Sternarchorhynchus, CASTELNAU, 1855

Santana, Carlos David De & Vari, Richard P., 2010, Electric fishes of the genus Sternarchorhynchus (Teleostei, Ostariophysi, Gymnotiformes); phylogenetic and revisionary studies, Zoological Journal of the Linnean Society 159 (1), pp. 223-371 : 272-274

STERNARCHORHYNCHUS CASTELNAU, 1855

Sternarchorhynchus Castelnau, 1855: 91 [Campos-da- Paz, 2000: 525, concerning page numbering; type species: Sternarchorhynchus mulleri, Castelnau, 1855 (= Sternarchus oxyrhynchus, Müller & Troschel, 1848 ); type by monotypy; masculine].

Rhamphosternarchus Günther, 1870: 4 (type species Sternarchus oxyrhynchus, Müller & Troschel, 1848 ; type by subsequent designation of Jordan, 1919; proposed as subgenus of Sternarchus, Bloch & Schneider, 1801 ; masculine).

Sternarchorchynchus Mago-Leccia, 1978: 14 (misspelling).

Diagnosis: Synapomorphies for Sternarchorhynchus were discussed under the preceding phylogenetic reconstruction and are summarized in the synapomorphy list for the genus in Appendix 2 (clade 3). Although these characters delimit the genus as monophyletic, they overwhelmingly involve internal characters and are therefore inappropriate to determine whether whole specimens at hand are members of Sternarchorhynchus . The following combination of derived and plesiomorphic characters serve to delimit members of the genus.

Sternarchorhynchus shares with other species of the Apteronotidae the presence of a caudal fin and a fleshy dorsal electroreceptive filament, features that readily distinguish the family from other groups in the Gymnotiformes . Species of Sternarchorhynchus differ from other genera in the Apteronotidae by the combination of the possession of an elongate, tubular snout that is compressed laterally and slightly to strongly curved ventrally anteriorly; the length of the snout (usually more than 50% of HL), the small mouth (usually less than 12% of HL), the pronounced distance from the posterior naris to the eye (usually more than 37% of HL), and the posterior position of the origin of the midsaggital electroreceptive filament (approximately at, or posterior to, the middle of TL and situated approximately at 55 to 70% of TL).

Remarks: Campos-da-Paz (2000: 527) detailed the history of the recognition, or lack thereof, of Sternarchorhynchus by authors following Castlenau’s (1855) proposal of the genus. As discussed by Campos-da- Paz, various authors failed to recognize Sternarchorhynchus for nonspecified reasons, perhaps because they were not aware of Castlenau’s description of the genus. Günther (1870: 4), for example, was apparently unaware of Sternarchorhynchus because he proposed Rhamphosternarchus as a subgenus of Sternarchus , with that genus including three species amongst which was S. oxyrhynchus , the type species of Sternarchorhynchus . The Rhamphosternarchus of Günther was based on the presence in member species of a ‘snout produced into a long tube’, one of the distinguishing attributes for Sternarchorhynchus . Eigenmann & Ward (1905: 166) rectified the problem by explicitly noting that Rhamphosternarchus was a synonym of Sternarchorhynchus .

Common features of species of Sternarchorhynchus: The species of Sternarchorhynchus share a distinctive bauplan with many of the features delimiting species involving meristics and specific morphometric features and/or coloration rather than significant changes in overall external head and/or body morphology. In the interest of space efficiency, we describe external features common to all species of Sternarchorhynchus in this section rather than reiterating them in each species description. Conditions of the intragenerically variable external features for each species are detailed in each species account.

Body elongate and distinctly laterally compressed (cultiform), more so in region posterior to abdominal region. Greatest body depth located in area of abdominal cavity or slightly posterior to that region. Dorsal profile of body ranges between straight to very slightly convex. Anteriormost perforated lateral line scale located along vertical through pectoral-fin origin.

Head laterally compressed, widest in opercular region and deepest at nape. Eye very small, laterally positioned on dorsal half of head, and completely covered by thin membrane. Cephalic pores small. Snout distinctly elongate, overall tubular but somewhat compressed laterally. Premaxilla of small size. Anterior naris located at end of small tube and proximate to tip of snout. Posterior naris ellipsoid, without tubular extension. Posterior naris located distinctly closer to tip of snout than to anterior margin of eye. Branchial opening constricted to short vertical aperture situated along posterior margin of opercle and slightly anterior to vertical through pectoral-fin origin. Branchial membranes joined at isthmus. Anus and urogenital papilla adjacent and ventrally positioned. Location of both structures intragenerically and ontogenetically variable in many species and sexually dimorphic in some species.

Scales cycloid, small and present over all, or nearly all, of body from rear of head to anterior portion of base of caudal fin. Scales absent or sparse in some species along mid-dorsal line of body anterior to origin of electroreceptive filament. Lateral line scales often larger than those of scale rows immediately dorsal and ventral of lateral line series.

Pectoral fin long, broad, and distally pointed. Anal fin elongate and extending from under head posteriorly for most of length of body. Dorsal electroreceptive filament arising on posterior portion of body and progressively narrowing posteriorly and inserting into and attached to narrow mid-dorsal grove. Caudal and pectoral fins present, but dorsal and pelvic fins absent as in all gymnotiforms.

