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Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
Physicochemical characterization of the white, black, and clearwater
rivers of the Amazon Basin and its implications on the distribution of
freshwater stingrays (Chondrichthyes, Potamotrygonidae)
WALLICE PAXIÚBA DUNCAN1 & MARISA NARCISO FERNANDES2
1Departamento de Morfologia, Instituto de Ciências Biológicas, Universidade Federal do Amazonas, Avenida General
Rodrigo Octávio Jordão Ramos, 3000. Coroado I, 69.077-000, Manaus, Amazonas, Brazil. E-mail:
wduncan@ufam.edu.br
2Departamento de Ciências Fisiológicas, Centro de Ciências Biológicas e da Saúde, Universidade Federal de São
Carlos, Via Washington Luiz, km 235, 13.565-905 - São Carlos, São Paulo, Brazil.
Abstract: This study characterizes the spatial physicochemical variation of the surface water in
the main stems of three important rivers in the Amazon Basin and its influence on the distribution
of freshwater stingrays. The Amazon River is circumneutral pH (pH 6.6 ± 0.2), high conductivity
(44.8 ± 24.8 S/cm) and solute-rich (total dissolved solid, TDS= 23.9 ± 17.8 mg/l) in relation to
the Negro River. The Negro River is blackwater (black or tea-colored), acidic (pH 4.5 ± 0.9), low
conductivity (17.0 ± 15.2 S/cm) and solute-poor (TDS= 7.1 ± 6.7), while the Tapajós River is a
clearwater river with physicochemical characteristics (pH 6.5 ± 0.4; conductivity= 14.4 ± 13.1
S/cm; TDS= 7.7 ± 5.6 mg/l) lying between those of the Amazon and Negro rivers. The
longitudinal variability of the water of these rivers was attributed to the discharges of their
tributaries, which come from different geological provinces. The distribution of the Family
Potamotrygonidae was interpreted as a result of the physicochemical differences between bodies
water. Thus, the physicochemical characterization of the main water types in the Amazon Basin
may act as a hydrological barrier (or hydrological filter) for the dispersal of potamotrygonid rays.
Key words: Amazon River, Negro River, Tapajós River, potamotrygonid, hydrological barrier
Resumo: Caracterização físicoquímica dos rios de água branca, preta e clara da Bacia
Amazônica e suas implicações para a distribuição das arraias de água doce (Chondrichthyes:
Potamotrygonidae). Este estudo descreve o perfil espacial das variáveis físicas e químicas da
água em três importantes rios da bacia Amazônica. A água branca do Rio Amazonas possui pH
circumneutro (pH 6,6 ± 0,2), elevada condutividade elétrica (44,8 ± 24,8 S/cm) e rica em sólidos
totais dissolvidos (TDS= 23,9 ± 17,8 mg/l) em relação ao Rio Negro. O Rio Negro possui água
preta, ácida (pH 4,5 ± 0,9), baixa condutividade (17,0 ± 15,2 S/cm) e pobre em sólidos
dissolvidos totais (TDS= 7,1 ± 6,7 mg/l). As águas claras do Rio Tapajós possuem características
físicas e químicas (pH 6,5 ± 0,4; condutividade= 14,4 ± 13,1 S/cm; TDS= 7,7 ± 5,6 mg/l)
intermediárias entre o Rio Amazonas e Rio Negro. Nestes rios, a variabilidade longitudinal dos
valores de pH, condutividade e TDS da água foi atribuida às descargas dos seus tributários, os
quais se originam de diferentes províncias geológicas. A distribuição das arraias de água doce da
família Potamotrygonidae foi interpretada como o resultado das diferenças entre os tipos de águas.
Portanto, as características físicas e químicas das águas da bacia Amazônia podem agir como
barreiras ou filtros hidrológicos para a dispersão das espécies de potamotrigonídeos.
Palavras-chave: Rio Amazonas, Rio Negro, Rio Tapajós, potamotrigonídeos, barreira hidrológica
Introduction
The Amazon Basin is the largest world’s
hydrographic system, covering an area of 6,879,761
km2. The basin is drained by the 7,025 km Amazon
River. The Amazon’s average annual discharge of
220,000 m3 s-1 comprises about 20% of the Earth’s
The distribution of the freshwater stingrays as function of physicochemical parameters in Amazonian waters
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
455
superficial freshwater (Molinier et al. 1997). From
the headwaters to its mouth in the Atlantic Ocean,
the Amazon River is fed by more than 1,000
tributaries from different geological regions; thus,
the physicochemical characteristics of the rivers in
the Amazon Basin reflect the soil properties of the
provinces they drain (Salati & Vose 1984,
Konhauser et al. 1994).
