Comprehensive Description

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Apteronotus mariae (Eigenmann and Fisher)

Original description (Eigenmann and Fisher, 1914: 236):

“Head 7.1 type, (6.3 paratype) in the length to the end of the anal; depth.92 type, (.94 paratype) in the length of the head; anal rays 173 type, (176 paratype); snout 2.7 type, (2.5 paratype) in the length of the head; eye 5.5 type, (5.7 paratype) in the interorbital, 5 type (6 paratype) in the head; width of head 2.4 type (2.5 paratype) in the depth. Head decurved; gape reaching to vertical from the eye; ground color light buff, body closely pigmented with minute dark chromatophores, much more abundant dorsally. An uninterrupted whitish streak along the median dorsal line from the end of the snout to the base of the caudal.”

Sternarchus mariae ZBK : Eigenmann & Fisher (1914: 236) [original description; type locality: Girardot, Colombia]. Eigenmann (1922: 175, 334, plate XXXIV, fig. 6) [in key; first illustration of the species]

Apteronotus mariae : Miles (1947: 181, 182 and 185, fig. 134) [ new combination , in key]. Mago-Leccia (1994: 29, fig. 32) [listed; illustrated]. Albert & Campos-da-Paz (1998: 431) [listed]. Albert (2001: 76). de Santana (2003: 8) [in key]. Albert (2003: 499) [listed].

Holotype . FMNH 56774, 195.1 mm TL, female, Girardot, Colombia .

Paratype - CAS 62345 (IU 13375), 274.5 mm TL, female.

Additional material - IAVHP 3137, 284.3- 330.0 mm TL, 1 male and 1 female ; IAVHP 3998, 198.0-212.0 mm TL, 1 male and 1 female ; IAVHP 3999, 185.0-201.0 mm TL, 3 females ; IAVHP 4000, 223.0 mm TL, 1 female .

Diagnosis. Included in the Apteronotidae by having a dorsal mid-sagittal electroreceptor organ and a caudal fin (Mago-Leccia, 1994; Albert & Campos-da-Paz, 1998; Albert, 2001). Apteronotus mariae shares the following diagnostic features with the subfamily Apteronotinae (Albert & Campos-da-Paz, 1998; Albert, 2001): gape of mouth large, extending to the posterior edge of the eye; maxilla rhomboid in lateral view; anterior surface of mesethmoid concave, and lateral process of ventral ethmoid robust. Included in the genus Apteronotus ZBK (sensu Albert & Campos-da-Paz, 1998; Albert, 2001) by possessing the following characters: white mid-dorsal stripe; white band circling the caudal fin base; and elongate posterior limb of the anguloarticular. Apteronotus mariae can be diagnosed within the genus Apteronotus ZBK by the following combination of features: lateral ethmoid present and ossified (vs. lateral ethmoid absent or unossified in A. cuchillejo and A. rostratus ); and body coloration light brown, with small black spots distributed over the integument (vs. coloration even or mottled in the remaining members of Apteronotus ZBK ).

Apteronotus mariae can be included in the “ A. albifrons species-group ,” defined by two pigmentation characteristics: a pale pigment patch on the caudal peduncle and highcontrast pale or white mid-dorsal stripe and mental patch, as proposed by Albert & Campos-da-Paz(1998) and Albert (2001). Apteronotus mariae can be distinguished from A. eschmeyeri ZBK , A. magdalenensis and A. rostratus by possessing a light brown body with small black spots distributed over the integument (vs. blotchy in the first two species, and uniformly brown to black in the third one).

Description. Morphometrics for the holotype and paratype , plus two other specimens are presented in Table 1. Body laterally compressed, greatest body depth at abdominal cavity or slightly posterior to this area. Dorsal profile nearly straight. Lateral line extending to tail, absent on caudal fin. First anterior perforated scale above pectoral fin origin.

Head laterally compressed, widest at opercular region and deepest at dorsal region; profile nearly straight, snout somewhat curved. Eyes small, located laterally on head, completely covered by a thin membrane. Mesethmoid short and narrow, its anterior tip reduced, flexed ventrally, and concave. Lateral ethmoid present and ossified. Premaxilla of moderate size, with four irregular rows of teeth; 18 teeth in total (n=1). Maxilla crescentshaped, with an ossified anterodorsal head and anteroventral shelf, an anterior process, and the ventral margin of its descending blade curved. Dentary long and deep (Fig. 2), with irregular rows of teeth, 12 to 13 teeth in total (n= 2). Anguloarticular elongated. Mandibular canal ossified, shaped as a tube. Mouth large, terminal, rictus passing a vertical through posterior border of eye. Upper jaw longer than lower jaw. Endopterygoid narrow, without teeth, and with an ascending process connecting the orbitosphenoid, its base ossified and the remaining portion unossified. anterior portion of the endopterygoid narrow, extending to midlength of dentary. Four branchiostegal rays, first and second almost filamentous, the remaining large and laminar. The 5th and 6th infraorbital bones ossified, their shape as a slender tube. Anterior nares at the end of a small tube, close to tip of snout; posterior ones ellipsoid, without a tube and closer to tip of snout than to eyes. Branchial opening slightly anterior to pectoral-fin origin; branchial membranes joined to isthmus. Anus and urogenital papilla adjacent, ventral, located in a vertical posterior to eye, but without a noticeable forward displacement with age. Pectoral fin somewhat elongated, broad and pointed distally, with ii+13 (holotype) to ii+16 rays. Anal fin origin slightly anterior to opercular region; 22-27 (n=4, mode= 27) unbranched anal-fin rays, 162-180 (n= 8, mode= 171) total anal-fin rays. The number of anal fin rays, we counted 169 in the holotype (vs. 173 in the original description) and 171 in the paratype (vs. 176).

