Distribuição espacial de peixes characidíneos (Teleostei, Crenuchidae) no sistema do Ribeirão Grande, um tributário da bacia do rio Paraíba do Sul, Brasil

This study analyses the spatial distribution of characidiin fishes Characidium lauroi and Characidium alipioi (Crenuchidae) in a forest stream system located in southeastern Brazil. Fish were sampled from July 2001 to April 2002. Collections were made with an electro-fishing device in five stream reaches of the Ribeirao Grande system. Conductivity, pH, water temperature and dissolved oxygen were measured at each site. The species have different distributions in Ribeirao Grande system. Characidium lauroi is abundant in montane-piedmont zones and Characidium alipioi occurs mainly in piedmont-plain zones. Streams’ different features contribute to these species’ distribution in the system


Introduction Introduction Introduction Introduction
The neotropical genus Characidium is a group of small fishes that inhabit streams and creeks.This group was recently removed from Characidae and included in a separate monophyletic family Crenuchidae, widely distributed between Panama and eastern Argentina (Buckup and Reis, 1997).
Characidium lauroi was described from Rio das Pedras, State of Rio de Janeiro, southeastern Brazil (Travassos, 1949).This stream is also a tributary of the Paraíba do Sul drainage, and the type-locality is at about 2,000 m above sea level.Characidium alipioi was described from the river Paraíba do Sul, State of Rio de Janeiro, in the Paraíba Vale, at low altitude (Travassos, 1955).
Characidim lauroi and Characidium alipioi have been found in the Ribeirão Grande system, a tributary of the rio Paraíba do Sul located at the Serra da Mantiqueira southeastern slope, southeastern Brazil.The East-Brazilian basin is formed by small rivers with a high number of endemic species (Lowe-McConnell, 1975).
In eastern State of São Paulo, Brazil, the landscape is topographically expressed by Serra do Mar and Serra da Mantiqueira, with the river Paraíba do Sul running between them.
Fish communities in streams have their diversity, ecology and dynamics not satisfactorily studied in Brazil, and their habitats are vulnerable to degradation.Paraíba do Sul tributaries have been partially destroyed and well preserved sites have become rare (Garavello and Santana, 1998).Some studies were carried out in southeastern Brazil, in small tributaries of river Paraná basin (Castro and Casatti, 1997;Uieda and Barreto, 1999;Lemes and Garutti, 2002a, b), concerning ichthyofauna structure and composition.However, ecology and population dynamic of fish communities in Serra da Mantiqueira are not discussed in detail.
The present work aims at characterizing the distribution and length structure of two Characidiinae species, Characidium lauroi and Characidium alipioi, in Ribeirão Grande system, tributary of the Paraíba do Sul left margin, southeastern Brazil.

Study area Study area Study area Study area
The slope of the Serra da Mantiqueira is covered by remains of Atlantic forest.The forest is formed by species that belong to Leguminosae, Bignoniaceae, Sapotaceae, Lauraceae, Myrtaceae and others (Hueck, 1972).The streams that form the Ribeirão Grande system (and other systems around it) run between crests of slopes, formed by faults during the late Pleistocene to Holocene tectonic reactivations (8,000 -13,000 years BP) (Modenesi-Gauttieri et al., 2002), surrounded by Atlantic forest.From the base of the slope through the valley (piedmont), there are remains of natural forests and also farms and plantations.
Five sites were surveyed in the Ribeirão Grande system (22 o 47' 08'' S, 45 o 28' 17'' W).There are five major tributaries that form the Ribeirão Grande system: the medium Ribeirão Grande stream, Cedro stream, Cajarana stream, Ferraz stream and Cachoeirão stream (not sampled) (Braga, 2004).Stream order segments in Ribeirão Grande system follow the Horton-Strahler classification (Allan, 1995;Matthews, 1998).The Serra da Mantiqueira is an east-west mountain range composed of granitic and metamorphic rocks.Streams in the Mantiqueira uplift have steep gradients and coarse substrata, bordered by rain forest (Atlantic forest).The low parts of the streams move on the Vale do Paraíba plain.The streams and creeks that form the Ribeirão Grande system are separated from other neighboring systems by slope faults, forming a left-side Paraíba do Sul drainage basin.

