A New Species of Gymnotus (Gymnotiformes, Gymnotidae) from Uruguay:
Description of a Model Species in Neurophysiological Research
Mathilde M. Richer-de-Forges
, William G. R. Crampton
, and James S. Albert
Gymnotus omarorum is described from coastal and interior drainages of Uruguay, where it is locally abundant in streams
and lagoons, and is not known to occur sympatrically with congeners. This species has been used for more than 30 years
as a model organism in neurophysiological research, where it has been referred to as G. carapo or G. cf. carapo.Gymnotus
omarorum is a member of the G. carapo species group, with which it shares the presence of two pores in the dorsolateral
portion of the preopercle, irregular (wavy) dark pigment bands which usually become broken and/or lose contrast with
the ground color through growth, a clear patch at the caudal end of an otherwise darkly pigmented anal fin, and more
than four arrowhead-shaped (anteroposteriorly compressed) teeth in the anterior portion of the dentary. Gymnotus
omarorum is readily differentiated from other members of the G. carapo species group by the following unique
combination of character states: a short distance to first ventral lateral-line ramus (39–45%TL vs. 47–58%), few pored
scales to first ventral ramus (27–35 vs. 40–78), many ventral lateral line rami (16–37 vs. 0–14), and ovoid (vs. elongate)
scales on the posterior portion of body.
GYMNOTUS is the most species-rich genus of Neo-
tropical electric fish with 33 species currently
recognized, and many additional undescribed spe-
cies known from museum collections (Albert et al., 2004;
Albert and Crampton, 2005). Gymnotus exhibits consider-
able morphological and ecological diversity, ranging in
mature body size by an order of magnitude (approximately
10–100 cm), and occupying most freshwater habitats
throughout the lowland Neotropics. Gymnotus has the
widest range of all gymnotiforms, from the Rı´o San Nicolas
in southern Mexico to as far south as the Rı´o Salado in the
pampas of Argentina. Individual specimens of Gymnotus
generate a continuous train of weak (,0.5 V) pulsed Electric
Organ Discharges (EODs) from an electric organ located in
the hypaxial musculature along the ventral margin of the
For more than two decades, a team of neurophysiologists
at the Instituto de Investigaciones Biolo´ gicas Clemente
Estable in Montevideo, Uruguay, has exploited a locally
abundant species of Gymnotus as a model species for
understanding electric organ physiology and electrocom-
munication. Pioneering work by Omar Trujillo-Cenoz and
Omar Macadar elucidated the detailed sequence of neural
and electrocyte discharge patterns that, in concert, generate
the complex and temporally variable three-dimensional
external electric field (Trujillo-Cenoz et al., 1984; Macadar
et al., 1989; Trujillo-Cenoz and Echague, 1989). These
studies led to the development of a sophisticated neuroan-
atomical model of the mechanisms by which the nervous
system and electric organ generate the EOD (Caputi et al.,
1993; Caputi, 1999; Capurro et al., 2001).
Papers published from the Montevideo group have referred
to this species as Gymnotus carapo or G. cf. carapo. Likewise,
until recently, many species of Gymnotus from other regions
of South America have been assigned to G. carapo in the
literature, based on color pattern and external body propor-
tions. However, Albert and Crampton’s (2003) redescription
of G. carapo sensu stricto reported that this species is restricted
to the Amazon and Orinoco basins, the coastal drainages of
the Guyanas, and some coastal basins of northeastern Brazil.
A subsequent phylogeneticanalysis of the genus (Albert et al.,
2004) regarded G. carapo as the type species of the G. carapo
species group, a clade diagnosed by the presence of two pores
in the dorsolateral portion of the preopercle, irregular, wavy
dark pigment bands which usually become broken and/or
lose contrast with the ground color through growth, a clear
patch at the caudal end of an otherwise darkly pigmented
anal fin (more pronounced in juveniles), and more than four
arrowhead-shaped (anteroposteriorly compressed) teeth in
the anterior portion of the dentary.
Here we describe the species that has been the focus of
neurobiological research in Uruguay as a new member of the
G. carapo species group. This description, which is based on
external morphology, pigmentation, osteology, and the
EOD, brings the number of species in the G. carapo species
group to 20 and the total number of Gymnotus species to 34.