Alfred Russel Wallace (1853) was the first
to classify the Amazon and its tributaries into white,
black, and clearwater rivers types. However, this
classification was based only on color. Sioli (1984)
showed that these waters are chemically and
physically heterogeneous. Whitewater rivers (such
as the Amazon and Madeira rivers) have a
characteristic muddy color, relatively high
concentrations of dissolved solutes, an alkaline to
neutral pH, and a high sediment load originating
from Andean regions (Konhauser et al. 1994, Alcour
et al. 2003). Blackwater rivers (for example, the
Negro and Uatumã rivers) are black or tea-colored
due to a high concentration of dissolved organic
carbon, have negligible suspended sediment loads
and medium transparencies, are very dilute in
dissolved ions, and are usually acidic (Rickey et al.
1990). Other characteristics of blackwater include
extremely weathered sandy podzolic soil, bed
stability, and low erosion (Klüchler et al. 2000).
Finally, clearwater rivers (for example, the Tapajós
and Xingu rivers) are relatively transparent and
olive-green in color, typically have lower dissolved
sediment loads (as well as ion loads), exhibit low
values for electric conductivity, and range from
acidic to alkaline (pH 5-8). These rivers usual drain
the weathered soil of the Precambrian Shield which
explains their low dissolved sediment loads (Sioli
1984, Konhauser et al. 1994).
As a result of these differences between
aquatic environments, the Amazon Basin is a mosaic
of different water types connected by the main stem
of the Amazon River. Therefore, it has been
hypothesized that hydrographic conditions form a
strong hydrographic barrier constraining the
dispersion of fish acting as selective forces that drive
allopatric speciation (Lovejoy & Araújo 2000,
Hubert & Reno 2006, Willis et al. 2007). In the case
of the Amazon main stem, it serves as a major
corridor for the dispersal of aquatic biota among
different water types. However, the main channel of
the Amazon River may not be a free dispersal
corridor but rather a zoogeographical filter for the
interchange of fish species, at least for freshwater
stingray species (Potamotrygonidae) such as those
that occur in the Casiquiare River, which acts as a
corridor and geographical filter for fish dispersion
between the Orinoco and Amazon basins
(Winemiller et al. 2008).
Potamotrygonids are endemic to South
America and are the only extant elasmobranch
family restricted to a freshwater environment. The
family Potamotrygonidae comprises three genera:
Paratrygon, Plesiotrygon, and Potamotrygon.
Paratrygon is monotypical, represented by
Paratrygon aiereba Müller & Henle 1841 and is
widely distributed in all water types, both in the
Amazon River and Orinoco River. Plesiotrygon is
also monotypical, represented by Plesiotrygon
iwamae Rosa, Castello & Thorson 1987 which
occurs in the Amazon River drainage, but is usually
distributed only in the main stem and marginal lakes
of the Amazon River (Rosa et al. 1987).
Potamotrygon contains 18 described species, most
of them restricted to a single river and its tributaries
(Rosa et al. 2008). For example, Potamotrygon
leopoldi Castex & Castello 1970 and Potamotrygon
henlei Castelnau 1855 are endemic to the Xingu and
Tocantins Rivers and its tributaries, respectively.
Both rivers are typically classified as clearwater.
However, Potamotrygon sp. (an undescribed specie
known as cururu ray) is endemic to the acidic, ion-
poor, and blackwater of the Negro River. In contrast,
Potamotrygon motoro Müller & Henle 1841 and
Potamotrygon orbignyi Castelnau 1855 are
widespread in different water types of the Amazon
Basin (Martin 2005). This clearly shows that
potamotrygonid species show marked allopatric
distribution patterns; four of them are widely
distributed throughout the Amazon Basin, and
several species are restricted to a single river and its
tributaries. Recently, studies have revealed large
genetic differentiation between several populations
of potamotrygonids, suggesting that the river may be
a barrier to gene flow (Toffoli et al. 2008).
