Gorgocephalus yaaji

Huston, Daniel C., Cutmore, Scott C., Miller, Terrence L., Sasal, Pierre, Smit, Nico J. & Cribb, Thomas H., 2021, Gorgocephalidae (Digenea: Lepocreadioidea) in the Indo-West Pacific: new species, life-cycle data and perspectives on species delineation over geographic range, Zoological Journal of the Linnean Society 193 (4), pp. 1416-1455 : 1425-1430

Gorgocephalus yaaji

This species was described by Bray & Cribb (2005) based on adult specimens obtained from Kyphosus vaigiensis , collected from off Lizard Island, GBR, Australia. Gorgocephalus yaaji is easily distinguished from all other species of the family, largely in having a more robust, dorsoventrally flattened body shape and in having the vitellarium extend into the forebody. Previously, Huston et al. (2016) generated ITS2 and 28S rDNA gene sequences from adult and intramolluscan specimens of G. yaaji from the type locality. In the present study, additional adult gorgocephalids morphologically consistent with G. yaaji were obtained from the type locality and from Rangiroa Atoll in French Polynesia and Sodwana Bay in South Africa ( Fig. 1 ). Despite extensive investigation, including traditional morphometrics, PCA analyses and SEM imaging, we were unable to find any significant morphological differences between specimens from these three localities. Intramolluscan gorgocephalids from three Echinolittorina cinerea collected from Rangiroa Atoll, French Polynesia were molecularly matched to adults consistent with G. yaaji collected from the same locality. Furthermore, intramolluscan gorgocephalids have previously been collected from Austrolittorina unifasciata , from multiple locations along the coast of New South Wales, Australia ( O’Dwyer et al., 2015; Huston et al., 2016), but they had not previously been connected to an adult form. BLAST analyses showed that the sequences generated from intramolluscan gorgocephalids collected from Kioloa, NSW, matched sequences of ‘ Gorgocephalus sp. Aus’ from the study of O’Dwyer et al. (2015) with identity scores of 98–100%, demonstrating that gorgocephalids collected from A. unifasciata in the studies of O’Dwyer et al. (2015) and Huston et al. (2016) are conspecific. The present phylogenetic analyses ( Figs 3–6 ) demonstrate that these infections are representative of G. yaaji .

Despite adult gorgocephalids consistent with Gorgocephalus yaaji collected from across the IWP being morphologically indistinguishable, phylogenetic analyses of the ITS2 and 28S rDNA single-gene datasets did not recover a monophyletic G. yaaji clade ( Figs 4 , 5 ). In the ITS2 single-gene tree ( Fig. 4 ), specimens morphologically consistent with G. yaaji were paraphyletic and formed three well-supported clades: specimens from Lizard Island (adults and intramolluscan stages), specimens from French Polynesia (adults and intramolluscan stages) + those from Kioloa, NSW, Australia (intramolluscan stages only), and those from South Africa (adults only). The first two of these clades were recovered in polytomy with the third + G. euryaleae + G. kyphosi , with G. graboides sister to all of these clades together. All ITS2 sequences for G. yaaji were identical for specimens within each locality and differed by 0–5 bp (0.0–1.1%) between localities ( Table 5 ). In the 28S rDNA single-gene tree ( Fig. 5 ), G. yaaji was also recovered as paraphyletic, but with the South African representatives resolving as basal to all clades. The 28S rDNA sequences of G. yaaji differed by 0–6 bp (0.0–0.6%) between localities.

In contrast to analyses of ITS2 and 28S rDNA, those for the COI mtDNA single-gene dataset ( Fig. 3 ) resolved all specimens morphologically consistent with Gorgocephalus yaaji as a monophyletic group, sister to the remaining lineages of the Gorgocephalidae , albeit without support in ML analysis. Sequences differed by only 0–3 bp (0.0–0.6%) within individual localities but differed by 12–62 bp (~3.0–13%) between localities. The largest difference (62 bp) was between specimens from French Polynesia and South Africa, representing opposite sides of the IWP marine region, although the South African specimens were also relatively divergent from those collected in Australia (50–58 bp or ~11–12%; Supporting Information, Table S5 ). The concatenated COI + ITS2 + 28S analyses ( Fig. 6 ) also resolved representatives of G. yaaji in a monophyletic clade, with lower BI posterior probability support but higher ML bootstrap support.

Similar to the pattern observed for Gorgocephalus kyphosi, COI and ITS 2 sequences of G. yaaji , generated from one Australian locality (Kioloa, NSW) were more similar to those from French Polynesia than to those from another Australian locality, Lizard Island, GBR (Supporting Information, Table S5 ). Such molecular variation coupled with the lack of monophyly in the ITS2 and 28S rDNA trees suggests the possibility of multiple species. However, in the ITS2 and 28S trees, sequences associated with specimens morphologically indistinguishable from G. yaaji , result in a paraphyletic, rather than a polyphyletic, G. yaaji (i.e. sequences are more, or at least, as related to one another as they are to other species represented). There are also no hostlevel distinctions to delineate additional species within the G. yaaji concept; the species appears to have a broad definitive and intermediate host range (within the confines of the Kyphosidae and Littorinidae ). The issues with paraphyly in the ITS2 and 28S trees might be alleviated with sequences from additional specimens collected from localities between Australia and South Africa. However, without further evidence there appears no justification for splitting the G. yaaji clade into multiple morphologically cryptic species. Sequences of G. yaaji differ from the others species of the Gorgocephalidae recognized here by 66–88 bp (14–19%), 5–16 bp (1.1–3.5%) and 12–21 bp (1.2–2.1%) in the COI, ITS2 and 28S gene regions, respectively.

Figures

Figure 1. Map of Indo-West Pacific collection localities where specimens of the digenean family Gorgocephalidae were obtained for the present study. Adult specimens were obtained at all localities except for Kioloa, NSW, Australia. At this locality, intramolluscan specimens had been obtained from littorinid gastropods for the study of Huston et al. (2016); genomic DNA from this previous study was used for the generation of additional COI mtDNA gene sequences in the present work.

Figure 3. Bayesian majority-rule consensus tree of the COI mtDNA single-gene alignment. Bayesian inference (BI) posterior probabilities and maximum likelihood (ML) bootstrap support shown at nodes. A ‘-‘ symbol indicates the node was not recovered in ML analysis. The scale-bar indicates the number of substitutions per site.

Figure 4. Bayesian majority-rule consensus tree of the ITS2 single-gene alignment. Bayesian inference (BI) posterior probabilities and maximum likelihood (ML) bootstrap support shown at nodes. An asterisk indicates different topology recovered in ML analysis. The scale-bar indicates the number of substitutions per site.

Figure 5. Bayesian majority-rule consensus tree of the 28S rDNA single-gene alignment. Bayesian inference (BI) posterior probabilities and maximum likelihood (ML) bootstrap support shown at nodes. A ‘-‘ symbol indicates the node was not recovered in ML analysis. The scale-bar indicates the number of substitutions per site.

Figure 6. Bayesian majority-rule consensus tree of the concatenated COI + ITS2 + 28S alignment. Bayesian inference (BI) posterior probabilities and maximum likelihood (ML) bootstrap support shown at nodes. A ‘-‘ symbol indicates the node was not recovered in ML analysis. The scale-bar indicates the number of substitutions per site.