Invasive mollusks Tarebia granifera Lamarck , 1822 and Corbicula fluminea Müller , 1774 in the Tuxpam and Tecolutla rivers , Mexico : spatial and seasonal distribution patterns

The Tuxpam and Tecolutla rivers in the Gulf of Mexico, are located in the Mesoamerican biodiversity hotspot and support different human activities: crude oil extraction, agriculture and livestock, that provoke environmental disturbances. Aquatic mollusks, physicochemical water characteristics and substrate type were examined along the main watercourse of these rivers during three periods of one year: the wet season, dry season, and during the northern winds season, when hurricanes are more frequent. In both rivers, physicochemical water characteristics, types of substratum and mollusk fauna demonstrated environmental gradients and differentiate two river zones: freshwater and estuarine. Differences were also found between the dry season, with higher inorganic salts content, and the wet and northern winds and hurricane seasons, when inorganic and organic nutrient inputs occurred. The mollusk fauna is composed of nine and eleven taxa in the Tecolutla and Tuxpam rivers, respectively. In both rivers, the introduced gastropod Tarebia granifera is the dominant species in the freshwater zone with regard to population density and the area it covers within the ecosystem, followed by the introduced bivalve Corbicula fluminea. Native mollusk species were confined to point-locations and attained very low densities. The gastropod Neritina virginea and the bivalve Brachidontes exustus were dominant in the estuarine zone of both rivers. Mollusk population densities declined during the wet, northern winds and hurricane seasons, while in the dry season both alien species reached higher densities, which could indicate that alien mollusks are removed by effect of climatic events. T. granifera and C. fluminea exhibit traits characteristic of invasive species and pose a risk to native mollusk biodiversity in these rivers.


Introduction
The Mexican rivers Tecolutla and Tuxpam drain into the Gulf of Mexico within the Mesoamerican biodiversity hotspot (Myers et al. 2000).Throughout much of the Mexican Gulf coast region human activities such as crude oil extraction, intensive agriculture, water extraction and inputs of untreated wastewater have degraded various aquatic ecosystems (Ortíz-Lozano et al. 2005).Lake (2000), states that several environmental disturbances have a negative impact on native species, through eradication or removal of sensitive species, and can create spaces available for colonization.
Other human activities including aquaculture, recreation, transport, and trade globalization intentionally or accidentally also contribute to the spread of invasive aquatic species (Cohen and Carlton 1998;Carlton and Geller 1993) that can use such available spaces.Invasive species are considered the second major cause of biodiversity loss in the world (Vitousek et al. 1996;Leung et al. 2002) and their spread is one of the major problems in ecosystem conservation (Ruiz et al. 2000).Adverse effects of invasive species may take different forms: once they have achieved a wide distribution, these species produce global homogenization of the patterns of distribution, altering the once distinctive regional biota, and they may affect aquatic ecosystems through competitive exclusion of indigenous biota (Rahel 2000;Sousa et al. 2008).Some invasive species displace native species through direct competition, predation, introduction of diseases, changes in biological and geochemical cycles, and alteration of the trophic web, thus causing adverse effects in the invaded habitats (Davis et al. 2000;Mack and Lonsdale 2001;Sax et al. 2002).
In Mexico, the National System on Invasive Species established by the National Commission for the Understanding and Use of Biodiversity (CONABIO 2008) has preliminarily identified at least 1000 invasive species of plants, mollusks, crustaceans, insects, fishes, amphibians, reptiles, birds and mammals.These include 13 species of invading mollusks: 11 bivalves (four freshwater) and two gastropods (one freshwater).CONABIO (2008) states that these species affect the natural wealth of Mexico, deteriorate environmental conditions in ecosystems and may have negative consequences on human health, production and the economy and recognizes the urgent need for additional knowledge of the distribution of these species and their impact on native biota, and the need to identify high-risk biodiversity areas.