MATERIALS AND METHODS
Specimens of the type series were captured in Laguna del
Sauce, a reservoir formed by the impounding of the Rio
Cisne, about 15 km northwest of the town of Maldonado on
the southeastern coast of Uruguay. Laguna del Sauce is a
natural setting with marginal habitats that closely resemble
other localities where the new species is found. Specimens
were located with a portable amplifier for detecting electric
discharges, and captured with a dip net. Measurements and
counts follow Albert and Crampton (2003), except for a new
measurement reporting the distance between the tip of the
snout and the first lateral-line ramus (SLR). All measure-
ments were taken with digital calipers to the nearest
0.1 mm. Osteological data were taken from specimens
cleared and stained (CS) following the technique described
by Dingerkus and Uhler (1977). Size of specimens is reported
as total length (TL) in mm from the snout to tip of tail.
Sexual maturity was determined from examination of
gonads in freshly preserved specimens under a dissecting
microscope. Measurements are reported as percentage TL
and were not taken from specimens with a damaged caudal
appendage or those with incomplete regeneration. Institu-
tional abbreviations follow Leviton et al. (1985), with the
Department of Biology, University of Louisiana at Lafayette, Lafayette, Louisiana 70504-2451; E-mail: (MMR) firstname.lastname@example.org; and
(JSA) email@example.com. Send reprint requests to JSA.
Department of Biology, University of Central Florida, P.O. Box 162368, Orlando, Florida 32816-2368; E-mail: firstname.lastname@example.org.
Submitted: 24 April 2007. Accepted: 7 April 2009. Associate Editor: C. J. Ferraris.
F2009 by the American Society of Ichthyologists and Herpetologists DOI: 10.1643/CI-07-103
Copeia 2009, No. 3, 538–544
addition of ZVC-P, Zoological Vertebrate Collection-Peces,
Museum of Natural History, Montevideo, Uruguay.
The EODs were recorded from the center of an 80 340 3
40 aquarium at 27 60.1uC after acclimation in holding
tanks at 27 60.2uC for a minimum of 12 hours. Signals were
amplified from silver/silver-chloride electrodes placed at the
tank ends using an AC-coupled differential amplifier (Signal
Recovery 5113), and digitized using a National Instruments
6052E digitizer at a sampling rate of 250 kHz and a
resolution of 16 bits. EODs were not recorded for specimens
with a history of damage to the caudal appendage. EOD
durations were calculated with the beginning and end of the
EOD taken at a 1%threshold of the amplitude of the
dominant positive phase (P1) following normalization to
the P1 amplitude.
Recordings of resting pulse repetition rate were taken from
60 s recordings between 1000 and 1400 from fish subjected
to subdued natural lighting (10 s recordings for post-larval
fish). Recordings of nocturnal pulse rate were taken from
recordings taken one to three hours after sunset. The mean,
standard deviation and coefficient of variance for pulse rates
were calculated for individual fishes and reported here as
averages among all specimens to characterize diurnal and
Gymnotus omarorum, new species
Figure 1, Table 1
Holotype.—ZVC-P 6480, male, 254 mm, Uruguay, Maldo-
nado Department, Rı´o Cisne basin, Laguna del Sauce,
34u50.3289S, 055u06.8699W, 12 December 2006, A. Caputi,
M. Castello, P. Aguilera, A. Rodriguez-Cattaneo, W. Cramp-
ton, J. Albert, and M. Richer-de-Forges.
Paratypes.—Uruguay, all collected with holotype: AMNH
239656, 16, 3 CS, 32–222 mm, 5 female, 4 male, 5
immature, 2 post-larval; ZVC-P 6481, 33, 3 CS, 23–
201 mm, 2 female, 5 male, 8 immature, 18 post-larval;
MCP 41266, 3, 1 CS, 150–190 mm, 1 female, 2 male.
