Host and geographical records for the three acanthocephalan taxa were collected from literature. Manual searches were conducted using Pubmed, Web of Science and Google Scholar. Additionally, we used the R (R Core Team 2019) package helminthR (Dallas 2016) to extract records from the Host-Parasite Database of The Natural History Museum in London (Gibson et al. 2005). Geographical records for each Corynosoma species were summarised by MEOW ecoregions (Spalding et al. 2007) and then plotted as a distribution map using the meow R-package (Byrnes 2016).

For species names, we followed the nomenclature given by the Catalogue of Life (2019 Annual Checklist: Roskov et al. 2019) and WoRMS (WoRMS Editorial Board 2019).

The term accidental host was used here for a host, where the parasite usually is not found, because the host is not suitable for the parasite's development. This can mean that the accidental host also becomes a dead-end, because the life cycle of the parasite is blocked (e.g. no observed gravid female and/or mature individuals) and the parasite does not reach its definitive host. Since in many literature references, this detailed information was not given, we used the term accidental host as generic term for all of the dead-end/accidental/incidental host records we found.

Our literature survey on the zoogeography showed that the three species of Corynosoma (C. magdaleni, C. semerme and C. strumosum) and their definitive hosts are restricted to the Northern Hemisphere (Figs 1, 2, 3). The relatively few records for C. magdaleni suggest that this taxon is restricted to the North Sea, Baltic Sea and northwest Atlantic. By contrast, C. semerme has a circumpolar distribution, having been reported from the Arctic Ocean, north Atlantic and north Pacific (e.g. Popov and Fortunato 1987, Kaimoto et al. 2018, Waindok et al. 2018) (Fig. 2). The geographical range of C. strumosum encompasses the distributions of the other two taxa, but also extends into more southern regions, such as the Mediterranean Sea and coast of California (Fig. 3).

Figure 1.  

Geographical distribution of Corynosoma magdaleni.

Figure 2.  

Geographical distribution of Corynosoma semerme.

Figure 3.  

Geographical distribution of Corynosoma strumosum.

In most of the Baltic Sea studies, C. semerme was the species most commonly found in seals, followed by C. magdaleni and only rare infections were found for C. strumosum (Nickol et al. 2002, Sinisalo and Valtonen 2003, Leidenberger and Bäcklin 2008, Waindok et al. 2018). Co-infections have been observed (Nickol et al. 2002, Leidenberger and Bäcklin 2008, Leidenberger et al. 2019), most commonly involving C. semerme and C. magdaleni (Sinisalo and Valtonen 2003, Leidenberger and Bäcklin 2008, Waindok et al. 2018). Interestingly, land-locked seal species show no mixed infections, but only single infections, for example, Phoca hispida saimensis in Lake Saimaa only by C. magdaleni (Sinisalo and Valtonen 2003) and Pusa caspica in the Caspian Sea only by C. strumosum (Amin et al. 2011). García-Varela et al. (2005) concluded that the Corynosoma species found in the Caspian Sea is C. caspicum and not C. strumosum. Indeed, C. strumosum from the land-locked Caspian seal recorded by Amin et al. 2011 was characterised with proboscis hooks and trunk spines distinct from other Northern European morphological descriptions (Nickol et al. 2002, Waindok et al. 2018, Leidenberger et al. 2019), suggesting that they may not be conspecific. Surprisingly, Corynosoma caspicum Golvan and Mokhayer, 1973, described based on immature specimens from sturgeons in Caspian Sea (Golvan and Mokhayer 1973) and often found in three-spined stickleback (Gasterosteus aculeatus) (Niksirat et al. 2006), has not yet been reported in Pusa caspica or another definitive host. Waindok et al. (2018) described some individuals of C. strumosum from the German North and Baltic Seas with different proboscis morphology, similar to those described by Amin et al. (2011) and stated this as C. magaleni isolate Pv1NS instead. Additionally, they found a cryptic species refered to as "Candidatus Corynosoma nortmeri sp. nov."' Waindok et al. (2018) in the European study area.

The extent of the geographical and host ranges of the three taxa correlates with the year of their description. Corynosoma strumosum was the first species of the genus to be described (Rudolphi 1802) and has, by far, the broadest geographical and host distributions. Corynosoma semerme, described by Forssell (1904), appears to have a slightly more limited geographical range and far fewer paratenic host records. Relatively few hosts have been reported for C. magdaleni, described by Montreuil (1958) and its geographical range appears to be restricted to the northern Atlantic, in contrast to the circumpolar distributions of the other two species. Up until its description in 1958, specimens of C. magdaleni were almost certainly assigned to the morphologically similar C. strumosum. The potential bias resulting from taxon age should be kept in mind when comparing the reported host and geographical distributions of the three taxa (Fig. 3).

The heteroxenous life cycle of Corynosoma species involves a peracaridan intermediate host, a paratenic host (fish) and a mammalian definitive host. While there are numerous studies on C. semerme and C. strumosum (Tables 3, 4, 5, 6, 7, 8), there have been few studies on C. magdaleni, besides its original description by Montreuil 1958 (Tables 1, 2). The intermediate host of C. magdaleni is unknown. Lake Saimaa may be a good location to search for the intermediate host of C. magdaleni, because the other Corynosoma species appear to be absent from this land-locked waterbody (Sinisalo and Valtonen 2003, Sinisalo et al. 2003). A total of five species of paratenic host (all Actinopterygii) have been reported for C. magdaleni; three from Canada and two from the Baltic Sea (Table 1). In Canada, only C. magdaleni is reported together with C. wegeneri Heinze, 1934. In the definitive hosts (Table 2), the species seems to prefer the last part of the small intestine (ileum) and the colon. Corynosoma semerme has been reported from the whole intestine, but is typically found at a higher density in the large intestine, especially the cecum and rectum (Nickol et al. 2002, Leidenberger and Bäcklin 2008), where C. strumosum and C. magdaleni are not generally observed. Microhabitat segregation might facilitate reproductive isolation of the three species, especially in mixed infections that are often observed in seals from the Baltic Sea (Nickol et al. 2002, Sinisalo and Valtonen 2003, Leidenberger and Bäcklin 2008), but less commonly in the North Sea (Waindok et al. 2018). While the three Corynosoma species share most of their definitive hosts, they show some differences in their paratenic hosts. We found that C. strumosum shares three paratenic hosts with both other species, none with C. magdaleni only, but > 20 paratenic hosts are shared with C. semerme. More than 80 paratenic hosts were described for C. strumosum and these are, up to now, not reported from any of the other two species (Tables 1, 4, 6).