The goal of this study was to characterize
the physicochemical variables along the longitudinal
gradient in the main stem of the Amazon
(whitewater), Negro (blackwater), and Tapajós
(clearwater) rivers and to discuss the geographical
pattern of potamotrygonid species distribution in the
context of environmental differences to evaluate the
potential of Amazonian basin water to function as a
geological barrier that drives allopatric speciation.
Material and Methods
Water and stingray sampling was carried out
in the Amazon River, Negro River and Tapajós
River (Fig. 1) during the high water period (May-
June) of 2007 and 2008.
W. P. DUNCAN & M. N. FERNANDES
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
456
Figure 1. Study area. The names of the main stem and major tributaries are shown. Cross-
sectional composites were performed on the main stem of the Amazon River (whitewater type,
from São Caetano de Odivelas/Colares to Manaus), the Negro River (blackwater type, from
Manaus to Barcelos), and the Tapajós River (clearwater type, from Santarém to the São Luís do
Tapajós rapids). The fishing sampling are shown as black squares.
Study area - For the Amazon River, water
samples were collected upstream river from town of
São Caetano de Odivelas and Colares Island (0o44’
S; 48o00’ W) near to Manaus City at the confluence
of the Solimões (Lower Solimões) and Negro rivers
(03o08’ S; 59o55’ W), a distance of 1,540 kilometer.
In the Amazon Basin, the water upstream from this
confluence of the Amazon River is known as the
Solimões River (Upper, Middle and Lower
Solimões), while downstream from this confluence
(the main stem) the river is named the Amazon
River (Upper, Middle and Lower Amazon). Both the
Solimões and Amazon rivers are typically
whitewater types. The Amazon River receives a
volumetrically high amount of blackwater (from the
Negro, Urubu and, Uatumã rivers) and clearwater
(from the Trombetas, Tapajós, and Xingu rivers) and
has only one important whitewater tributary, the
Madeira River. The water at the region of São
Caetano de Odivelas and Colares Island is typically
brackish and affected by daily tides. It is also
influenced on a seasonal basis by the hydrological
pulse of the Amazon River.
For the Negro River, the samples of water
were collected from the river mouth at Manaus
(3o29’ S; 59o55’ W) upstream to the Middle Negro
River (Mariuá Archipelago) at Barcelos (0o41’ S;
63o09’ W). The sample comprised a span of 480
kilometer. The Negro River extends approximately
1,700 kilometer, and its basin spreads over an area
of 715,000 km2, which represents 14% of the total
Brazilian Amazon Basin. It is the largest and most
important source of blackwater.
For the Tapajós River, water samples
collected on the main stem of the Tapajós River
were conducted along a stretch of 312 kilometers
from the mouth of the Tapajós River at Santarém
(2o25’ S; 54o53’ W) upstream to the rapids in the
village of São Luís do Tapajós (4o19’ S; 56o03’ W).
The Tapajós River is 851 kilometer long, with many
rapids and waterfalls that extend over 500 kilometer.
The São Luís do Tapajós Rapids are the most
important of these geological barriers.
Water sampling - Water sampling was
carried out along the main stem of the Amazon
River (n = 67 sites), the Negro River (n = 54 sites),
and the Tapajós River (n = 37 sites). Four sub-
samples were collected manually using a
polyethylene flask beneath the water’s surface. The
measured distance between each site collection was
The distribution of the freshwater stingrays as function of physicochemical parameters in Amazonian waters
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
457
calculated using a Garmin Plus III global position
system. Field measurements of physicochemical
parameters were analyzed immediately after the
collection of samples from each site. The pH values,
electrical conductivity (µS/cm), total dissolved solid
(TDS, mg/l), and salinity (psu) were measured using
a Consort C535 multiparameter analyzer.
Stingray - Stingrays were collected in the
Amazon River (a whitewater system), near Colares
Island in Marajó Bay (0o55’ S; 48o17’ W) and Lake
Janauacá (3o21’ S; 60o15’ W). In the Negro River (a
blackwater system), rays were collected between the
Arirahá River (0o30’ S; 63o32’ W) and the Cuiuni
River (0o45’ S; 63o06’ W). In the Tapajós River (a
clearwater system), rays were collected near the
town of Aveiros (3o38’ S; 55o20’ W) and in the São
Luís do Tapajós rapids (4o25’ S; 56o15’ W). The
rays were caught during rising and high water levels.