Non-type specimens.—All from Uruguay. Artigas Department:
ZVC-P 16, 1, 185 mm, Rı´o Uruguay basin, del Yucutuja´ , Rı´o
Fig. 1. Gymnotus omarorum head and body, ZVC-P 6480, holotype, 254 mm, Laguna del Sauce, Uruguay, 34u50.3289S, 055u06.8699W. Scale bars 5
Richer-de-Forges et al.—A new species of Gymnotus 539
Cuareim, rancho El Ombu´; ZVC-P 105, 2, 100–170 mm,
laguna en can˜ada Invernada; ZVC-P 404, 2, 160–190 mm, Rı´o
Uruguay basin, Rı´o Cuareim, Picada Tareira; ZVC-P 2906, 3,
95–205 mm, Rı´o Uruguay basin, Rı´o Cuareim, rancho Pereira
Reverbell; ZVC-P 3407, 3, 195–240 mm, Rı´o Uruguay basin,
Catala´n Chico, rancho Martine. Lavalleja Department: ZVC-P
950, 2, 123–158 mm, Rı´o Cebollatı´-Laguna Merı´n, rancho
Sosa Dı´az, can˜ada Mariscala. Rivera Department: ZVC-P 1351,
1, 245 mm, Rı´o Negro, can˜ada Cun˜ apiru´ . Florida Depart-
ment: ZVC-P 303, 2, 173–215 mm, Rı´o Uruguay basin, Rı´o
Santa Lucia; ZVCP-5502, 1, 227 mm, Rı´o Uruguay basin, Rı´o
Santa Lucia, can˜ada Casupa; ZVCP-3423, 1, 146 mm, Rı´o
Uruguay basin, can˜ada Milano. Lavalleja Department: ZVC-P
1917, 1, 82 mm, Laguna Merin basin, Villa Serrana, can˜ada
los Chanchos. Tacuarembo´ Department: ZVC-P 7430, 1,
222 mm, Laguna Lavalle. Treinta y Tres Department: ZVC-P
7429, 3, 227–269 mm, Laguna El Tigre.
Diagnosis.—Gymnotus omarorum differs from all other mem-
bers of the G. carapo species group (sensu Albert et al., 2004)
in the following unique combination of character states: a
short distance from the tip of the snout to the position of
the first ventral lateral-line ramus (39–45%TL vs. 47–58%),
many pored lateral-line scales to the first ventral lateral-line
ramus (27–35 vs. 40–78), and ovoid (vs. elongate) scales on
the posterior portion of body.
Description.—Body shape and pigmentation illustrated in
Figure 1. Morphometric and meristic data presented in
Table 1. Size to 254 mm in males and 222 mm in females.
Minimum known size at sexual maturity 155 mm for males
and 187 mm for females. No secondary sexual dimorphism
known in body shape or anatomy. Anterior narial pore pipe-
shaped, partially or entirely included within gape in large
narial fold. Scale shape in mature specimens slightly ovoid
above lateral line at midbody. Lateral line dorsal rami few (1–
5) and sparse, irregularly formed, and asymmetrically
arranged. Circumorbital laterosensory series tubular, forming
ovoid series around eye in lateral view, infraorbital canal
contacts supraorbital canal at weakly acute angle. Premaxilla
with four or five arrowhead-shaped (anteroposteriorly com-
pressed) teeth anteriorly and additional five or six conical
teeth with recurved tips posteriorly, arranged in single row
along oral margin. Oral margin of premaxilla curved. Maxilla-
palatine position near anterior tip of endopterygoid. Maxilla
orientation vertical. Maxilla rod-shaped, narrow distally with
straight ventral margin. Maxilla length equal to width of
about four to six dentary teeth. Dentary with four or five
arrowhead-shaped teeth anteriorly, and additional 10 or 11
conical teeth with recurved tips posteriorly, arranged in
single row along oral margin. Dorsoposterior and ventropos-
terior dentary processes abut at midlength of dentary.
Dentary ventral posterior process almost as long as dorso-
posterior process; dorsoposterior process narrow distally.
Anteroventral margin of dentary without hook-shaped
process near mental symphysis. Anguloarticular ventrolateral
lamella expanded, extending over retroarticular. Anguloarti-
cular ascending process long, extending beyond ventral
margin of dentary dorsoposterior process. Retroarticular
posterior margin square.
Table 1. Morphometric and Meristic Data for Gymnotus omarorum. Head length, body depth, body width, anal-fin length, and distance from the
snout to first lateral-line ramus are reported as percentage of total length; other measurements as percentage of head length.