Distribution: A distinct majority of the species of Sternarchorhynchus inhabit portions of the Amazon basin (22 of 32 species, Table 1), with the genus occurring across major portions of that drainage system. Many of the apparent major gaps in the distribution of the genus across the Amazon basin are most likely to be a function of the lack of collecting in habitats appropriate for the members of the genus. This issue is general for the Neotropical freshwater fish fauna ( Vari & Malabarba, 1998). Seven species of Sternarchorhynchus occur in the Río Orinoco system and the independent river systems of north-eastern Venezuela that drain into the Golfo de Paria ( Table 1). One of these species, S. mormyrus , is the only member of the genus shared with the Amazon basin (comments under that species description).

The sparse records of Sternarchorhynchus in the rivers of the Guianas (two species) may be in part a function of the limited collecting efforts in appropriate habitats, but the genus is unknown even in some relatively well sampled basins in this region ( Mol et al., 2007). Similar gaps in distributions at the generic level occur in other components of the ichthyofauana in the Guianas. A single species of Sternarchorhynchus occurs in the Essequibo River system of Guyana, one species in the Lawa River of the Marowijne River (= Fleuve Maroni) system along the border between Suriname and French Guiana. Sternarchorhynchus has also been reported from the Fleuve Oyapock (= Rio Oiapoque ) along the boundary between French Guiana and Brazil ( Planquette, Keith & Le Bail, 1996: 406). We were unable to examine the specimens that served as the basis for that report. A single species of Sternarchorhynchus , S. britskii , is known from the upper portions of the Rio Paraná basin in Brazil; that being the only member of the genus occurring in a drainage system south of the Amazon basin.

Diversity: The 32 recognized species of Sternarchorhynchus represent approximately 40% of the present known diversity at the species level within the Apteronotidae , making it by far the most speciose genus in the family, with the approximately 15 recognized species of Apteronotus (sensu stricto; Albert & Crampton, 2005: 363) making it the next most diverse genus in the family. As we note under the Possible additional undescribed species section, the 32 recognized species of Sternarchorhynchus undoubtedly represent an underestimate of the actual diversity in the genus. Under present concepts of generic limits and species richness, Sternarchorhynchus is one of the most speciose genera within the Gymnotiformes . Only Gymnotus in the Gymnotidae , that is estimated to include at least 35 species (Albert & Crampton, 2005; Albert et al., 2005) is as speciose as Sternarchorhynchus .

An evaluation of the diversity of Sternarchorhynchus relative to that of Gymnotus shows some notable differences in the proportional density of species in the two genera across their range. Most obviously, Sternarchorhynchus is restricted to the cis-Andean portions of South America with only one species, S. britskii , known from a river basin south of the Amazon, whereas Gymnotus has a massive distribution from Mexico to Argentina to both sides of the Andean Cordilleras. Gymnotus is, furthermore, able to tolerate a much broader range of habitats, being able to absorb atmospheric oxygen via a portion of the gas-bladder ( Campos-da-Paz, 2003: 483); an adaptation that opens up low oxygen-level backwaters and swamps as potential habitats. Sternarchorhynchus lacks such modifications and is apparently limited to well-oxygenated habitats. The more restricted geographical range and habitat tolerances of the species of Sternarchorhynchus render its speciose nature all the more impressive in terms of proportional density of species. In part this density is made possible by the ability by many species of Sternarchorhynchus to exploit deep river habitats, which contrasts with the River systems and regions are as follows: A, Rio Orinoco basin ; B, Guianas; C, Rio Negro ; D, Rio Tocantins/Araguaia; E, Rio Trombetas ; F, mainstream Amazon below Manaus; G, mainstream Amazon above Manaus; H, Rio Madeira ; I, western Amazonian tributaries; J, upper Rio Paraná; K, Rio Xingu ; L, Rio Branco.

absence of all genera of the Gymnotidae from such habitats ( Crampton, 2007: 302).

Remarks on secondary sexual dimorphism: Various species within one clade within Sternarchorhynchus together with various other Gymnotiformes (e.g. Sternarchogiton ; de Santana & Crampton, 2007: 390: fig. 1a; Compsaraia, Albert & Crampton, 2009 : fig. 1) demonstrate particularly striking sexual dimorphic remodelling of the lower jaw of various forms. Some males with well-developed testes amongst the Sternarchorhynchus species in clade 17 have the dentary anteriorly lengthened and transversely expanded into a dorsally bulbous structure that is rounded from a dorsal view ( Fig. 25A ; comments on Sexual dimorphism under Discussion above). This enlarged portion of the dentary bears a patch of distinctly enlarged, typically slightly posteriorly recurved dentition. This patch of enlarged teeth in males of this clade may be utilized in confrontations during breeding contests ( Marrero & Taphorn, 1991: 129).

Such dramatic elaborations of the lower jaw and dentary dentition are unknown in some species of the clade represented by limited numbers of specimens and larger samples are necessary to determine whether the condition is indeed absent in these species. We have examined conspecific males of the species characterized by such dentary modifications with well-developed testes that nonetheless lack elaborations of the lower jaw and associated dentition. This is despite those individuals being of body sizes comparable to conspecific males with those attributes. Thus, one cannot assume that large specimens of a species within clade 17 that lack an expansion of the dentary and elaborations of the dentary dentition are females. The significance of these differences amongst conspecific males of comparable sizes cannot be elucidated from museum specimens, but potentially reflects social structure amongst groups of males. Alternatively, it may be a function of seasonality in the manifestation of this secondary sexually dimorphic feature.

Figures

Figure 25. Radiograph of head and snout of Sternarchorhynchus yepezi, holotype, MCNG 44286; showing A, expanded portion of dentary and elaboration of associated dentition in sexually dimorphic males; B, point of articulation between quadrate and lower jaw; and C, elongate mesethmoid.