Mexico has been ranked as one of the five megadiversity countries in the world (Bezaury et al. 2000), however, knowledge of the diversity of certain groups within its territory is limited because large areas remain unstudied, especially in the case of freshwater mollusks (Naranjo 2003).Based on worldwide diversity estimates, Strong et al. (2008) recognize Mollusca with as many as 80,000-100,000 described species, as an extraordinarily diverse phylum, second in diversity only to arthropods.In Mexico, 16 families of freshwater mollusks have been recorded, representing five orders, four subclasses (Naranjo 2003) and 211 species (Contreras-Arquieta 2000).Furthermore, in a study of the continental gastropods of Mexico and Central America, Thompson (2008) found 168 aquatic operculate and 84 aquatic pulmonate species and emphasized that the mollusk fauna of Mexico is poorly studied.Thus, the number of continental mollusk species may exceed current records, particularly since two zoogeographic regions, Nearctic and Neotropical, each with its own characteristic fauna, converge in Mexican territory sharing a broad transition zone (Morrone and Márquez 2001).To the paucity of knowledge on the native freshwater mollusks of Mexico must be added the lack of information on the distribution of species catalogued as invasive by CONABIO (2008).It is therefore essential for studies to focus on the impact of invasive species on the native biota and invaded habitats.This paper contributes to the understanding of patterns of distribution of native and introduced mollusk species in the Tuxpam and Tecolutla rivers, in the Mexican subtropics.The different forms taken by the invasion of two alien species in each of these rivers are explained and the risks these species pose are evaluated.

Study area
Tuxpam and Tecolutla rivers cross the Gulf of Mexico coastal plain in the northern part of the state of Veracruz (Figure 1).Both are perennial rivers with high discharges arising in the Eastern Sierra Madre (more specifically the Sierra Norte de Puebla) and flowing through a region of hot humid and sub-humid climate with abundant rainfall throughout the year, particularly in summer.Discharges of both rivers are affected by the impact of northern winds and hurricanes.
The Tecolutla drainage includes the major tributaries Necaxa, Tenango, Laxaxalpan, Zempoala, Apulco and Chichicatzapa rivers within its total drainage area of 7,903 km 2 to discharge a mean of 6,885 million m 3 , into the Gulf of Mexico at Barra de Tecolutla (CONAGUA 2007).The mean air annual temperature is 21-26°C, total annual rainfall ranges from 1,200 to over 4,000 mm, and the rate of evaporation is 1,064-1,420 mm (Sanchez and De la Lanza 1996).The vegetation cover is semi-deciduous tropical forest, mangrove, halophytic vegetation and palm grove (Arriaga et al. 2002).Tecolutla River is listed by CONABIO (Arriaga et al. 2002) as a priority hydrologic region since it flows through an area of high biodiversity, and is catalogued as threatened because it supports diverse land uses (agriculture, livestock, fishing and tourism).
Tuxpam River watershed covers a surface area of 5,899 km 2 and has a mean annual flow of 2,580 million m 3 (CONAGUA 2007).Its principal headwater known as Río Pantepec is joined by several tributaries before reaching the coastal plain named Tuxpam to discharge into the Gulf of Mexico at Barra de Tuxpam.Human activities in this basin include livestock, agriculture, fishing and industry.There is a power plant in Barra de Tuxpan, and a shipyard and the seaport of Tuxpam are located near the river's mouth.

Material and Methods
A total of 11 sites in the Tuxpam River (Tx) and nine in the Tecolutla River (Tc) were surveyed (Figure 1, Annex 1).Collection of aquatic mollusks was made during three different periods of the year: the wet season (August 2007 in Tc, September 2007 in Tx), the northern winds and hurricane season (December 2007 in Tc, January 2008 in Tx) and the dry season (March 2008in Tc, April 2008 in Tx).Surveys covered the main water-courses of the basins including their upper, middle and lower reaches.Different collection methods were used depending on stream bed characteristics and presence/absence of aquatic vegetation (Wetzel and Likens 2000): collection within 1-m 2 quadrats, sieving of sediments, 1 m 2base Surber net, and collection among roots, stems and foliage in areas of the banks with aquatic vegetation.Specimens were fixed in 70% ethanol.