Measurement nHolotype Mean Range
Total length (mm) 32 254 — 116–254
Head length (mm) 32 28.3 — 14.3–28.3
Head length 31 11.1 11.6 10.3–13.3
Head depth 32 67 66.1 60.2–71.2
Head width 32 67 60.1 52.7–68.6
Preorbital distance 32 40 36.4 32.6–41.4
Postorbital distance 32 60 58.4 52.7–63.0
Interorbital distance 32 45 39.8 35.5–45.2
Mouth width 32 48 44.6 38.8–51.8
Body depth (BD) 18 10.2 10.7 9.4–12.2
Body width (BW) 18 7.5 6.9 5.9–7.5
BW/BD at anal-fin origin 18 0.73 0.65 0.64–0.68
ramus distance 18 39 42.0 39.0–45.0
Anal-fin length 18 81 73.2 78.0–82.8
Pectoral-fin length 30 46 45.6 41.2–51.5
Preanal length 18 86 79.9 71.0–94.3
Bands 22 22 23 22–24
Scales above lateral line 9 7 7 7
Pored lateral-line scales to 1
ramus 19 31 31 27–35
Total pored lateral-line scales 18 98 93.5 85–100
Anal-fin pterygiophore scales 9 8 7 7–8
Lateral-line ventral rami 16 28 29 16–37
Pectoral-fin rays 12 16 16 16–17
Anal-fin rays 11 230 250 230–265
Precaudal vertebrae 3 — 34 34–34
540 Copeia 2009, No. 3
Mandible shape short and compressed. Adductor man-
dibula intermuscular bones absent. Endopterygoid ascend-
ing process long and straight, base shorter than length, and
with simple distal tip. Endopterygoid superior portion
ossified less than to anterior margin of inferior portion.
Hyomandibula with two separate foraminae on dorso-
lateral surface for passage of supraorbital and infraorbital
trigeminal nerve rami. Hyomandibula with foramen at
posterodorsal margin for passage of posterior lateral-line
nerve. Preopercle with two dorsoposterior laterosensory
pores, anterior notch, and broad median shelf extending
more than half width of symplectic. Subopercle dorsal
margin concave. Opercle dorsal margin straight or slightly
Mesethmoid anterior margin concave with small, paired
anterolateral processes. Anterolateral margin of frontal
straight, continuous with margins of adjacent roofing
bones. Frontal postorbital process broad, more than two
times width of supraorbital canal. Frontal narrow, width at
posterior articulation of infraorbital series less than that of
parietal. Lateral ethmoid unossified or absent. Parasphenoid
shape narrow, length more than 2.2 times width. Vomer
long, extending more than half distance to parasphenoid
lateral process. Parasphenoid posterior processes robust,
stout, posterior margin with shallow convexity at midline.
Parietal shape rectangular, length less than width. Ptero-
sphenoid anteroventral portion robust, extending ventral to
lateral margin of parasphenoid.
Pectoral fin wide and long, with four large cartilaginous
radials, and single large unbranched anterior ray. Mesocor-
acoid proximal portion broad and distal portion ossified to
partially ossified. Cleithrum broad, ventral margin curved.
Cleithrum anterior limb long, about twice length of
ascending limb, and with shallow anterior notch. Cleithrum
dorsoposterior facet small. Supratemporal paddle-shaped.
Fifth rib robust along entire extent, less than three times
width of sixth rib.
Single hypaxial electric organ, extending along entire
ventral margin of body. Four rows of electrocyte tubes
present in caudal region. Caputi et al. (1993), Lorenzo et al.
(1988), and Trujillo-Cenoz and Echague (1989) provide
complete anatomical and physiological descriptions of
electric organ of adult G. omarorum, and Pereira et al.
(2007) describe larval electric organ.
Color in alcohol.—Ground color of body hyaline or light gray
ventrally grading to dark brown/dark gray dorsally. Juveniles
and adults with 21 to 24 obliquely oriented bands or band
pairs, with wavy irregular margins, extending from tip of tail
to pectoral-fin base. Dark bands evenly pigmented with
distinct band margins in juveniles. Bands divide with
growth into band-pairs that break into spots above lateral
line and band margins become irregular and wavy. Contrast
between dark bands and pale interbands increases ventrally
and caudally and fades with growth on anterior half of body.
