The west margin of the Patos Lagoon is characterized by the presence of Pleistocene-Holocene sedimentary deposits, with the predominance of silty-clayey sand (Tomazelli and Villwock 2000; CPRM 2007). The swamps we studied are located around Lagoa Pequena, a lacustrine system with an area of approximately 4,000 km², in the boundaries of the Pelotas and Turuçu municipalities (Fig. 1). Lagoa Pequena is connected to the Patos Lagoon estuary on its west margin and is subject to estuarine physical-chemical and biological influence (Alves et al. 2009).

Figure 1. 

Four main geological formations from the state of Rio Grande do Sul, Southern Brazil. The study area (Quaternary deposits) as well as previous studies used in Dice Similarity Index analysis is entirely inserted in Coastal Plain. Key: 1 present study 2 Turuçu River basin; Burns et al. 20153 Corrientes Stream; Volcan et al. 20124 Tapes Butiazais; Becker et al. 20075 Fortaleza lagoon; Schifino et al. 20046 Lagoa do Peixe; Loebmann and Vieira 20057 Peat forest; Quintela et al. 20078 Rio Grande coastal streams; Tagliani 1995 9 Cassino coastal streams; Bastos et al. 201310 Taim wetland; Garcia et al. 200611 Lagoa Mangueira; Artioli et al. 2009.

Fishes were sampled in four marginal swamps: swamp 1 (S1) -31°56'96"S, 52°11'78"W – emergent vegetation (Juncus sp. L.), with higher incidence of floating species (Azolla sp. Lam.) during warm periods; swamp 2 (S2) -31°56'90"S, 52°12'02"W – predominance of floating macrophytes (Azolla sp., Pistia stratiotes L., Salvinia sp. Ség.); swamp 3 (S3) -31°56'50"S, 52°13'10"W – margins sparsely covered by Juncus sp. and higher concentration of floating species (Salvinia sp.) during warm periods; swamp 4 (S4) -31°56'80"S, 52°13'82"W – predominance of Nymphoides indica (L.) Kuntze. Distances between the swamps and Lagoa Pequena are respectively 101 m, 395 m, 1,386 m and 2,229 m.

The ichthyofauna of the study area was sampled seasonally during the year of 2010 with the use of a 5 m long, 2 m high seine net with a 5 mm mesh between adjacent nodes. We applied an effort of three seines by seasonal sampling campaign in each swamp, totalizing an effort of 48 seines. Captured individuals were euthanized in clove oil solution, fixed in 10% formalin, preserved in 70% ethanol and housed in the Fish Reference Collection of the Instituto de Ciências Biológicas at the Universidade Federal do Rio Grande (CIFURG) (Appendix 1).

Aiming to evaluate similarities on species composition between the study area assemblage and other fish assemblages recorded in RS Quaternary deposits (Tagliani 1995; Schifino et al. 2004; Loebmann and Vieira 2005; Garcia et al. 2006; Becker et al. 2007; Quintela et al. 2007; Artioli et al. 2009; Volcan et al. 2012 [partial: P12-P15]; Burns et al. 2015 [partial: potamon zone]), we calculated the Dice Similarity Index (DSI) based on a matrix of species presence/absence, using 1,000 bootstrap resampling. Obtained values were submitted to a cluster analysis based on the Unweighted Pair Group Method with Arithmetic Means (UPGMA) and similarity relationships were expressed in the form of a dendrogram. Grouping significance was tested through an Analysis of Similarity (ANOSIM). Analyses were performed in PAST version 1.79 (Hammer et al. 2001).

Finally, we compiled data regarding fish species recorded in limnic systems of RS Quaternary deposits from the list of the binary matrix used in the similarity analysis and from the literature, including information on additional species contained in punctual records, references on type material and references on comparative systematic/taxonomic studies (Malabarba and Isaia 1992; Buckup and Reis 1997; Costa and Cheffe 200120022005; Costa 20022006; Giora et al. 2008; Malabarba and Dyer 2002; Lucinda 20052008; Costa and Lanés 2009; Claudino et al. 2010; Volcan et al. 2010; Corrêa et al. 2011; Carvalho and Reis 2011; Malabarba et al. 2013; Lanés et al. 2015; Giora and Malabarba 2016).