Environmental parameters were measured in situ at all study sites using a Quanta multiparametric sonde: water temperature (°C), turbidity (NTU), salinity (PSU) dissolved oxygen (mg/L) and pH.A General Oceanic flow meter was used to record current velocity (m/sec) in transects across the sampled area, while altitude (masl) and geographic coordinates were determined with a Sport Trak Magellan GPS.
Two 500-mL samples of water from each site were transported in acid washed glass containers at 4°C to the laboratory in order to evaluate water quality parameters.Two sediment samples from each site were collected in 1000-mL flasks for subsequent sediment and organic matter analyses and for specimen collection.
Sediment samples were dried at 60°C for 24 h and processed for analysis according to EPA (1986) procedures.The mesh size (mm) of sieves was: 4. 76, 3.36, 2.38, 2, 1.68, 1.41, 0.841, 0.5, 0.42, 0.297, 0.25, 0.21, 0.177, 0.149 and 0.106.The variable φ (-Ln 2 of the diameter of particle) was plotted against the cumulative weight of materials retained in each sieve in order to obtain the mean particle size and percentages of each soil texture according to the Wentworth scale.Organic matter was quantified using the Walkey and Black method (NOM-021-RECNAT 2000).
Separate sediment samples for specimen collection were sieved and examined with a Zeiss stereoscopic microscope.The specimens obtained were separated and fixed in 70% ethanol.
Importance value index (IVI) was determined according to Krebs (1989) for the whole river, taking into account the frequency of occurrence (proportion of times that a species occurs in all the samples taken) and relative abundances (proportion of specimens for each species in the total specimens collected) of all mollusk species for each river: IVI= %FO + relative Ab, where: %FO = Percent frequency of occurrence and relative Ab = Relative abundance.
Zones and environmental gradients along each river were visualized and characterized by principal component analysis (PCA) using a correlation matrix of the log transformed (x+1) values of physicochemical factors, water quality parameters, organic matter and mean sediment particle size at each of the sites during the three study seasons.Canonical correspondence analysis (CCA) was used to examine the correlation of mollusk species with habitat conditions in each river, using a correlation matrix of the log (x+1) transformed values of in situ recorded factors, water quality, sediment analysis and organic matter, and a second matrix with the relative abundance of each mollusk species, transforming values to square roots.Statistical analyses were performed with XL-STAT v. 2008software (XL-STAT 2008).

Environmental characterization of rivers Tecolutla River
PCA revealed an environmental gradient with spatial and seasonal variations.The ordination biplot (Figure 2) shows the first two axes which together accounted for 51.43% of the total variance (PC1: 34.21% and PC2: 17.22%).Three clusters of sites corresponding to each of the study seasons are apparent.In August (wet season) there are high levels of total coliforms, nutrients (nitrate, nitrite, orthophosphate, P T ), suspended solids and turbidity.No marked spatial variations occurred in this month, but at sites Tc9 and Tc8 the highest content of coliforms and nutrients was recorded, while sites Tc1 to Tc6, located at higher altitudes above sea level, showed higher levels of orthophosphate, P T and turbidity.In December (the cold, northern winds season), there was a wider scatter of sites expressed as spatial variation in the biplot (Figure 2), as a result of the higher variation of measured parameters.Tc8 and Tc9, influenced by sea water, were highest in salinity, total dissolved solids, chloride and sulfate.Tc1 to Tc7, on the other hand, were characterized by higher flow velocity, pH, N T , color, and organic matter content as well as larger particle size.In March (dry season), the highest levels of hardness, alkalinity, BOD 5 , ammonia and dissolved oxygen were recorded.Spatial variations occurred in this season.Tc9 was found to be the most different site due to high salinity, total dissolved solids, sulfate and chloride, while at site Tc8, the highest content of ammonia was recorded.PCA differentiated the marine influence (estuarine zone Tc8 and Tc9 in December and March, Tc9 in August) from the freshwater zone (Tc1 to Tc7), and revealed the seasonal effect of torrential rains when the salinity was lower and almost all the river showed environmental homogeneity.