Head color pattern not banded nor blotched. Head slightly
countershaded with freckling over branchiostegal and
opercular regions and with even dark brown coloration
dorsally. Pectoral-fin rays brown or gray, interradial mem-
brane hyaline. Anal-fin membrane hyaline and anal-fin rays
dark drown, anal fin with clear patch near the posterior end
in juveniles, becoming dusky in mature specimens. Pre-
served specimens with less bright coloration than fresh
Electric organ discharge.—Gymnotus omarorum generates a
continuous train of pulse-type EODs. Adult specimens (100–
262 mm TL, Pereira et al., 2007) captured at the Laguna del
Sauce generated EODs at a mean repetition rate (per
individual) of 29.1–49.7 Hz during the day (mean among
all specimens 40.6 Hz, SD 3.4, n531 individual fishes), with
a standard deviation (SD) of 0.5–9.7 Hz (mean 2.40, SD 2.34,
n531) and coefficient of variance of 1.3–21.4%(mean
5.8%, SD 5.4, n531). Adult specimens generated EODs at a
mean repetition rate (per individual) of 46.3–62.0 Hz (mean
among all specimens 56.7 Hz, SD 3.4, n530) during the
hours of peak activity, one to three hours after sunset, with a
standard deviation of 1.9–7.1 Hz (mean 2.9, SD 1.1, n530)
and a coefficient of variation of 3.4–13.9%(mean 5.2%, SD
2.2, n530). Post-larval specimens in the size range 23–
32 mm TL in the type series generated EODs at an averaged
repetition rate of 25.0–44.2 Hz during the day (mean 33.6,
SD 5.0, n520), with a standard deviation of 0.4–5.5 (mean
1.6, SD 1.3, n520) and coefficient of variance of 1.4–12.4%
(mean 4.4%, SD 3.3, n520). The nocturnal pulse rates of
post-larval specimens were not measured.
The EODs of adult specimens (.100 mm TL) comprise
three components of alternating polarity (P0, P1, and P2)
sensu Crampton and Albert (2006) and vary in duration from
2.595–3.729 ms (mean 3.187, SD 0.308, n530; Fig. 2). This
initial negative component, P0, is considered in G. omar-
orum to comprise two distinct sub-components: a long
shallow negative decline in voltage (V1, sensu Caputi, 1999)
followed by a more distinct negative component (V2, sensu
Caputi, 1999). Several papers discuss the cellular anatomical
and physiological basis of the four waveform phases V1–V4
in G. omarorum (reviews in Caputi, 1999 and Caputi et al.,
2005). The peak power frequency of the EOD of adult
specimens ranges from 0.7896–1.0338 kHz (mean 0.8676,
SD 0.0595, n530).
The EOD waveforms of post-larval specimens in the size
range 24–32 mm are dominated by a large P1 (V3) followed
by a weaker P2 (V4). P0 is partially represented in some
specimens: V1 is absent; V2 is absent in smaller specimens
and present, but very weak, only in those in the upper size
range (Fig. 2). The EOD duration of post-larval specimens
ranges from 1.652–2.967 ms (mean 2.042, SD 0.424, n514),
and the EOD peak power frequency ranges from 0.3090–
0.6218 kHz (mean 0.5024, SD 0.0869, n514).
Ecology.—Gymnotus omarorum is known from lakes and
small streams, where it lives in submerged stems and roots
of aquatic vegetation. The type series of G. omarorum was
captured in dense beds of water hyacinths (Eichhornia
crassipes) in a wind and wave protected inlet. The water
hyacinths extended from the shore margin approximately
30 m, to a depth of approximately 1 m. The substrate of the
lake comprised coarse gravel and rock fragments on a
granite platform. The following water quality parameters
were measured: electrical conductivity 170–190 mScm
pH 7.6–7.8, temperature (at midday) 27–28uC in open water
(although temperatures of 23–26uC were recorded under-
neath the water hyacinths).
One large male specimen (235 mm, ZVC-P 6481) was
found in the immediate proximity of approximately 40
post-larval specimens in the size range 24–30 mm. This
situation resembles that of paternal care previously noted by
Crampton and Hopkins (2005) in G. carapo (from Trinidad
and Venezuela) and G. mamiraua (from the Central Amazon
Richer-de-Forges et al.—A new species of Gymnotus 541
of Brazil). Potential predators of G. omarorum include
Synbranchus sp., which is common in the water hyacinth
root mats, Crenicichla sp., and water snakes of the family
Colubridae. No other gymnotiform fishes occur in Laguna
del Sauce. However, G. omarorum occurs sympatrically with
Brachyhypopomus pinnicaudatus and B. bombilla (Loureiro
and Silva, 2006) in some areas of northern Uruguay (A. Silva,
Distribution.—Known from the Santa Lucia and Cisne coastal
drainages of southeastern Uruguay and the Cuariem drain-
age of the Rio Uruguay in northern Uruguay (Fig. 3).
