Pomphorhynchus laevis (Zoega in Muller, 1776)

Perrot-Minnot, Marie-Jeanne, Guyonnet, Emilie, Bollache, Loïc & Lagrue, Clément, 2019, Differential patterns of definitive host use by two fish acanthocephalans occurring in sympatry: Pomphorhynchus laevis and Pomphorhynchus tereticollis, International Journal for Parasitology: Parasites and Wildlife 8, pp. 135-144 : 137

2.3. Infection patterns of P. laevis and P. tereticollis

We first established the proportion of P. laevis and P. tereticollis in each fish species and compared it to the one expected from their distribution in amphipod intermediate hosts. Because the number of individual parasites genetically identified per individual fish was low (median=3, min-max=1–43, 1 rst and 3rd quartiles=1–5), we pooled all parasites collected from at least 5 individual hosts per fish species, and calculated the proportion of P. laevis and P. tereticollis harboured by each fish species. We also compared the relative proportion of P. laevis and P. tereticollis in each fish species according to fish ecology: benthic (bottom-feeders) or bentho-pelagic (i.e. drift-feeders; Oberdorff, 1995).

We then tested the hypothesis that differences in parasite abundance among fish species could be determined by host species abundance, by regressing log10-transformed mean parasite abundance per fish species on log10-transformed fish biomass ( Arneberg et al., 1998; Buck and Lutterschmidt, 2017). We also illustrated the transmission efficiency of parasites within the local network of definitive hosts by estimating the flow rate of P. laevis and P. tereticollis among fish species. Flow rate was calculated as the product of the mean P. laevis or P. tereticollis abundance per fish species and the density of each fish host species in the community.

Host diversity and specificity were quantified for each Pomphorhynchus species using three indices: Shannon - Wiener's diversity index (H′) as a measure of structural host diversity, phylo-structural index of specificity STD * ( Poulin and Mouillot, 2005; Poulin et al., 2011), and Paired Difference Index (PDI) as a quantitative measure of specialization ( Poisot et al., 2012). The Shannon-Wiener index was calculated from relative host use by P. laevis and P. tereticollis , i.e. their relative abundance within each fish species. The phylo-structural index of specificity, combining the taxonomic hierarchy of hosts to observed prevalence, was calculated after Poulin and Mouillot (2005). This index varies with the number of host species and the prevalence in each species, according to the taxonomic distance between species: increasing the number of species with taxonomic redundancy decreases STD * while adding species with a distant taxonomic position increases STD *, especially at high prevalence. Therefore, STD * index is inversely proportional to specificity. To calculate STD *, we used six levels of taxonomic hierarchy (Genus -all distinct-, Subfamily, Family, Order, supra-Order, and infra-Class Teleostei). Within the Order Cypriniform , we obtained information about host's taxonomic rank from Gaubert et al. (2009). The Paired Difference Index contrasts a species' strongest link on a resource with those over all remaining resources ( Poisot et al., 2012; R package ‘bipartite’, Dormann et al., 2017). Here we used fish species as resource, and the relative mean abundance of intestinal P. laevis or P. tereticollis as a measure of link strength between parasite and fish host. The Paired Difference Index ranges from 0 (generalist) to 1 (perfect specialist; Poisot et al., 2012). Finally, the degree of niche overlap between the two Pomphorhynchus species was estimated by calculating Renkonen's similarity index. This index was chosen based on the recommendations of Wolda (1981) for samples with contrasted abundance. It was calculated using the relative flow rate of each Pomphorhynchus species among fish host species as a descriptor of parasite niche, after log-transformation.

To assess relative compatibility towards fish hosts, we focused on the two main hosts of P. laevis and P. tereticollis , based on their use (mean abundance) and on their local abundance within the fish community: the barbel Barbus barbus and the European chub Squalius cephalus . We estimated worm development by measuring parasite size (body length) on thawed worms in water, from photographs taken under a stereomicroscope (SMZ 1500, Nikon) and using the image analysis software LUCIA G 3.81. We also recorded the position of each individual worm along the intestinal tract. We then measured several reproductive parameters to assess male and female reproductive success according to parasite and fish species. We measured testes volume in adult males ( Fig. S3c ), and, the number and volume of ovarian balls (free ovaries) and the number of eggs in females. Ovarian balls are produced by ovarian fragmentation in the definitive host, and increase in size and cell number during their development, before the progressive release of mature oocytes upon insemination ( Crompton, 1985). Therefore the number of ovarian balls and the number of eggs might well reflect host suitability to sustain ovarian development ( Crompton, 1985). Testes volume and the number and volume of ovarian balls were measured from photographs. The ovoid volume (testes and ovarian balls) was estimated from length and width measurements using the formula of ellipsoid volume (V = π*L*D 2 /6; L: length, D: diameter). We collected eggs from female body cavity and stored them in 200μl of 10% formaldehyde at 8̊C. The number of eggs in the suspension was estimated using an automatic particle counting machine (Coulter º Multisizer™). Egg size distribution was automatically partitioned into 70 size categories ranging from 11.55μm to 33.59μm, revealing three clusters of size categories gathering more than 65% of total egg number (peak 1, 2, 3 in Fig. S3a ).

Figures

Fig. 3. Relative flow rates of P. tereticollis and P. laevis in their hosts in the Ouche (a) and Vingeanne (b) localities, taking into account the abundance of adult intestinal parasites and the relative abundance of each fish host species in the local community. The thickness of arrows and percentages indicate the estimated relative value of each fish species as a resource for each Pomphorhynchus species.