Hook and line, throw-net, beach seine, harpoons,
and a long line were used to capture the stingrays
(an effort of five hours per day). Species
determination was carried out according to Rosa
(1985). In this study was considered only the
presence and absence of the potamotrygonid species
collected in each river sampled. Herein, in this issue
forward the species of the genus Paratrygon and
Plesiotrygon are cited as full names, while for the
genus Potamotrygon is abbreviated.
Statistical analysis - All data are reported as
the means ± standard deviation (sd). To perform the
correlation between pH and electric conductivity
parameters, the conductivity data were log-
transformed prior to performing analysis.
Results
The variations of physicochemical
characteristics in the Amazon River along the stretch
upstream from the Amazon estuary to confluence of
the Negro and Solimões rivers are shown in the
Figure 2. The physicochemical patterns were similar
values in both years sampled (2007 and 2008). In
general, an increase of the conductivity and total
dissolved solids (TDS) was observed along the
stretch studied. But, the pH values showed little
spatial variation along the stretch studied, except in
the sites near the margin in confluences with acidic
rivers such as the Guamá River (1o30’ S; 48o31’ W)
and the Negro River (3o04’ S; 59o40’ W), which is
characterized by a strong variation from acidic (pH 4
- 5) to slightly neutral (pH 6 - 7) water. At the
Amazon River mouth, the physicochemical variables
were relatively high near Colares Island (pH 6.3 ±
0.3; conductivity 104.3 ± 0.3 µS/cm, and TDS 67.0
± 0.2 mg/l). In these locations, the Amazon River
water is strongly influenced by seawater from the
Guyana Current (an ascendant of the South
Equatorial Atlantic Current). P. scobina Garman
1913 and P. orbignyi are common potamotrygonid
species found in the Colares Island (Marajó Bay,
Amazon River mouth), while Paratrygon aiereba is
occasionally captured. In the Lake Janauacá (Lower
Solimões/Upper Amazon), the occurrence of
Paratrygon aiereba, P. motoro, P. scobina, and P.
orbignyi was also recorded in this study.
The Negro River was characterized by high
heterogeneity in its physicochemical parameters,
except in its mid-channel and around of the island,
which was typically homogeneous and characterized
by acidic with low conductivity and diluted water
(very low TDS values). The river-margin was highly
heterogeneous in its physicochemical variables; at
the Mariuá Archipelago this was largely due to the
discharge of the Demini and Branco rivers (Fig. 3).
In the flooded forest, the electrical conductivity was
negatively related to pH value (n = 15, r2 = 0.80, p <
0.05 in May-June 2007; n = 21, r2 = 0.72, p < 0.05 in
May-June 2008) (Fig. 4). The flooded forest (locally
know as the igapó forest) was characterized by
acidic water (pH 3.7) and is the preferential habitat
of two potamotrygonids, Potamotrygon sp. (cururu
ray) and P. motoro. The rays Paratrygon aiereba, P.
orbignyi, and P. schroederi Fernández-Yépez 1958
showed a preference for habitats near the beaches of
the islands in the mid-channel of the Negro that
exhibited highly water quality (i.e.
highly transparent and oxygen-rich water, 5.1±0.6
mg/l). At its mouth the Tapajós River is
constrained by the water of the Amazon River.
Consequently, its physicochemical parameters are
strongly affected by the whitewater of the Amazon
River, and it exhibits high pH, conductivity, and
TDS values until 50 kilometer upstream. In general,
the clear water of the Tapajós River was relatively
homogeneous along the stretch studied (Fig. 5),
except for the high conductivity and TDS values
recorded in Aveiros (3o38’ S; 55o20’ W).
Physicochemically, the water of the Tapajós River
exhibited characteristics lying between those of the
Amazon and Negro rivers waters (see Fig. 4). The
fauna of elasmobranch between São Luís do Tapajós
Rapids and Aveiros was characterized by
Paratrygon aiereba, P. motoro, and P. orbignyi
(these species are also commonly found in the
Amazon and Negro rivers). A summary of the
physicochemical variables on the surface water of
the three Amazonian rivers studied (the Amazon,
Negro and Tapajós rivers) is shown in Table I, as
well as the potamotrygonid species that occur in
each river.