The marginal-lacustrine swamps sampled in the present study host a considerable ichthyofaunistic diversity, showing a species richness within the range observed in limnic systems of RS Quaternary deposits. For example, Garcia et al. (2006) recorded 57 species in lagoons of the Taim Ecological Station in the Rio Grande restinga, while Artioli et al. (2009)sampled 52 species in Lagoa Mangueira. Smaller systems in this restinga have shown lower richness, such as 31 species in a set of three coastal streams (Tagliani 1995) and 18 species in a peat forest fragment (Quintela et al. 2007). Loebmann and Vieira (2005) recorded 67 species (among limnic, estuarine and marine forms) in Lagoa do Peixe at the restinga of São José do Norte, while Schifino et al. (2004) listed 22 species in Lagoa Fortaleza, at the Tramandaí River basin. Becker et al. (2007) recorded 55 species in alluvial deposits of swamps, ponds and streams on the west margin of the Patos-Mirim complex, at the Butiazais region in Tapes, while Volcan et al. (2012) found 49 species in the Corrientes Stream’s lower course. Such a high species richness registered for characids, which represent 34% of the species recorded in the study area, corroborates with the pattern found in other fish assemblages previously investigated in RS Quaternary deposits (e.g. Tagliani 1995; Schifino et al. 2004; Loebmann and Vieira 2005; Garcia et al. 2006; Becker et al. 2007; Quintela et al. 2007; Artioli et al. 2009; Volcan et al. 2012; Bastos et al. 2013).

Regarding system comparisons, the assemblage of marginal-lacustrine swamps is more similar on its species composition to the assemblage of the lower course of the Corrientes Stream (Volcan et al. 2012), followed by the ichthyocenose of the Lagoa do Peixe National Park (Loebmann and Vieira 2005). The Corrientes Stream, however, is closest to the study area when compared to the other systems, distancing from 50 m up to 590 m from the marginal swamps. Thirty-four of the forty-two species (81%) recorded in the marginal swamps were also found by Volcan et al. (2012) in the Corrientes Stream’s lower course and the following species were found exclusively in the swamps: Odontesthes argentinensis, Platanichthys platana, Mugil liza, Astyanax aff. fasciatus, Hyphessobrycon togoi, Brachyhypopomus draco, Hisonotus laevior and Phalloptychus iheringi. Such similarity may be related to the proximity and possible connection between the stream and marshes during rainfall periods, leading to an exchange of species. The second most similar assemblage of the Lagoa do Peixe National Park (Loebmann and Vieira 2005), on the other hand, is located in deposits at the eastern margin of the Patos Lagoon. The fact that higher similarity is found between assemblages from the study area and Lagoa do Peixe is remarkable, considering that other systems such as the Turuçu basin (Burns et al. 2015) and the systems of Rio Grande (Tagliani et al. 1995; Quintela et al. 2007; Bastos et al. 2013), are closer to the study area. These results suggest that environmental factors may exert greater influence on the species composition in these systems compared to the effect of geographic distances.

With the exception of Characidium orientale, all other species found in the study area have been recorded in limnic systems of the coastal restingas of Rio Grande and São José do Norte (Malabarba and Isaia 1992; Tagliani 1995; Loebmann and Vieira 2005; Garcia et al. 2006; Quintela et al. 2007; Artioli et al. 2009; Bastos et al. 2013; Malabarba et al. 2013). Characidium orientale was described among specimens collected in Arroio Chasqueiro and the type series includes specimens from several localities in the RS Shield (Precambrian) and the Central Depression (Permian-Triassic) (Buckup and Reis 1997). Posteriorly, the species was recorded by Volcan et al. (2012) in the Corrientes Stream and by Burns et al. (2015) in the Turuçu River basin. Therefore, this species’ distribution in the RS Quaternary may be restricted to alluvial deposits of the west margin of the Patos-Mirim complex.

The compilation of bibliographic information associated to our sample data revealed a total of 156 species (114 limnic, 15 marine/estuarine/limnic, ten marine/estuarine, nine estuarine/limnic and eight marine) occurring in limnic systems inserted in RS Quaternary deposits (Table 1). Comparatively, 160 species (including undescribed forms) can be found in the Patos Lagoon basin (Malabarba et al. 2009), which includes rivers and streams originated from distinct geological formations in RS such as the Shield and the Paraná Basin (Central Depression and Meridional Plateau), located beyond the Patos Lagoon itself, which in turn, covers an area of approximately 10,360 km² (Vieira 1984). Approximately 100 species were recorded in the Tramandaí River basin, which is formed by rivers and streams of the Serra Geral slops (eastern Meridional Plateau) and several lakes and channels of the northern coastal plain (Malabarba et al. 2013). Therefore, limnic systems in the RS Quaternary host relevant ichthyofaunistic diversity, compassing approximately 27% of the freshwater ichthyofauna recorded in RS (Bertaco et al. 2016). Furthermore, this study recorded a low percentage (around 8%) of the marine species usually found in the RS coastal zone (Seeliger et al. 1998).

