2.1 Species studied

The material comprises 10 oxudercine species representing eight genera, i.e. Apocryptes bato (Hamilton, 1822), Apocryptodon madurensis (Bleeker, 1849), Boleophthalmus dussumieri Valenciennes, 1837, Oxuderces dentatus Eydoux & Souleyet, 1850, Pn. freycineti (Quoy & Gaimard, 1824), Pn. schlosseri (Pallas, 1770), Ps.  novemradiatus (Hamilton, 1822), Ps. waltoni Koumans, 1955, P.  borneensis (Bleeker, 1855), and S.  tenuis (Day, 1876) (see Table 1 for details). Species identification was verified in each case based on the species-diagnostic characters, the sampling location and further information provided in Murdy (1989). A specimen of Gobius niger Linnaeus, 1758 from the NE Atlantic Sea (collection of the National Museum Prague, Czech Republic, number NMP6V 146072) was used as a representative species of the 'gobiine-like' Gobiidae sensu Agorreta et al. (2013). Digital X-ray images were taken with a Faxitron Bioptics (LLC-Vision NDT version 2.2.5, 45 k.v. and 30 s) housed in the State Zoological Collection, Munich for all specimens and served as basis for the osteological study.

2.2 Study of osteology

Figure 1 provides an overview of the vertebral column and caudal skeleton, the osteological terminology follows Schultze and Arratia (2013). The vertebral column comprises an abdominal and a caudal part (Figure 1a). Each vertebra consists of a centrum, and, except the last vertebra (= terminal centrum, TC), a neural arch and a neural spine (Figure 1a, b). Abdominal vertebrae, except the first one or two, are associated with ribs; caudal vertebrae (except TC) are characterized by the presence of a closed haemal arch and a haemal spine (Figure 1a, b). Following Schultze and Arratia (2013), solely the penultimate and antepenultimate caudal vertebrae are designated here as preural vertebrae (PU) as their spines contribute to the support of the caudal rays. The penultimate caudal vertebra represents PU2 and the antepenultimate one is PU3 (Figure 1b). The usual condition in gobioids is that PU2 possesses an expanded haemal spine (which supports caudal rays, see Figure 1b), but a shortened neural spine (see Fujita, 1990). Also the neural and/or haemal spine of PU3 are visibly differentiated from a 'usual' caudal vertebra - i.e. more elongated, or more expanded, or more bent - and the distal tips of the PU3-spines are associated with (or near to) the caudal cartilage that supports the dorsal and ventral procurrent caudal rays (see Fujita, 1990). Further elements of the caudal skeleton of gobiids are two large hypural plates (Hy1+2, Hy3+4), an autogenous small hypural plate (Hy5), a parhypural (Php) and one or two epural bones (Ep1, 2) (Figure 1b). The hypural plates and the parhypural support the principal caudal rays and the epural bones are associated with the support of the dorsal procurrent caudal rays (Figure 1b).

For the purpose of this study, we use as additional term 'modified caudal vertebra' (MC), which is numbered from the posteriormost to the anteriormost (like the preural vertebrae). This term is used for each vertebra that has, like a preural vertebra, a visible modification of its neural and/or haemal spine in comparison to a 'usual' caudal vertebra, but differs from a preural vertebra as it is not involved in caudal ray support (Figure 1b). The same terminology and definition are used in Charmpila et al. (in press).

3.1 The posteriormost vertebral column

The presence of two preural vertebrae (PU2, PU3), as seen in Gobius niger (Figure 2a), and the absence of modified caudal vertebrae in the definition of the present study can be considered as the usual configuration in the Gobiidae; it also occurs in As. bato, O. dentatus, and An. madurensis (Figure 2b–d, Appendix: Figure A1).

The addition of a single modified caudal vertebra (MC1) is seen in the posterior vertebral column of P. borneensis and B. dussumieri (Figures 2e, 3a, Appendix: Figure A1). Note that B. dussumieri exhibits a neural spine of MC1 that is not only slightly broadened (as in P. borneensis), but also relatively closer positioned to the neural spine of PU3 (Figure 3a). Two modified caudal vertebrae (MC1, MC2) occur in S. tenuis (Appendix: Figure A1) and Pn. freycineti (Appendix: Figure A2), and 3–6 modified caudal vertebrae are recorded in Pn. schlosseri (MC1–MC3, Appendix: Figure A2), Ps. novemradiatus (MC1–MC5, Appendix: Figure A2) and Ps. waltoni (MC1–MC6, Appendix: Figure A2).

Furthermore, the shape, size and inclination of the neural and haemal spines of PU2 and PU3 revealed clear differences between the species (Figures 2-4), of which the details are specified in Table 2. Most noticeable is a particular close association between the neural spines of PU2 and PU3 in Ps. novemradiatus (Figure 3d) and Ps. waltoni (Figures 3e, 4i): the PU2 spine has a convex anterior margin that dovetails with the concave posterior margin of the PU3 spine.

3.2 Caudal skeleton

The configuration of the hypural plates is largely the same in all ten species (Figures 2, 3, Appendix: Figures A1 and A2). There are two large hypural plates (Hy1+2, Hy3+4), approximately equal in size and separated by a gap; HY3+4 is fused to the terminal centrum. Hypural plate 5 (Hy5) is a small, elongate or needle-shaped autogenous bone. Two relatively robust epural bones are present in all species. The parhypural is slender and not connected to the terminal centrum. In P. borneensis (Figure 2e), S. tenuis (Figure 2f), Pn. freycineti (Figure 3b), Pn. schlosseri (Figure 3c), Ps. novemradiatus (Figure 3d) and Ps. waltoni (Figure 3e), the parhypural is distinctly smaller than in the other species.

The principal caudal rays are relatively uniform in As. bato (Figure 2b), An. madurensis (Figure 2d), O. dentatus (Figure 2c) and P. borneensis (Figure 2e). In contrast, the principal caudal rays show an asymmetrical configuration in B. dussumieri (Figure 3a), Pn. freycineti (Figure 3b), Pn. schlosseri (Figure 3c), Ps. novemradiatus (Figure 3d), Ps. waltoni (Figure 3e) and S. tenuis (Figure 2f), with thickened and shortened ventralmost rays compared to their dorsal counterparts. This is most pronounced in Ps. waltoni, and least developed in S. tenuis.