Apteronotus acidops , new species of long snouted electric fish ( Teleostei : Gymnotiformes : Apteronotidae ) from the upper rio Paraná basin in Brazil , with a key to the apteronotid species from the area

Apteronotus acidops, new species, is described from the upper Paraná River basin, Brazil. It can be diagnosed from the remaining species of the genus by the following combination of characters: dorsal snout profi le pointed; snout length 46.4 – 63.7 % of head length; oculo-nasal distance 39.2 – 72.1 % of postocular distance; lateral ethmoid with both dorsal and ventral extremities expanded, strongly oblique in orientation; mouth rictus surpassing vertical through anterior eye margin, including its skin folds, in all ages; dorsal head profi le nearly horizontal from a vertical through anus to posterior extremity of occipital bone; absence of mid-dorsal white or clear stripe on head and body anteriorly; chin brown; upper lip clear to a vertical through anterior nostril; transversal unpigmented bar or bars posteriorly on the body absent; fl anks brown, darker dorsally and pectoral fi n hyaline, with chromatophores over rays. Moreover, a key for the apteronotid species from the upper rio Paraná is provided.


Introduction
Currently, 22 species are considered valid in the genus Apteronotus La Cepède, 1800(Froese & Pauly, 2011), although A. ellisi (Arámburu, 1957) and A. marauna (Triques, 1998) have been described and considered valid in other genera (e.g.Campos-da- Paz, 2005;Da Graça & Pavanelli, 2007;Triques, 2005).The Ilha Solteira Power Plant was built in the rio Paraná, between the states of São Paulo and Mato Grosso do Sul.In its coffer dam, in 1965, specimens of an unidentifi ed species of Apteronotus were col-lected.Additional specimens of the same species were collected in the rio Mogi-Guaçu at rio Pardo drainage, rio Paraná basin in the state of São Paulo, Brazil.The species resembles A. brasiliensis (Reinhardt, 1852), from the rio São Francisco and rio Paraná basins and A. leptorhynchus (Ellis, 1912) from the Guyanas and río Orinoco basins, in general morphology (differing clearly from this last species by the absence of a clear mid-dorsal stripe on head and anterior region of body, in the new species).Apteronotus brasiliensis is known to occur in the same area but skeletal and external morphology differences were found among them.Differences were also found from every other Apteronotus species.Furthermore, the new species clearly can not be included in any other long snouted genus of the family and is placed in Apteronotus.

Material and Methods
Measurements and counts were made as in Triques (1996), together with the "length to the end of anal fi n" (LEA), that is, the length from the anterior tip of snout to the base of the last anal-fi n ray.
Comparisons with already known valid species were made on the basis of original descriptions and other information present on literature (mentioned in the comparative diagnosis) and/or with comparative material (presented in the comparative material list).Apteronotus brasiliensis is morphologically similar to the new species, both occurring in Rio Paraná basin and presents the same color pattern of the new species, therefore skeletal differences were presented to differ both species more conclusively.Thus, fi ve specimens of A. brasiliensis and one of the new species were cleared and stained for bone and cartilage (annotated as C&S), according to Taylor & Van Dyke (1985) and dissected according to Weitzman (1974).Institutional acronyms follow Leviton et al. (1985) with the addition of DZUFMG for Departamento de Zoologia da Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais State, Brazil.For sexual dimorphism evaluation, permission for dissection to sexual identifi cation was requested, and obtained for seven specimens, including the holotype; data expressed on type material presentation included head and snout lengths only for holotype and a few sexed specimens.Although there are some proposals of species subgroups for Apteronotus, they are not consensual (e.g.Albert, 2003;Triques, 2005) and therefore a comprehensive diagnosis seems to be preferable and was thus made.