Tuxpam River
The first two PCA components accounted for 44.27% of the total variance (PC1: 27.43% and PC2: 16.84%).The biplot of these components (Figure 3) show the position of sites arrayed as environmental gradients.Tx8 to Tx11 in January and April and Tx11 in September were characterized by the highest levels of salinity, chloride, sulfate and total dissolved solids, making evident the reach with marine influence.In January (the cold, northern winds season), the upper reaches (Tx1 to Tx7) showed the highest flow velocity, alkalinity, dissolved oxygen and total coliforms values.In April (dry season), Tx1 to Tx6 were high in total coliforms and organic matter content, and Tx7 in BOD 5 , hardness, color, turbidity, N T and P T .In September (wet season), environmental conditions were uniform throughout the river.This season was characterized by peak of water temperatures and high nutrient (nitrite, nitrate, orthophosphate) and organic matter content.Only one site (Tx11) differed from the rest of the studied localities due to its marine influence (chloride, salinity, total dissolved solids), as well as the high levels of ammonia and NT.According to the results of PCA (Figure 3), the estuarine (Tx8 to Tx11) and freshwater (Tx1 to Tx7) zones were differentiated, as well as the seasonal dynamic of the system.

Mollusk species and patterns of distribution
Nine mollusk species were collected in the Tecolutla and 11 in the Tuxpam basins (Table 1).Three alien species occurred in both rivers: two gastropods (Tarebia granifera and Melanoides tuberculata) and one bivalve (Corbicula fluminea).The species collected showed marked differences in their pattern of distribution along the rivers.
Distribution of species in Tecolutla and Tuxpam rivers are shown in Table 2.In Tecolutla River, T. granifera and C. fluminea were widely distributed in the freshwater zone, from Tc1-8.M. tuberculata was found only in the upper reaches from Tc1-3.Haitia mexicana is present at Tc4 and Tc5.Brachidontes exustus and Neritina virginea are restricted to the lower reaches near the river mouth (Tc8).Species with a more limited distribution were Pomacea flagellata, in the upper reaches at Tc1, and Succinea sp. and Biomphalaria sp. in the upper Mollusk species showed marked variations in densities and dominance.In the Tecolutla River, T. granifera attained mean densities > 100 indivuals/m 2 .The species C. fluminea, N. virginea, Haitia mexicana and Melanoides tuberculata had mean densities ranging from 1 to 100 individuals/m 2 .Brachidontes exustus, P. flagellata, Succinea sp. and Biomphalaria sp.all had mean densities ≤1 individual/m 2 (Figure 4).In the Tuxpam River, T. granifera and Brachidontes exustus reached mean densities of 100 to 1,000 individuals/m 2 .N. virginea, H. mexicana, C. fluminea, Micromenetus dilatatus and Melanoides tuberculata had mean densities of 10-100 individuals/m 2 while mean values for Hebetancylus excentricus and P. flagellata ranged from 1 to 10 individuals/m 2 .Melampus coffeus and O. palmula had the lowest mean densities, with ≤ 1 individual/m 2 (Figure 4).
Very marked spatial and seasonal variations occurred in mean densities of mollusk species.In the Tecolutla River, the lowest densities occurred in December and the highest in March (Figure 5a).In the freshwater zone, Tc5 and Tc7 had the highest densities in all three study seasons.Seasonal density changes produced a  5b).In the middle reaches, in spite of seasonal variations, the species with the highest densities was T. granifera, while in the lower reaches N. virginea had the highest densities in all three seasons (Figure 5b).
In the Tuxpam River, the lowest densities occurred in January (Figure 6a) while peak values were attained in April.In the freshwater zone, Tx3 to Tx6 showed the highest densities.In spite of seasonal variations in population densities, there was no change in the higher ratios at which T. granifera was present in the freshwater zone and the species N. virginea and B. exustus in the estuarine zone (Figure 6b).