W. P. DUNCAN & M. N. FERNANDES
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
458
Figure 2. The Amazon River (a whitewater river): longitudinal trends along a 1,540 kilometer
stretch of the Amazon River main stem from Manaus (at the confluence of the Solimões and
Negro rivers) to Colares Island (Amazon estuary), showing a general view of Marajó Várzea (A);
values of pH (B), total dissolved solids (C), and conductivity (D) along the same river stretch in
2007 (closed circles) and 2008 (open circles) during increasing water levels. In (B), note the low
pH value at the confluence of the Negro River (arrowhead) and the Guamá River (arrow), which
are both acidic tributaries.
Figure 3. Longitudinal trends along a 480 kilometer stretch of the Negro River (a blackwater
system) main stem from Manaus (at the confluence of the Solimões and Negro rivers) to
Barcelos (Mariuá Archipelago), showing the igapó flooded forest (A), longitudinal variation
in the pH (B), total dissolved solids (C), and conductivity (D) during increasing water levels
in 2007 (closed circles) and 2008 (open circles).
The distribution of the freshwater stingrays as function of physicochemical parameters in Amazonian waters
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
459
Figure 4. Relationship between electric conductivity (log-transformed values) and
pH from data collected in 2007 (closed symbols) and 2008 (open symbols) during
increasing water levels of the Amazon River (squares), Negro River (circles) and
Tapajós River (inverted triangles). Note that the physicochemical characteristic of
the Tapajós River is intermediate between the Amazon and Negro rivers. In the
Negro River, a negative correlations were observed both in 2007 (solid line; n =
15; r2 = 0.80; p < 0.05) and 2008 (dash line; n = 21; r2 = 0.72; p < 0.05).
Figure 5. Longitudinal transect along a 312 kilometer stretch of the Tapajós River main stem
from Santarém (at the confluence of the Amazon and Tapajós rivers) to Itaituba: São Luís do
Tapajós rapids in (A), and pH (B), total dissolved solids (C), and conductivity (D) during
increasing water levels in 2007 (closed circles) and 2008 (open circles).
W. P. DUNCAN & M. N. FERNANDES
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
460
Table I. A summary of physicochemical variables (mean ± sd; min – max; number of samples) and occurrences of
potamotrygonid species in the Amazon River, Negro River and Tapajós River.
Rivers pH Conductivity
(µS/cm)
TDS
(mg/l)
Potamotrygonidae
Amazon River
6.6 ± 0.2
(6.0 - 6.8)
n = 40
44.8 ± 24.8
(17.4 - 104.3)
n = 40
23.9 ± 17.8
(8.8 - 55.0)
n = 40
Paratrygon aiereba#
Plesiotrygon iwamae*
Potamotrygon motoro#
Potamotrygon orbignyi#&
Potamotrygon scobina#&
Potamotrygon ocellata*
Potamotrygon humerosa*
Potamotrygon constellata*
Negro River
4.5 ± 0.9
(3.7 - 6.4)
n = 15
17.0 ± 15.2
(7.7 - 35.4)
n = 15
7.1 ± 6.7
(4.4 - 19.4)
n = 15
Paratrygon aiereba#
Potamotrygon motoro#
Potamotrygon orbignyi#
Potamotrygon sp. 1, n.sp.
(cururu ray) #
Potamotrygon schroederi#
Tapajós River
6.5 ± 0.4
(6.0 - 8.0)
n = 23
14.4 ± 13.1
(11.7 - 25.2)
n = 23
7.7 ± 5.6
(6.2 - 13.3)
n = 23
Paratrygon aiereba#
Potamotrygon sp. 2, n. sp.
(P14-Itaituba ray)¥
Potamotrygon sp. 3, n. sp.
(jabuti ray)¥
Potamotrygon sp. 4, n. sp.#@
Potamotrygon motoro¥
Potamotrygon orbignyi#
#Occurrence recorded in situ; *Occurrence according to Compagno & Cook (1995); &Collected in the Amazon estuary;
¥M.L.G. Araújo, personal communication; @New species (n. sp.) according M.R de Carvalho (personal
communication).
Discussion
In the Amazon Basin, the annual water-level
fluctuation is seasonally dependent (Junk, 1997).
The average flooding amplitude is about 10 m and
the flooding occurs during the rainy season between
May and June and corresponds to the maximum
monthly Amazon River discharge to the Atlantic
Ocean (Ffield 2007, Birkett et al. 2002). At this
time, the main stem of the Amazon River receives a
high volume of water from different tributaries
(Martinelli et al. 1993). Consequently, the river
demonstrates physicochemical variables along its
longitudinal gradient. Furthermore, during the years
in which the inundation period is more severe, such
as in 2008, a consistent dilution in the dissolved
solutes was observed. As a result, some limnological
variables were reduced.