The Siluriformes and Characiformes orders, which comprise respectively 38 and 36 species, were the most speciose orders in RS Quaternary deposits, corroborating with the pattern already found for the Neotropical region (Lowe-McConnell 1987; Reis et al. 2003a). With the exception of Acestrorhyncus pantaneiro, a species typical for the Paraná, Uruguay, Paraguay and Mamoré River basins (Menezes 2003), and Pachyurus bonariensis, native from the Paraná, Uruguay and Paraguay River basins (Casatti 2003), all other recorded species are characteristic of Atlantic basins in RS, which include the Patos-Mirim basin and Tramandaí and Mampituba rivers (Buckup and Reis 1997; Melo and Buckup 2006; Malabarba 2008; Malabarba et al. 2013). Acestrorhyncus pantaneiro was recorded for the first time in RS Atlantic basins by Saccol-Pereira et al. (2006), who reported the capture of three individuals in the Parque Estadual Delta do Jacuí during the years of 2004 and 2005. Posteriorly, Artioli et al. (2013) reported the capture of three more individuals during the year of 2008 in lagoons in Fortaleza and Malvas and in the Tramandaí basin. Recently, Einhardt et al. (2014) registered the species in the Chasqueiro Stream micro-basin, an integrant of the Mirim Lagoon sub-basin. Before these records, the occurrence of A. pantaneiro in RS was known only for the Uruguay River basin, where it is considered a native species (Menezes 2003). Thus, this phenomenon represents a case of recent invasion and dispersion of an aloctone species in RS Atlantic basins, which according to Artioli et al. (2013), could have been favored by the geomorphology of the coastal plain, associated to connectivity between systems during flooding periods and the opening of artificial channels used for drainage and irrigation.

An analysis of the geographic distribution of species occurring in RS Quaternary deposits reveals distinct patterns. One group of “subtropical” species is distributed mainly in the Pampa biome, although some species also spread out to peripheral systems of subtropical Atlantic Forest and to other contacting biomes (e.g. Astyanax eigenmanniorum, Cheirodon interruptus, Hyphessobrycon meridionalis, H. igneus, H. togoi, Oligosarcus jenynsii, Cyphocarax voga, Loricariichthys anus, Odontesthes bonariensis, Phalloceros caudimaculatus). These species are typical for the Tramandaí, Patos-Mirim, Uruguay and lower Paraná basins (Malabarba 1998; Dyer 2003; Ferraris 2003; Lima et al. 2003; Lucinda 2008; Malabarba et al. 2013). Most of them are widely distributed in the RS Quaternary, occurring in systems from all segments of the coastal restingas and also in alluvial plains at west of the Patos-Mirim complex (Loebmann and Vieira 2005; Artioli et al. 2009; Volcan et al. 2012; Malabarba et al. 2013; present study). One sub-group of “subtropical” species (e.g. Australoheros acaroides, Cichasoma portalegrense, Gymnogeophagus gymnogenys, Oligosarcus robustus, Heptapterus sympterygium) is restricted to Atlantic discharge basins in RS (Reis and Malabarba 1988; Schindler et al. 2010; Malabarba et al. 2013) and is also widespread in RS Quaternary deposits (Artioli et al. 2009; Bastos et al. 2013; Malabarba et al. 2013; as well as the present study). Meanwhile, another sub-group is restricted to specific segments of the RS Quaternary deposits. For example, Rineloricaria quadrensis, Microglanis cibelae and Pachyurus bonariensis, natives from the Paraná, Uruguay and Paraguay basins (Casatti 2003), occur exclusively in lagoons and channels of the northern coastal plain and rivers of the Tramandaí basin (Malabarba et al. 2013); the second also occurring in the Mampituba basin (Malabarba and Mahler 1998).

The RS Quaternary deposits are also marked by endemism. Odontesthes ledae, O. piquava, O. bicudo, Gymnogeophagus lacustris and Gymnotus refugio occur only in systems of the Tramandaí basin (Reis and Malabarba 1988; Malabarba and Dyer 2002) and consequently represent endemic species of the northern coastal plain. Two other cases of endemism in the central and southern segments of coastal restingas are known: Cynopoecilus fulgens is known only in its typical locality (municipality of São José do Norte) (Costa 2002) and in the Lagoa do Peixe National Park (municipalities of Tavares and Mostardas) (Keppeler et al. 2015; Lanés et al. 20142015), therefore occupying a restrict portion of the central coastal plain; Austrolebias minuano was described among specimens collected in the Rio Grande municipality (Costa and Cheffe 2001). Posteriorly, this same species was recorded in new areas of Rio Grande (Porciuncula et al. 2006), in a restinga of São José do Norte, in the municipalities of Tavares and São José do Norte (Costa 2006), and in the Lagoa do Peixe National Park (Corrêa et al. 2009; Keppeler et al. 2015; Lanés et al. 2014, 2015).