Dorsal mid-sagittal electroreceptor organ origin on posterior half of body and inserted into a narrow middorsal groove almost extending to, or slightly passing, the end of the anal fin. Compressed and short tail, ending in a small and somewhat elongated caudal fin, with 21-22 rays (n= 2). Number of precaudal vertebrae 16, with 12 anterior and 4 transition vertebrae (n= 2).

Coloration in alcohol. Body coloration light brown with fine black pigmented spots distributed over the integument; clear band from tip of snout to beginning of dorsal midsagittal electroreceptor organ; chin with white band; two clear bands encircling caudal peduncle, (only observed in some females); or a clear band surrounding caudal fin base, males and females.

Sexual Dimorphism. We have not observed secondary sexual dimorphism in the specimens examined.

Electric organ discharges. The family Apteronotidae differs from the other gymnotiforms in that the EOD is generated (usually at higher frequencies) by the action potential of neurons (Bennett, 1971; Kramer et al., 1981; Kirschbaum, 1995; Crampton, 1998; Alves-Gomes, 2001, Crampton & Albert, in press). Although there is growing evidence for species-specific or species typical EOD waveforms in some taxa of pulse generating gymnotiforms (Hopkins & Heiligenberg 1978; Heiligenberg & Bastian, 1980; Crampton & Albert, in press), little has been published about the species specificity of EOD waveforms in apteronotids. Kramer et al. (1981) and Crampton & Albert (in press) noted that many sympatric species of river-dwelling Apteronotidae from the Central Amazon share similar EOD waveforms. As such, the waveforms are not generally used as a taxonomic tool in apteronotids.

EOD fundamental frequencies and variations in the fourier power spectra have been used to separate sympatric assemblages of apteronotids (e.g., Crampton, 1998; Kirschbaum, 1995; Crampton & Albert, in press). At the moment, Orthosternarchus tamandua (Boulenger) presents the lowest EOD frequency in the family at 421 Hz, while Magosternarchus raptor Lundberg, Cox-Fernandes & Albert ZBK and Sternarchella schotti (Steindachner) have the highest EOD frequencies, at 1969 Hz and 2179 Hz respectively (Carmpton & Albert, in press). It is thought that fish possessing low frequencies have a diet based primarily on aquatic invertebrates, while high-frequency species are believed to be primarily or at least partially fish predators (Crampton, 1998). In addition it is believed that high frequency EODs may represent a better strategy or advantage to species inhabiting fast-flowing waters (Crampton, 1998; Alves-Gomes, 2001; Crampton & Albert, in press).

As do most apteronotids, Apteronotus mariae has a quasi-sinusoidal EOD waveform. Electric organ discharges were recorded from a female captured at Honda with a water temperature of 27° C and a pH of 7.0. The waveforms of sympatric apteronotids captured at Honda had the same basic waveform. Apterontous mariae had the highest EOD frequency among the apteronotids captured at 1440 Hz (n=1, female) vs. 840 Hz in A. eschmeyeri ZBK (n= 3, females) and 750 Hz in A. magdalenensis (n=1, female). As both high and low frequency species apparently were found living in fast-moving water (however, they might inhabit different microhabitats), no correlation could be drawn between water velocity and EOD frequency. Examination of the stomach contents in A. mariae showed the diet to consist mostly of aquatic invertebrates.

Distribution. Apteonotus mariae is known from Girardot, Apulo and Honda in the Rio Magdalena basin, Colombia (Fig. 3), the last locality being a new record for the species. Additional gymnotiform species occurring in that same basin are Apteronotidae: A. eschmeyeri de Santana, Maldonado-Ocampo, Severi & Mendes ZBK , A. magdalenensis (Miles) and A. rostratus (Meek & Hildebrand) ; Sternopygidae: Eigenmannia humboldtii (Steindachner) , Eigenmannia cf. virescens (Valenciennes) and Sternopygus aequilabiatus (Humboldt) ; Hypopomidae: Brachyhypopomus occidentalis (Regan) ; and Gymnotidae: Gymnotus ZBK sp. (Maldonado-Ocampo & Albert, 2003).

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