Except for pH values, there was a clear
longitudinal decrease in conductivity and TDS
values downstream. The Amazon River receives
volumetrically high discharges of highly diluted
water from the Negro, Urubu, and Uatumã rivers (all
blackwater types) and the Trombetas, Tapajós, and
Xingu rivers (all clearwater types). According to
Santos & Ribeiro (1988), the Solimões River shows
a consistent dilution in its major nutrients along a
stretch from Tabatinga (Upper Solimões River,
Brazil-Colombia border) until Santarém (Middle
Amazon), at the mouth of the Tapajós River. This is
consistent with the hypothesis in which dissolved
The distribution of the freshwater stingrays as function of physicochemical parameters in Amazonian waters
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
461
nutrients originating in the Andean headwaters
exhibit a dilution effect downstream, where large-
scale variations are controlled largely by the relative
contribution of larger diluted tributaries (Richey et
al. 1990). There is no well-defined pattern for pH
values; this variable often breaks down at the river-
margin after the confluence of acidic rivers like the
Negro, Urubu, Uatumã, and Guamá. The slight
longitudinal variation in pH along the Amazon main
stem may reflect the maintenance of conservative
dissolved characteristics such as alkalinity, which
originates in the Andes and alluvial foreland rivers
and is present primarily in the main stem. These
waters thus avoid the acidification effect farther
downstream that occurs as a result of the input of the
larger acidic tributaries cited above.
Two main channels comprise the estuary of
the Amazon River. The southward channel
constitutes a long bay named Marajó Bay at the
confluence of the Pará and Tocantins rivers. During
the dry season, Marajó Bay is strongly affected by
seawater due to the reduced discharge of the
Amazon, Tocantins and Guamá Rivers. As a result,
the water of Marajó Bay is a typically brackish
environment. In contrast, freshwater habitats extend
to Colares Island during the rainy period. The waters
along the main stem at Marajó Bay are then more
diluted compared to the northward channel of the
Amazon River at the Marajó Archipelago.
According to Charvet-Ameida et al. (2005), the
Colares region is characterized by a high population
density of freshwater stingrays, mostly represented
by four species: Plesiotrygon iwamae, Paratrygon
aiereba, P. orbignyi, and P. scobina. The latter two
stingrays were the most abundant species (96%) in
Soure region (Amazon estuary), near of Colares
Island (Almeida et al. 2009). During the dry season,
most areas in the Marajó Bay exhibit brackish
characteristics, including Colares Island, where
salinity increases from 0.1 during the rainy season to
7 psu during the dry season due to the simultaneous
discharge reduction of the Amazon and Tocantins
rivers (Almeida 2003). According to several authors,
potamotrygonids do not disappear completely from
this area (Charvet-Almeida et al. 2002, Charvet-
Almeida et al. 2005; Almeida et al. 2009). However,
there is evidence of effects on fauna of
elasmobranch composition during seasonal periods,
in which some species are more tolerant to increases
in environmental salinity than others. Paratrygon
aiereba may be less tolerant to salinity than P.
scobina and P. orbignyi (Charvet-Almeida et al.
2005). According to Almeida et al. (2009), P.
orbignyi is the most salt tolerant species found at the
mouth of the Amazon. These authors recorded the
occurrence of this species in the brackish water of
the Soure region in Marajó Bay (Amazon River
mouth), which conductivity and salinity levels were
as high as 20,800 S/cm and 12.4 psu. However,
despite the occurrence of P. orbignyi in the Soure
region, the salt water (salinity >18 psu) of São
Caetano de Odivelas (which is only 20 kilometer
downstream from Colares Island and 40 kilometer
west of the town of Soure) may act as a geographical
barrier for potamotrygonids. Thus, an imaginary line
between Colares Island and Soure region may be the
distributional limits of potamotrygonids in the
southern channel of the Amazon estuary during the
rainy season.
In spite of the controversial phylogenetic
relationships within the Potamotrygonidae family
Plesiotrygon iwamae is a sister-species of
Potamotrygon sp. “cururu ray” (Toffoli et al. 2008).