Importance value indicates that T. granifera is the dominant species in both rivers, with index values of 128 and 96 in the Tecolutla and Tuxpam rivers, respectively (Figures 7a and 7b).In the Tecolutla River, T. granifera, C. fluminea and M. tuberculata (all three non-native species), account for a cumulative index value of 179 out of a possible 200.Importance values below 7 were obtained for all other mollusk in this river (Figure 7a).In the Tuxpam River, T. granifera, two brackish species (N.virginea, B. exustus) and two freshwater species (C.fluminea, Haitia mexicana) have index values ranging from 10 to 26.Species with IVI< 10 include Micromenetus dilatatus, P. flagellata, Hebetancylus excentricus, Melanoides tuberculata and Melampus coffeus (Figure 7b).

Relation of environmental parameters, substratum characteristics and mollusk species
The first two components in the CCA of the Tecolutla River account for 81.30% of the explained variance (Axis 1: 64.93% and Axis 2: 16.37%).The biplots show the relations of mollusk species and sites, as well as water quality conditions and type of substratum.Three groups of species can be seen in the biplot in Figure 8a.The smallest one is formed by N. virginea and B. exustus, which reached their highest densities at Tc9 in December and March.Similarly, Figure 8b shows these species are closely correlated with high salinity, total dissolved solids, chloride, sulfate and hardness.In the upper left quadrant are T. granifera, Haitia mexicana and C. fluminea, which attained maximum densities in Tc4, Tc5, Tc6 to Tc7 in March and in Tc3 and Tc7 in August.In both seasons, these sites had the highest levels of coliforms, nitrite, nitrate, ammonia, BOD 5 , color, alkalinity, current velocity and water temperature, with fine and very fine sandy substrata.On the right side of the biplot are P. flagellata, which is closely correlated with Tc1 in March, a site in the upper reaches with a substratum of gravel and very coarse sand; Melanoides tuberculata, related with Tc1 and Tc2 in August and with fine gravel and very fine sandy substrata; and Biomphalaria sp. and Succinea sp., occurring at Tc2 in March and related with fine gravel and coarse sandy substrata; seasons and conditions that are exactly opposite those of the first group of species.CCA showed species assemblages, their divergent patterns of distribution and most importantly their relation with habitat conditions.
In the CCA of Tuxpam River, the first two components accounted for 75.89% of the explained variance (Axis 1: 55.37% and Axis 2: 20.53%).Biplots show the species separated into two groups.The group on the right side includes B. exustus, Micromenetus dilatatus, Melampus coffeus and N. virginea.These species were present at Tc7, Tc8, Tc9, Tc10 and Tc11 in all study seasons (Figure 9a) and are related with high levels of total dissolved solids, salinity, chloride, ammonia, sulfate, N T , P T and BOD 5 , and very fine sandy and coarse to very fine silty substrata (Figure 9b).On the left side of the biplot are T. granifera, C. fluminea, Hebetancylus excentricus, Haitia mexicana, P. flagellata and Melanoides tuberculata.These species are distributed in the freshwater zone (Tx1-6) with high levels of nitrite, nitrate, orthophosphate, alkalinity, pH, turbidity, water temperature and coliforms, and also at higher altitudes where the substratum is predominantly coarser materials such as pebbles, gravel and coarse to fine sand (Figures 9a and 9b).CCA identified two groups of mollusks with entirely different distributions as well as the relation of each group with contrasting environmental conditions and substrata.

Discussion
Water quality characteristics indicate that the Tuxpam and Tecolutla rivers receive inputs of organic matter, nutrients (orthophosphate, nitrogenous compounds) and coliform bacteria.Both rivers are located in a region with several environmental disturbances by human activities (Ortíz-Lozano et al. 2005), which contribute to the inputs of diverse compounds originating in wastewater discharges, leaching materials and basin runoff.Despite the nutrient and organic matter enrichment found by this study, the water of these rivers has been rated good and excellent by the CONAGUA (2007) monitoring network.This agency, however, uses only three parameters (BOD 5 , Chemical oxygen demand and total suspended solids) to rate water quality.
Our results, on the other hand, indicate both rivers sustain nutrient enrichment, although they are less degraded than the Coatzacoalcos and Pánuco rivers which are also located in the Gulf of Mexico coast area (CONAGUA 2007).