Sampling methods and Sampling methods and Sampling methods and Sampling methods and treatments treatments treatments treatments data data data data
Fish were sampled four times in each site, from July 2001 through April 2002: winter (July 2001), spring (October 2001), summer (February 2002) and autumn (April 2002).Quantitative collections were made with an electro-fishing device, powered by a generator with a maximum capacity of 1,500 V and 8.7 A of 60 Hz alternating current.Five stream reaches were sampled.These reaches were selected to represent the range of habitats in the stream system.All specimens were preserved in 10% formalin and returned to the laboratory for identification and biometry.Conductivity, pH, water temperature and dissolved oxygen were measured in each site, and elevations in meters were taken by a GPS.Mann-Whitney U-tests were used to determine the significance (P=0.05) of the variables in distinguishing between locals occupied by one or other species, according to Sokal and Rohlf (1981).

Discussion Discussion Discussion
Characidiin fishes include species with morphological traits associated with life in torrential environments, such as small length and a large number of reductive characters (Buckup, 1993).Braga ( 2004) described the absence of a swim bladder and a reduction in testis and ovary for Characidium lauroi and Characidium alipioi and concluded that morphological features in stream mountain fishes were used to predict the niche adaptation for torrent-dwelling characidiins.
Shelford (apud Sheldon, 1968) explained fish distribution with the geological concept of stream bed aging.During the late Pleistocene-Holocene, the Campos do Jordão Plateau, which includes the Serra da Mantiqueira geomorphological zone, has been subject to slow, but continuous, tectonism.This fact had a strong influence upon the present drainage, being preferential sites for the stream piracy phenomena observed in headwaters along the Mantiqueira divide (Modenesi-Gauttieri et al., 2002).This allowed contacts among populations of Characidium lauroi in Serra da Mantiqueira eastern upland parts.On the other hand, the population of Characidium lauroi has been separated from the population of Characidium alipioi according to the parapatric model.Centofante et al. (2003) studied the evolutionary differentiation in sex chromosomes of Characidium lauroi and Characidium alipioi in the Ribeirão Grande stream, concluding that evolutionary differentiation may be associated with a biogeographic barrier.
In small streams, fish communities can change within relatively short distances, and fish distribution and diversity can be controlled by the habitat structural features (Sheldon, 1968).The explanations of parapatric speciation along an environmental gradient and allopatric speciation with later secondary contact could cause complementary distribution, because both species are adapted to different environments (Winston, 1995).As mentioned above, a transitional zone exists between montane and piedmont (ecotone).Two populations are parapatric if they occupy contiguous territories that overlap only very narrowly (Futuyama and Mayer, 1980;Key, 1982;Taylor and Lienesch, 1996).According to Bull (1991), parapatric boundaries can occur in less extreme gradients of elevation, climate, soil, water speed and vegetation.These conditions have been found in the ecotone at the montane-piedmont zone.
In an ecotonal change, each species is better adapted to conditions on its own side.In Ribeirão Grande system, the streams exhibit a downstream decrease in gradient along their length; slopes are steep in the headwaters and become less so as one proceeds downstream.Many factors influence the physical environment of running waters, causing variations from place to place; temperature, for example, usually varies due to elevation (Allan, 1995), and an important cause of change in faunal composition is marginal vegetation loss (Karr and Schlosser, 1978).Stream chemistry also varies in stream chemical composition across a gradient from humid to arid condition; a high concentration of total dissolved salts is found in streams draining arid areas (Allan, 1995) just as observed in sites 3, 4, and 5.In the piedmont, the contact colluviumweathered bedrock is usually marked by stone lines, as can be seen on Ferraz stream right margin (22 o 47 ' 08 '' S; 45 o 28 ' 17 '' W).These accumulations of coarse clasts from a regional paleopavement has been attributed to the widespread aridity of the last glacial maximum, ca.13,000 -23,000 years BP (Ab ' Saber,  1977, 1979; Modenesi-Gauttieri et al., 2002).During this period, the forest stayed in the slope, covering the Serra da Mantiqueira upland.Widespread aridity occurred through the piedmont up to Vale do Paraíba; soils structure confirms this hypothesis.The reasons for the complete absence of the Characidium alipioi in site 1, and C. lauroi in site 5, could be explained by a separation of an ancient characidiin during the late Pleistocene, attributed to the widespread aridity occurred in this time.Both species are closely related and appear to be involved in a genetic interaction (Centofante et al., 2001(Centofante et al., , 2003)).

Table 3 .
Medians and ranges of variables for sites where Characidium lauroi and Characidium alipioi occurred and significance of Mann-Whitney U-tests in distinguishing among both sets of sites based on five variables.

Table 1 .
Abundance of characidiin fishes in Ribeirão Grande, in number of individuals.

Table 2 .
The order of the streams, sampled site, elevations and their features.