The former is typically found in the main stem of the
Amazon River and is absent in the clearwater of the
Tapajós River and the blackwater of the Negro
River, while the cururu ray, however, is endemic to
the acidic blackwater of the Negro River. It has been
widely hypothesized that the absence of any
paleogeological event to explain the allopatric
speciation between these species led several authors
to describe the differences among water types as a
geographic barrier driving speciation (Toffoli et al.
2008). The Negro River is known as one the most
extreme aquatic environments in the world. The
waters are typically acidic and ion-poor with low
conductivity and very few dissolved solids.
Moreover, the conductivity is associated with a H+
concentration, as observed by Furch et al. (1982).
During the rainy season, the water of the Negro
River spreads into the forest, forming a habitat
locally known as Igapó forest. Igapó forest is
characterized by submersed litter, acidic and tea-
colored water, and a low dissolved oxygen
concentration. Interestingly, this is the preferential
habitat of the most common potamotrygonids of the
Negro River, the cururu ray. The cururu ray exhibits
an unusual epithelial morphology in their gills
(Duncan et al. 2010) and physiological traits to
tolerate acidic and diluted water (Wood et al. 2002).
Paratrygon aiereba, P. schroederi, and P. orbignyi
are commonly found in the shallow water near the
beaches of the Negro River islands. The occurrence
of Paratrygon aiereba, P. schroederi, and cururu ray
in the Negro River Basin indicates that an ancient
lineage of potamotrygonid was well established in
this area, and this evidence suggests that some
habitats in the Negro River may be hydrographical
relics of the paleo-Amazon-Orinoco River in which
W. P. DUNCAN & M. N. FERNANDES
Pan-American Journal of Aquatic Sciences (2010), 5(3): 454-464
462
the potamotrygonid ancestor has evolved during
the Miocene (65-23 million of years ago). There
is substantial evidence that the ancestor of
P. motoro was responsible for the dispersion and
radiation of potamotrygonids in different bodies of
water in the Amazon Basin (Toffoli et al. 2008).
This species is widely distributed in the Amazonian
rivers surveyed, which suggests a high degree of
phenotypic plasticity due to the capacity to
osmoregulation in a wide range of ion
concentrations, as reported for Paratrygon aiereba
(Duncan et al. 2009).
In the Tapajós River, Paratrygon aiereba, P.
motoro, and P. orbignyi are sympatric and syntopic
species. In addition, there are at least three
undescribed endemic species in the clearwater of the
Tapajós River (Marcelo de Carvalho, personal
communication). One of these species may be
constrained by the whitewater of the Amazon River
at the mouth of the Tapajós River and by the rapids
at the village of São Luís do Tapajós. It is important
to emphasize the absence of volumetrically
significant tributaries in the stretch of water that
extends from the river mouth to the São Luis do
Tapajós rapids. This absence could explain the
relative homogeneity in the physicochemical
parameters along the same stretch (except near the
town of Aveiros, whose variations could be
associated with local pollution or mining).
In general, the Tapajós River is a clearwater
river and may be physicochemically characterized as
exhibiting an intermediary pattern lying between
those of the Amazon River and the Negro River
water types. The Amazon River is whitewater river
that is solute-rich and with a slightly alkaline to
neutral pH, while the Negro River is a blackwater
(or tea-colored) river that is acidic and solute-poor.
This difference between water types could help to
explain the zoogeographical distribution of the
family Potamotrygonidae in the Amazon Basin,
thus facilitating a better understanding of the role
of the rivers as geographic barriers or even as
selective corridors for the dispersion of
potamotrygonid rays.
Acknowledgments
The authors wish to thank M. L. G. Araújo,
O. T. F. Costa, N. F. Verany, A. Kalinin, F. L.
Marques, and B. Baldisserotto for their insightful
comments and inspiring discussions.
Potamotrygonid specimens were collected with a
wild animal license issued by IBAMA
(IBAMA/DIREC, Lic. No.#127/2004; IBAMA/
SISBIO No.#10085-1/2007 and No.#15068-1/2008).
This work was funded by the Fundação de Amparo à
Pesquisa do Estado do Amazonas (FAPEAM), the
Fundação de Amparo à Pesquisa do Estado de São
Paulo (FAPESP), and the Conselho Nacional de
Pesquisa Científica e Tecnológica (CNPq). Thanks
also to two anonymous referees whose comments
and suggestions were most helpful.
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Received August 2009
Accepted November 2009
Published online July 2011
