Hypopygus, HOEDEMAN, 1962

De Santana, Carlos D. & Crampton, William G. R., 2011, Phylogenetic interrelationships, taxonomy, and reductive evolution in the Neotropical electric fish genus Hypopygus (Teleostei, Ostariophysi, Gymnotiformes), Zoological Journal of the Linnean Society 163 (4), pp. 1096-1156 : 1121

HYPOPYGUS HOEDEMAN

Tateichthys La Monte, 1929: 1–3 [in part, see Nijssen & Isbrücker, 1972].

Hypopygus Hoedeman, 1962: 99 , fig. 4 [type species by original monotype, Hypopygus lepturus ]. – Mago- Leccia, 1994: 50. – Albert & Crampton, 2003: 595.

Stegostenopos Triques, 1997: 2 , fig. 5 [type species by monotype and original designation, Stegostenopos cryptogenes ]. – Campos-da-Paz, 2007: 123 [distribution in check list; Brazil]. – Albert & Crampton, 2003: 596 [in listing of taxa].

Diagnosis: Proposed synapomorphies for Hypopygus were discussed under the synapomorphy list for the genus. Although these characters serve to delimit the genus as monophyletic, they mostly involve internal characters and are thus inappropriate for the purpose of determining whether whole specimens at hand are members of Hypopygus .

Species of Hypopygus can be more readily distinguished from other gymnotiform genera, including all Rhamphichthyoidea, by the absence of the posterior nares and by the presence of a postpectoral accessory EO with dorsal and ventral anteriorprojecting grooves (See Fig. 15 ).

Common features of species of Hypopygus : Hypopygus share a distinctive bauplan, and most species are delimited by a combination of meristic and morphometric characters and/or pigmentation, rather than major changes in external head and/or body morphology. For brevity, we describe external features common to all species of Hypopygus 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 laterally compressed, more so posterior to abdominal region. As with other gymnotiforms, coelomic cavity is confined to a restricted area posterior to the head, extending no more than two head-lengths posterior to occiput. Viscera rotated so that anus is positioned below operculum, adjacent to isthmus. Greatest body depth located in area of abdominal cavity or slightly posterior to abdominal cavity. Dorsal profile of body straight or nearly straight.

Head widest in opercular region and deepest at nape. Eyes small, laterally positioned on dorsal half of head, and completely covered by thin membrane. Anterior nares circular, located at upper lip. Snout short. 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. Mouth small and terminal, upper and lower jaws approximately equal, or upper jaw slightly longer than lower jaw.

Scales cycloid, small, and present over all (or nearly all) of body, from rear of head to caudal filament. Scales absent in some species along mid-dorsal line of body. Anterior-most perforated lateral line scale located at vertical through pectoral-fin origin. Lateral line discontinuous.

A postpectoral accessory EO present. Pectoral fin short to moderate, broad and distally pointed. Anal fin elongate and extending from under end of pectoral fin posteriorly for 45 to 76% of TL. Caudal filament moderate to long (21–81% of length to end of anal fin in intact specimens). Dorsal, pelvic, and caudal fins absent.

Distribution: Species of Hypopygus are known to occur in most major river systems of the Amazon and Orinoco basins, from several coastal drainages of the Guianas, and from some northerly tributaries of the Rio Paraguay drainage in Brazil.

Figures

Figure 15. Postpectoral accessory electric organ and associated dorsal and ventral grooves of Hypopygus nijsseni MCP 44737, 75 mm; left side, lateral view, anterior to left.