Local name.—Morenita, a name also used for Gymnotus
species in Argentina.
Remarks.—Gymnotus omarorum further differs from G. in-
aequilabiatus, a congener present in other portions of the Rio
Uruguay drainage, in the absence of spots or stripes at the
caudal end of the anal fin, and in the long and irregular (vs.
short and regular) configuration of the ventral lateral line
Etymology.—Species named to honor Omar Macadar and
Omar Trujillo-Cenoz, both pioneers in the anatomical and
physiological study of electrogenesis in Gymnotus.
Fig. 2. Electric Organ Discharge (EOD) waveform (above) and Fourier Power Spectrum (below) for (A) 30 juvenile and adult specimens (120–
262 mm) of Gymnotus omarorum from the type locality (all undamaged specimens; AMNH 239656, ZVC-P 6480, ZVC-P 6481). (B) 14 post-larval
specimens (24–32 mm) from the type locality (all undamaged specimens; AMNH 239656, ZVC-P 6481). EODs plotted with head positivity upwards,
normalized to the amplitude of the dominant positive phase (P1), and aligned at the P1 peak. Scale bar 51 msec. Black labels P1–P3 refer to EOD
phase 1–3 (Crampton et al., 2005). Gray labels V1–V4 refer to phase labeling scheme used by the Montevideo research group (Caputi, 1999).
Fig. 3. Map of Rio Uruguay and portions of adjacent river basins
showing the known geographic distribution of Gymnotus omarorum.
Square indicates the type locality. Symbols may represent more than
one locality. Scale bar 5100 km.
542 Copeia 2009, No. 3
Here we list species of the G. carapo species group that occur
south of the Amazon basin. Further materials examined
used for the diagnoses are listed in Albert and Crampton
(2003), Crampton et al. (2005), Maldonado-Ocampo and
Albert (2004), and Fernandes et al. (2005). Data are
presented alphabetically by species, country, and museum
lot, followed by number of specimens, number cleared and
stained (CS), size range, and locality.
Gymnotus bahianus: Brazil: MCP 18110, 2 (2 CS), 90–
92 mm, Minas Gerais, Rio Jequitinhonha, Padre Paraı´so;
MCZ 9386, 7, 89–219 mm, Rio de Janeiro, ranch Santa Cruz;
MCZ 9373, 15, 117–210 mm, Rio de Janeiro basin, Rio
Itabapoana at Itabapoana; MNRJ 12316, 1, holotype,
177 mm, near Ilhe´ us, Bahia.
Gymnotus chimarrao: Brazil: UFRGS 6774, holotype,
118 mm, Rio Grande do Sul, Taquarı´ drainage; UFRGS
6770, 1 paratype, 191 mm, collected with holotype; UFRGS
6771, 1 paratype, 237 mm, collected with holotype; UFRGS
6772, 1 paratype, 177 mm, collected with holotype; UFRGS
6773, 1, 206 mm, collected with holotype; UFRGS 6775, 1
paratype (CS) 193 mm, collected with holotype; UFRGS
6776, 1 paratype, 124 mm, collected with holotype.
Gymnotus inaequilabiatus: Argentina: MNHN 4615, 1,
syntype, 1,000 mm, ‘‘La Plata.’’ Brazil: MCP 6956, 1,
602 mm, Rio Grande do Sul, Rio Uruguai Santana Velha;
MZUSP 46001, 1, 998 mm, Sa˜o Paulo, Parana´, Porto Prima-
vera; MZUSP 51268, 1, 270 mm, Sa˜o Paulo, Rio Capivara
basin, affluent of Rio Paranapanema; MCP 13257, 1, Rio
Grande do Sul, Rio Uruguai, Porto de Santo Izidro; MCP
39375, 2, Rio Grande do Sul, Rio Uruguai, Rio Ijui, Pirapo´.
