranging from 17 to 56 % with relatively low intensities ( Nagasawa et al., 1994; Jakob et al., 2009). Combined with other stressors like low oxygen levels in the water, a high fish population density, and the presence of insecticides, A. crassus even caused mass mortalities of eels in Lake Balaton ( Hungary) and the Morava River system ( Czech Republic) ( Molnár et al., 1991; Baruš et al., 1999; Nemcsók et al., 1999; Lefebvre et al., 2012). As a result, the parasite was intensively studied both in wild eels (overview see Jakob et al., 2009) and in experimentally infected eels ( Haenen et al., 1989, 1991, 1996; De Charleroy et al., 1990a; Moravec et al., 1994b; Knopf et al., 1998; Ashworth and Kennedy, 1999; Knopf and Mahnke, 2004; Fazio et al., 2008; Weclawski et al., 2013).

The basic life cycle of the species is rather simple (see Fig. 1 ): Adult nematodes of this species settle in the swim bladder of eels, nourish on blood, mate and produce eggs with second stage larvae (L2). Through excretion, L2 are released into the water, where they are ingested by intermediate hosts (mostly copepods). Inside the copepod, larvae develop from the second to the third larval stage (L3). As soon as L3 are ingested by the final host (eel), the larvae penetrate its intestine in order to reach the swim bladder wall, where they molt into the fourth stage larvae (L4). L4 grow further, molt, enter the swim bladder lumen and develop into adult nematodes ( KØie, 1991; Kirk, 2003; Moravec, 2006; Kennedy, 2007).

Experimental infection studies of (potential) intermediate hosts revealed almost 20 suitable species in Europe, mainly cyclopoid copepods (overview see Moravec, 2006). Additionally, paratenic hosts were found to be facultatively involved in the life cycle of A. crassus in Europe. More than 30 fish species, but also tadpoles, as well as some aquatic invertebrates, have been recorded as paratenic hosts ( De Charleroy et al., 1989, 1990b; Thomas and Ollevier, 1992; Haenen et al., 1994; Moravec and Konecny, 1994; Pazooki and Székely, 1994; Székely, 1994; Moravec, 1996; Moravec and Skorikova, 1998). The blood-sucking feeding habit of adult parasites can lead to severe changes of the swim bladder wall tissue and to loss of lumen, both resulting in an impairment of the buoyancy regulation of the host ( Molnár et al., 1993; Molnár, 1994; Würtz et al., 1996; Barry et al., 2014). As a consequence of parasite infection, eels with impaired swim bladders may be unable to reach their spawning grounds in the Sargasso Sea (5500 km transoceanic migration) ( Sures and Knopf, 2004; Palstra et al., 2007).

Figures

Fig. 1. Life cycle of Anguillicola crassus. The basic life cycle (blue arrow) includes eels as final hosts and copepods as intermediate hosts. By integrating additional paratenic hosts (e.g. fish), the life cycle can be extended (white arrow).Source: (Moravec, 2006).