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criteria, the revalidation of
Sotalia guianensis
(van Bénéden 1864) was recommended for the
marine ecotype, while the riverine form holds the binomial
Sotalia fluviatilis
(Gervais 1853,
van Bree, 1974).
Another important finding was the presence of
S. guianensis
at the mouth of the Amazon
River. The freshwater load of the Amazon River reaches hundreds of kilometers into the sea
(Muller-Karger et al., 1988), so the animals sampled in Pará were actually living in
freshwater. It would be interesting to analyze samples from intermediate locations along the
Amazon River, to detect how far upriver
S.
guianensis
occurs, and verify if there is sympatry
in any region with
S. fluviatilis
. For that, a joint analysis of mitochondrial sequences and
microsatellites would be crucial, since it would allow not only the detection of any possible
hybridization in the area but also its polarity.
Recently, Caballero et al. (2007) analyzed sequences from introns of three nuclear genes
(lactalbumin, actin and glucocerebrosidase) and another mitochondrial marker (ND2)
including South America and Caribbean samples. Their study, based on a larger dataset both
in genes analysed and geographical breadth gave support to the conclusion of Cunha et al.
(2005), confirming the specific status of
S. guianensis
and
S. fluviatilis
.
Two issues related to genus
Sotalia
remain unclear: the range and species identity of
Sotalia
dolphins in the Orinoco River, and the taxonomic status of
Sotalia
dolphins from
southern Maracaibo Lake. In the Orinoco River, there are frequent records of
Sotalia
dolphins
at Ciudad Bolívar, some 300 km from the river's mouth, but those may correspond to
S.
guianensis
, which can reach several kilometers upriver (da Silva and Best, 1996; Mead &
Koehnken, 1991; Flores & da Silva, 2008). Boher et al.
(1995) reported a sighting in the
Middle Orinoco, 800 km inland. In addition, there are disputed reports of
Sotalia
dolphins in
the Upper Orinoco, and even in the Apure River (Hershkovitz, 1963; Borobia et al., 1991;
Boher et al. 1995). However,
Sotalia
dolphins were not recorded in the Upper Orinoco and
Apure Rivers, nor in the lower reaches of most of the major tributaries of the Orinoco, during
a long term study conducted between 1983 and 1990 (Mead & Koehnken, 1991). It is
believed that
Sotalia
dolphins cannot traverse the rapids at the Casiquiare channel, which
connects the Orinoco and Amazon River basins (da Silva and Best, 1996). This barrier has
existed since the uplift of the Mérida Cordillera (10 mya; Lundberg et al., 1998), which
predates the split between
Sotalia
species (see next section). Thus,
Sotalia
dolphins in the
Middle Orinoco are likely to be an isolated population of
S. guianensis
.
Another interesting issue concerns
Sotalia
dolphins found in the southern, freshwater,
portion of the Maracaibo Lake. That population is morphologically different from the marine
Sotalia
that inhabit the northern portion of the Lake, where it opens to the Gulf of Venezuela.
Dolphins from southern Maracaibo are smaller than marine
Sotalia
, and about the same size
as
S. fluviatilis
(Casinos et al., 1981; da Silva & Best, 1996; León, 2005). However, there is
no connection between the Maracaibo Lake and the present day known range of riverine
Sotalia
, and the Maracaibo Lake has been isolated from the Amazon basin for the last 8-10
million years (Hoorn et al., 1995; Días de Gamero, 1996). The morphological distinctiveness
of the southern Maracaibo Lake population could result from true phenotypic plasticity,
unlike that found between
S. guianensis
and
S. fluviatilis
. However it may also indicate a lack
of gene flow with the marine
Sotalia
from the mouth of the lake and the Gulf of Venezuela.
Indeed, genetic differentiation between those areas was reported by Caballero et al. (2006).
Those authors observed some exclusive haplotypes in samples from the lake, but did not
attribute the variation to specific differentiation.
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