Gymnotus pantanal: Brazil: MZUSP 67874, holotype,
196 mm, Mato Grosso do Sul State, Rio Miranda; MZUSP
67876, 2 paratypes, 189–264 mm; MZUSP 67876, 1,
251 mm; MZUSP 67875, 1, 192 mm, Mato Grosso do Sul
State, Rio Paraguai; UMMZ 206080, 21, 82–260 mm, Rı´o
Paraguay, Parque Nacional Ybycui; NRM 42397, 1, 171 mm,
Rı´o Paraguay; NRM 42830, 1, 240 mm, Rı´o Parana´.
Gymnotus paraguensis: Brazil: FMNH 108546, 1, 164 mm,
Mato Grosso do Sul, Aquiduana, Rio Novo in Brejo da Santa
Sofia; MUSM 16975, 2, 78–114 mm, Mato Grosso do Sul,
Corre 60 Sa˜o Joa˜o; MUSM 17114, 1, 82 mm, Mato Grosso do
Sul, Rio Vermelho. Paraguay: NRM 42380, 1, 240 mm,
Canindeyu, Rı´o Parana´, Rı´o Parana´ at Saltos del Guaira´;
UMMZ 206155, holotype, 222 mm, Parana´, stream Tembley,
near San Rafael; UMMZ 240700, 1 paratype, 193 mm,
Parana´, stream Tembley, near San Rafael; MCP 19044, 1,
Santa Catarina, Rio Uruguai basin, Rio do Engano; MCP
37298, Rio Grande do Sul, Rio Uruguai basin, Rio Ijuı´, Sa˜o
Gymnotus sylvius: Brazil: LGP P2468, 1, 209 mm, Sa˜o
Paulo, Chapeco´ ; LGP P2454, 1, 271 mm, Sa˜o Paulo,
Chapeco´; LGP P2346, 1 (CS), 160 mm, Sa˜o Paulo, Jacareı´;
LGP P2338, 1 (CS), 180 mm, Sa˜o Paulo, Jacareı´; LGP P2333,
1, 219 mm, Sa˜o Paulo, Jacareı´; LGP P2336, 1, 175 mm, Sa˜o
Paulo, Jacareı´; LGP 925.1, holotype, 259 mm, Sa˜o Paulo,
Miracatu, Ribeira de Iguape; LGP 925.2, 2 paratypes, 251–
307 mm, Sa˜o Paulo, Miracatu, Ribeira de Iguape; UMMZ
234347, 2 paratypes, 255–271 mm, Sa˜o Paulo, Miracatu,
Ribeira de Iguape; LGP 931, 1, 157 mm, Sa˜o Paulo, Sa˜o
Sima˜ o, Rio Tamandua´ , Pardo.
Gymnotus sp.: Brazil: UFRGS 6536, 1, 208 mm, UFRGS
8655, 3 (CS), 62–130 mm; UFRGS 9104, 1 (CS), 153 mm;
UFRGS 9106, 2 (CS), 152–187 mm, Rio Grande do Sul,
Viama˜ o, Lagoa Verde, in Itapua˜ State Park; UFRGS 6558, 1,
191 mm, Rio Grande do Sul, Sa˜o Gabriel, tributary of stream
Piraı´; MZUSP 87912, 1, 116 mm, Sa˜o Paulo, Ipeuna, rio Tieteˆ
basin, Rio Passa Cinco. Uruguay: UFRGS 7990, 2, 144–
206 mm, Artigas Department, stream Guaviyu´ , rio Uruguai.
We thank A. Caputi, M. Castello´ , P. Aguilera, and A.
Rodriguez-Cattaneo for help during field sampling and for
the exchange of ideas. We also thank M. Loureiro, L.
Malabarba, R. Reis, and A. Silva for access to specimens and
information, and B. Moon for assistance in preparing
Figure 3. We acknowledge A. de Navı´o of the Aeronaval
Base C/C Carlos A. Curbelo, Maldonado, Uruguay for
assistance in accessing the field site. This research was
supported by National Science Foundation grant 0614334
(PI W. Crampton, Co-PIs J. Albert, A. Caputi, N. Lovejoy). All
recordings were conducted at the Instituto de Investiga-
ciones Biolo´gicas Clemente Estable in Montevideo, Uru-
guay, in accordance with institutional animal care proto-
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