Cryptic species are defined as two

Cryptic species are defined as two or more
morphologically indistinguishable species that are incapable
of interbreeding (Bickford et al., 2007). Such species likely
arise through interespecific reproductive isolation, which
can be caused by pre-zygotic means, such as gametic
incompatibility and/or ecological isolation (Miyatake et
al., 1999; Landry et al., 2003), or by post-zygotic means,
such as hybrid inviability and/or sexual selection against
hybrids (Orr, 1995; Noor et al., 2001; Presgraves et al.,
2002). Generally, these types of speciation events occur in
organisms with little or no motility, making certain plants
(Presgraves et al., 2002), insects (Campbell et al., 1994),
fungi (Theodoro et al., 2008), small mammals (Green
et al., 1980), amphibians (Kozak et al., 2006) and fishes
(Moreira-Filho & Bertollo, 1991) suitable model organisms
for studying this phenomenon. Although morphological
differences between cryptic species are minimal, other
traits can be used to detect these species complexes,
including behaviour (Crossley, 1986), karyotype structure
(Moreira-Filho & Bertollo, 1991; Dobigny et al., 2002;
Amaro et al., 2012) and protein (Nakamoto et al., 1986;
Fong & Garthwaite, 1994) and DNA (Kazan et al., 1993;
Hebert et al., 2004) sequences.
Neotropical fishes are excellent models for studying
cryptic species, as they are distributed widely across
continents and have a propensity to form endemic and
isolated populations, often culminating in allopatric
differentiation (Lundberg et al., 1998; Ribeiro, 2006).
In addition, natural or unnatural random events, such as
headwater capture or changes in watercourses, can lead to
secondary contacts between previously separated species
(Blanco et al., 2009; Peres et al., 2009), providing ideal
scenarios to study novel species complexes. Indeed, such
complexes have already been studied in a variety of fish
orders, including Characiformes, Synbranchiformes, and
Gymnotiformes (Moreira-Filho & Bertollo, 1991; Torres et
al., 2005; Milhomem et al., 2008).
Fishes belonging to the genus Synbranchus
(Synbranchiformes, Synbranchidae) are currently divided
into three recognized species: (1) S. madeirae Rosen &
Rumney, 1972, which is restricted to the Madeira River
basin; (2) S. lampreia Favorito, Zanata & Assumpção, 2005,
which is restricted to Marajó Island; and (3) S. marmoratus
Bloch, 1795, which is widely distributed throughout
South and Central America (Rosen & Rumney, 1972).
Although S. marmoratus specimens appear to belong to a
single taxonomic group, some populations may display an
extensive karyotype diversity, with diploid numbers ranging
from 42 to 46 chromosomes; furthermore, variations in
chromosome morphology as well as in the distribution of
constitutive heterochromatin and rDNA genes have also
been observed (Foresti et al., 1992; Melilo et al., 1996;
Sanchez & Fenocchio, 1996; Torres et al., 2005). Although
distinguishing the intraspecific S. marmoratus groups
is relatively easy with cytogenetic tools, distinguishing
between these groups based solely on morphology is
often impossible (Rosen & Rumney, 1972), making the
identification of new species difficult. Furthermore, one
must say that karyotypic structural analyses are limited
in their utility; for example, whereas these analyses can
identify chromosomal diversity and suggest possible
rearrangements leading to this diversity, the chronological
order in which such events occurred and the evolutionary
relationships among the karyomorphs cannot be directly
determined.
Considering the wide distribution of S. marmoratus
throughout the waters of South and Central America, the
purpose of this study was to characterize the karyotypes of
distinct groups within this species and determine whether
divergent species exist within the current S. marmoratus
grouping, to analyze the relationships between the sampled
taxa and to investigate the history of chromosomal
rearrangements in this species as a whole.