In contrast, there is one group of species that is widely distributed in Brazil and in the Neotropical region and is also well distributed in the RS Quaternary, occurring both in coastal restingas and alluvial plains at the Patos-Mirim west margin. The callichthyids Hoplosternum littorale and Callichthys callichthys occur in a great portion of the cis-Adean South America (Reis 2003). The gobiid Ctenogobius shufeldti is spread from North Carolina (EUA) to southern Brazil (Malabarba et al. 2013). Among cichlids, Crenicichla lepidota occurs from the Guaporé River (Amazon basin) up to the Uruguay and Paraná basins, while Geophagus brasiliensis is distributed along eastern Brazilian coastal basins and Uruguayan systems (Kullander 2003). Other species that inhabit coastal basins of southeastern and southern Brazil are the characid Hyphessobrycon boulengeri and the anablepid Jenynsia multidentata, both also occurring in Uruguay and Argentina (Malabarba et al. 2013). However, some widespread taxa are currently recognized as a species complex, including Astyanax “fasciatus”, Characidium “zebra”, Hoplias “malabaricus”, Gymnotus “carapo”, Rhamdia “quelen” and Synbranchus “marmoratus” (Malabarba et al. 2013). Therefore, these forms lack an integrative systematic analysis and appropriate taxonomic definitions.

Biogeographic hypotheses on South American ichthyofauna date back to the early twentieth century (Ribeiro et al. 2013) and point to sea level oscillations and orogenetic effects as the main factors that shape distribution patterns (Malabarba and Isaia 1992; Ribeiro 2006; Ribeiro et al. 2013). The fact that few species from genera that are usually well represented in “inland basins” occur in condition of endemism in the coastal basin (e.g. Crenicichla and Gymnogeophagus) suggests the occurrence of vicariance events followed by cladogenesis. Indeed, speciation by vicariance involving the genera Gymnogeophagus (Reis and Malabarba 1988; Malabarba and Isaia 1992), Mimagoniates (Menezes and Weitzman 1990) and Odontesthes (Malabarba and Dyer 2002) is suggested as one of the evolutionary processes that occurred in the Tramandaí River basin. Molecular data (Beheregaray et al. 2001) revealed that three endemic Odontesthes species in the Tramandaí basin (O. bicudo, O. ledae and O. piquava) have probably diverged after the Pleistocene-Holocene marine regressions that shaped the complex of lagoons where these species occur almost allopatrically. The “Pattern C” proposed by Ribeiro (2006), which suggests the occurrence of recent intraspecific vicariance events between “inland basins of the Brazilian Shield” and “coastal basins”, is corroborated by the presence of species whose distribution is restricted to the Atlantic drainage basins in RS (Patos and Tramandaí) and the “adjacent inland basin” of the Uruguay River - those species being Astyanax aff. fasciatus, Gymnogeophagus rhabdotus and Mimagoniates inequalis. Therefore, the extant ichthyofaunistic composition of limnic systems in RS Quaternary deposits seems to be the result of both internal processes and evolutionary events triggered in older adjacent geological formations.

In conclusion, limnic systems of Quaternary RS deposits host a diversified ichthyofauna, including endemic species and species with restricted distribution (to the RS state). These systems are home to 15 endangered species at state level (State Decree 51.797/2014), which include 13 killifishes (Rivulidae), Odonthestes bicudo and Gymnotus refugio. Rivulids, as well as other representatives of the ichthyofauna, are affected by the destruction and alteration of aquatic environments. In RS, interferences caused by rice cultivation, livestock, silviculture and urbanization are the main threats to the freshwater ichthyofauna (Reis et al. 2003b; Volcan et al. 2010). Moreover, only two integral protection conservation units host populations of endangered rivulids along the entire domain of the RS Quaternary deposits: the Banhado do Maçarico Biological Reserve (Costa 2006) and the Lagoa do Peixe National Park (Corrêa et al. 2009; Lanés et al. 2015). Thus, most of the threatened killifish populations, as well as populations of Odonthestes bicudo and Gymnotus refugio, remain unprotected. In this context, conserving these and other species, as pointed out by Reis et al. (2003b) and Volcan et al. (2012), implies the creation of public and private conservation units.