Apteronotus acidops spec. nov.
Figs. 1 -3 Diagnosis.Apteronotus acidops is mostly similar to A. brasiliensis and sympatric to it, differing by the snout length being 46.4 -63.7% of head length (vs. 42,8 -48,5% in A. brasiliensis, n = 11 specimens, including males and females of rather large size, 290 -398 mm LEA; Fig. 4) ; oculo-nasal distance 39.2 -72.1% of postocular distance (vs.30.7 -45.0% in A. brasiliensis, n = 11 specimens); lateral ethmoid with both dorsal and ventral extremities expanded, strongly oblique in orientation (vs.expanded dorsally and pointed ventrally, vertically oriented in A. brasiliensis); mouth rictus surpassing vertical through anterior eye margin, in all ages (vs.not reaching a vertical through anterior eye margin, usually attaining a vertical between posterior nostril and eye in A. brasiliensis); snout dorsal profi le straight, pointed (vs.convex in A. brasiliensis; Fig. 4).Apteronotus acidops can be diagnosed from the other species of the genus with the following combination of characters: absence of white or clear stripe from snout tip backward to nearly the middle of body length or origin of fl eshy dorsal fi lament (vs.presence of this stripe in A. albifrons [Linnaeus, 1766], A. bonapartii [Castelnau, 1855], A. cuchillejo Schultz [1949], A. cuchillo Schultz [1949], A. galvisi de Santana, Maldonado-Ocampo & Crampton [2007], A. leptorhynchus, A. rostratus, A. spurrelli [Regan, 1914]); circular; lateral line complete, to base of caudal fi n, but not entering it; mid-dorsal body line covered with scales visible through skin, not organized into straight series; head, pectoral and anal fi ns naked; caudal fi n with circular scales, slightly elongated horizontally; approximately 14 rows of scales above lateral line through a vertical passing by the posterior end of body cavity.Dorsal body profi le straight, backward sloping ventrally.Ventral body profi le approximately straight.Dorsal and ventral body profi les converge posteriorly, ending in a compressed caudal peduncle, with a circular or lanceolated caudal fi n.Pectoral fi ns with i + 13 -18 rays, anterior margin convex, posterior border straight.Anal fi n with 162 -220 rays, its origin in a vertical through middle of opercle.Opercular aperture straight or curve, its concavity backward; inferior limit at inferior level of pectoral-fi n base.
Head dorsal profi le sloping strongly upward and backward, from the anterior extremity of snout to a little before a vertical through anus, straight or with a very slight concavity over posterior nostril; then horizontal and slightly convex to osseous mid-dorsal border of head.Mouth terminal, with lower jaw included in upper jaw only laterally; upper lip profi le usually convex, sometimes in an obtuse angle.Mentonian lateral lobe present, completely inside mouth when mouth closed, backward elongated to a point through vertical by anterior nostril.Premaxillary with 8 -11 elongated, backward directed teeth, not organized chin brown (vs.clear in A. albifrons, A. leptorhynchus, A. cuchillo, A. magoi de Santana, Castillo & Taphorn [2006]); transversal unpigmented bar or bars posteriorly on the body absent (vs.present in A. albifrons, A. camposdapazi de Santana & Lehmann [2006], A. caudimaculosus de Santana [2003], A. cuchillo, A. eschmeyeri de Santana, Maldonado-Ocampo, Severi & Mendes [2004], A. galvisi); fl anks brown, darker dorsally (vs.ground color light brown with black dots throughout body length in A. magdalenensis [Miles, 1945], A. milesi and A. cuchillo; caudal peduncle with ground color clear with several dark marks in A. magoi); pectoral fi n hyaline, with chromatophores over rays (vs.an hyaline stripe on the base of the pectoral fi n, otherwise black in A. jurubidae [Fowler,1944]); dorsal snout profi le pointed (vs.snout profi le roundish in A. apurensis Fernández-Yepes, 1968, A. macrolepis [Stein dachner, 1881] and A. rostratus [Meek & Hilde brand, 1913], head dorsal profi le convex in A. mariae [Eigenmann & Fisher, 1914] see Mago-Leccia, 1994).
Description.Morphometric data are in table 1. Head and body elongated, strongly compressed.Body uniformly covered with cycloid scales, with decreasing sizes from lateral line series of scales, upward and downward, which are three times smaller; lateral line scales vertically elongated; dorsal and ventral scales than two eye diameters.Posterior nostril slit like, horizontally elongated, shorter than anterior nostril aperture.Mouth rictus wide, with external skin folds that surpass vertical through most anterior eye margin (Fig. 2).Eye small, dorso-laterally on head, uniformly covered by head skin; ocular diameter roughly fi ve times (smallest specimens) to two times (largest ones) longer than length of posterior nostril aperture.
spatially.Dentary with two series of elongated teeth, backward directed.Anterior nostril tubular, distant from anterior extremity of snout about two eye diameters, and from nearest upper lip margin about a little more than two eye diameters; antero-dorsally with one lateral line pore and postero-dorsally with another one; distant from posterior nostril about a little less (small specimens) or a little more (largest specimens)  Studied specimens of A. acidops range in size from 90 to 321 mm LEA and its snout is longer than in A. brasiliensis, with studied males and females of about 400 mm LEA.Therefore, it is not possible to correlate the studied material of A. acidops with terminal males of A. brasiliensis.Lateral ethmoid characteristics distinguish both species clearly.Permission was obtained to clear and stain one paratype of A. acidops and four of A. brasiliensis, covering widely the distribution of this last species in rio Paraná drainage: rio Piracicaba in São Paulo state (MZUSP 22237), Ilha Solteira in rio Paraná (MZUSP 24460), río Araguari in Minas Gerais state (DZUFMG 068) and rio São Marcos at Serra do Facão, Goiás state (DZUFMG 091), all of them with the same state for A. brasiliensis (expanded dorsally and pointed ventrally, vertically oriented vs. with both dorsal and ventral extremities expanded and strongly oblique orientation in A. acidops).Other diagnostic characters concerning to A. brasiliensis regard to snout profi le and mouth rictus, so that all diagnostic characters between these two species are not correlated in nature among each other.
No other apteronotid species from Rio Paraná basin has the snout length as 46.4 -63.7% of head length, except for Sternarchorhynchus britiskii Campos-da-Paz, 2000 that in turn has a downward curved snout, prolonged below a horizontal through anal fi n base (vs.snout elongated and straight, downward directed, not surpassing a horizontal through anal fi n base in A. acidops).Tembeassu marauna Triques, 1998(Apteronotus marauna in Albert, 2003) has mentonian lateral lobe extremely developed, and the upper jaw presents concave profi le to fi t it (vs.mentonian lateral lobe small and upper jaw profi le straight in A. acidops).For a description and evolutionary interpretation of this feature, see Color in alcohol.Head and body brown, darker dorsally; unpigmented spot on base of caudal fi n; pectoral fi n and anal fi n hyaline, with chromatophores over rays; caudal fi n dark brown, with chromatophores over rays and membrane.

Discussion
Apteronotus acidops is promptly distinguishable from all other Apteronotus species -except the sympatric A. br a siliensis -by means of color pattern features and/or snout profi le, all external and evident characters.However, as no color pattern difference was found with A. brasiliensis and the presence of secondary sexual dimorphism was described for some species of the genus, a deeper analysis was required in order to differenciate these two species.

Table 1 .
Morphometric data of holotype and paratypes of Apteronotus acidops.N indicates number of measured paratypes.