Harpagiferidae T. Gill 1861

Harpagiferidae T. Gill 1861: 510

Type species.— Harpagifer bispinis (Forster in Bloch and Schneider, 1801: 45).

Definition.— The least inclusive clade that includes Harpagifer bispinis (Forster) and Artedidraco mirus Lönnberg (1905: 40– 41). The reference phylogeny is one inferred from a Sanger sequenced dataset comprising two mitochondrial gene regions and seven nuclear genes (Dornburg et al., 2017: fig. 2).

Morphological apomorphies.— (1) Gill membranes are united and joined at the isthmus but do not form a fold (Eakin, 1981; Balushkin, 2000) and (2) the presence of one or two epurals (Eakin, 1981).

Composition.— There are 18 valid and distinct species of Harpagiferidae with six species of Harpagifer and 12 species of Artedidraconinae. Duhamel et al. (2005: 328, 358) and Eastman and Eakin (2021) call into question the distinctiveness of the five additional species of Harpagifer described by V. P. Prirodina and A. V. Neyelov (H. andirashevi Prirodina 2000, H. nybelini Prirodina 2002, H. crozetensis Prirodina 2004, H. macquariensis Prirodina 2000, and H. permitini Neyelov and Prirodina 2006). Duhamel et al. (2005) warn that these species are diagnosed primarily by the degree of development of the supraorbital protuberance, which is known to vary widely within a single species (Eastman and Eakin, 2021). Given these concerns, we conservatively recognize six species of Harpagifer and suggest that species delimitation analyses based on morphological and molecular data are needed to confirm the distinctiveness of the five species described by V. P. Prirodina and A. V. Neyelov.

Phylogenetic analyses.— Consistent with previous molecular phylogenetic studies (Derome et al., 2002; Lecointre et al., 2011; Near et al., 2012, 2018), Artedidraco sensu lato (s.l.) is paraphyletic in the ddRAD phylogenies inferred from both concatenated data and species tree analyses (Figs. 2, 3). A clade containing Neodraco skottsbergi and N. loennbergi is resolved as the sister lineage of all other species of Artedidraconinae. All other species of Artedidraco sensu stricto (s.s.) included in this study (A. orianae, A. mirus, A. glareobarbatus, and A. shackletoni) resolve as a monophyletic group with strong node support (Figs. 2, 3). Relationships among species of Artedidraco are consistent and strongly supported across all analyses: specimens of A. shackletoni do not resolve as a monophyletic group because specimens of A. glareobarbatus are nested within the species (Figs. 2A, 3A). A clade containing A. shackletoni, specimens of A. glareobarbatus, and A. mirus is resolved as the sister lineage of A. orianae (Figs. 2, 3).

In both the concatenated and species tree analyses of the min84 dataset, Artedidraco is resolved as sister to a clade containing Histiodraco velifer and Pogonophryne (Figs. 2A, 3A). This clade, inclusive of Artedidraco, H. velifer, and Pogonophryne, is resolved as the sister lineage of Dolloidraco longedorsalis (Figs. 2A, 3A). These relationships among the major artedidraconine lineages are also resolved in the species tree analysis of the min126 dataset (Fig. 3B); however, the concatenated analyses of the min126 and min144 datasets as well as the species tree analysis of the min144 dataset result in slightly different topologies. In each of these analyses, D. longedorsalis is resolved as the sister lineage of the clade containing H. velifer and Pogonophryne, and this clade including D. longedorsalis, H. velifer, and Pogonophryne is resolved as the sister lineage of Artedidraco (Figs. 2B, C, 3C). These alternative phylogenetic hypotheses likely emerge from differences in the phylogenetic information content across our analyzed datasets. Specifically, the hypothesized placement of D. longedorsalis as sister to H. velifer and Pogonophryne is resolved only in datasets which contain fewer missing data and therefore also include fewer loci (Figs. 2, 3). It has been demonstrated that stricter thresholds on missing data may result in the filtering out of loci with the highest mutation rates, thereby producing datasets with lower phylogenetic information content (Huang and Knowles, 2016). The reduction of phylogenetic informativeness in the datasets with fewer loci is evident by the decreasing node support observed for bipartitions in the phylogenies as the number of loci in a dataset is reduced (Figs. 2, 3).

Morphology.— There are significant differences among species of Artedidraco and Neodraco in several meristic traits (Fig. 4; Tables 1–4; Supplemental Tables S3–4; see Data Accessibility). Neodraco loennbergi and N. skottsbergi exhibit a significantly lower mean number of tubular upper lateral-line scales compared with species of Artedidraco (pairwise rank sum Wilcoxon test: P, 0.05 for comparisons with all species except A. longibarbatus; Table 1, Supplemental Table S4f; see Data Accessibility). The mean number of spines in the first dorsal fin exhibited by A. shackletoni and A. glareobarbatus is significantly higher than that of species of Neodraco and A. mirus (P, 0.05 for all comparisons; Table 2; Supplemental Table S4b; see Data Accessibility). Artedidraco mirus, A. orianae, and A. longibarbatus exhibit fewer anal-fin rays than observed in other species of Artedidraco and the two species of Neodraco (Table 3). In addition, Artedidraco mirus and A. orianae exhibit a lower mean number of second dorsal-fin rays (Table 4).

The disparity in the meristic traits is reflected in the results of the PCA (Fig. 4). The first three PC axes account for 93.4% of the variance in meristic traits. The first PC axis (51.2% of the variation) mostly represents variation in the first dorsal-fin spines and the number of tubular scales in the upper lateral line, the second PC axis (30.2%) mostly describes variation in the number of second dorsal-fin rays and anal-fin rays, and the third PC axis (12.0%) mostly represents variation in the number of pectoral-fin rays. Plotting PC2 against PC1 reveals separation of Neodraco skottsbergi and N. loennbergi from all species of Artedidraco along both PC2 and PC1 (Fig. 4). The distribution of specimens in the PC meristic morphospace is consistent with the diagnosis of Neodraco by a lower number of tubular scales in the upper lateral line as well two or three spines in the first dorsal fin (Fig. 4; Tables 1, 2). The PC plot shows almost no separation of A. shackletoni and A. glareobarbatus (Fig. 4).