The Tuxpam and Tecolutla rivers show important differences in the estuarine zone due to their morphology, freshwater discharges and seasonal variations.Mean annual flow is 2,580 million m 3 in the former and 6,885 million m 3 in the latter (CONAGUA 2007).As a result of these differences in stream flow, the estuarine zone is larger in the Tuxpam River than in the Tecolutla River.In our study PCA allowed identification of environmental gradients along each river that are associated with changes in altitude, flow velocity, dissolved solids, organic matter, nutrients and salinity, that differentiate two zones: freshwater and estuarine.The rivers undergo seasonal variations: during the dry season, peak levels of alkalinity and BOD 5 occurred and sea front influence produced a marked increase in salinity in the estuarine zone.During the wet and the northerly wind seasons, and favored by the effect of hurricanes, both rivers showed a tendency to environmental homogeneity.Higher levels of nutrients (nitrate, nitrite, orthophosphate, P T ) were present and the estuarine zone decreased in extent.Turbidity rose while salinity fell markedly.Studies showing the effect of seasonal rainfall and drought in watercourses in intertropical areas are scarce.Brodie and Mitchell (2005) found higher nutrient loads during rain episodes and in correlation with land use for agriculture in tropical river basins in Australia.Agriculture is a major land use in the Tuxpam and Tecolutla basins and, along with discharges from small human settlements near the river, may explain nutrient inputs, particularly during the wet season.Mathooko and Mavuti (1992) state that hurricanes and rainy seasons not only alter the volume of stream flow, they also bring sediment loads and increase benthic organisms drift, which affect their population densities.In our study in both rivers density values were depleted during the months of rainy, northern winds and hurricanes seasons as opposed to the dry season when densities rose.
Aquatic mollusks of the Tuxpam and Tecolutla rivers showed strong dominance of two exotic species: T. granifera and C. fluminea.Only at one of nine Tecolutla study sites (Tc9) and in four of 11 Tuxpam study sites (Tx8 to Tx11) were both absent.The only areas not colonized by these alien species were the estuarine zones of the rivers, where N. virginea and B. exustus were dominant.In both freshwater courses these alien species reached higher densities than others.Melanoides tuberculata, another non-native species, had a limited distribution in both rivers as well as lower densities than others.
Contreras-Arquieta (2000), Naranjo (2003) and Thompson (2008) state that knowledge of aquatic mollusks is in an initial stage in Mexico.In the Tuxpam and Tecolutla rivers there are no historical records of the native aquatic mollusks.The number of mollusk species found in our study in both rivers is low, compared to numbers in more extensively studied areas of Mexico, such as Cuatro Ciénegas, Coahuila with 13 species; (Contreras-Arquieta 1998) and El Edén, Quintana Roo with 11 species (Cózatl-Manzano and Naranjo-García 2007).Several authors (Davis et al. 2000;Mack and Lonsdale 2001;Sax et al. 2002;Sousa et al. 2008), mention that introduced species are able to displace native species through different mechanisms, including competitive exclusion, introduction of disease and even habitat change.In the case of the Tuxpam and Tecolutla rivers the absence of historical malacological fauna records prevents comparison of the native mollusk assemblages before and after the introduction of alien species.
The gastropod T. granifera is native to India, Southeast Asia, the Philippines, Japan and Hawaii (Abbot 1952).Its introduction to the United States has been documented by Karatayev et al. (2007), who point out that the aquarium trade recognizes its presence in Texas since 1935.In our study, the wide distribution of this species in the Tuxpam and Tecolutla rivers shows it has been able to establish and is very successful in freshwater zones of both rivers, although the date of its introduction remains unknown.In this respect, only Naranjo and Olivera-Carrasco (2007) record its occurrence in Lake Catemaco, Veracruz, in the south of the Gulf of Mexico.Karatayev et al. (2007) report that this species tolerates a wide range of temperatures (10 to 38°C).Water temperatures in the Tuxpam and Tecolutla rivers (18°C min to 30°C max) are well within this range.Pointier and Jourdane (2000) stress the damage T. granifera has caused, including probable irreversible eradication of several native mollusks in Cuba (Pachychilus violaceus and various species of the genus Biomphalaria), and endangerment of others (Hemisinus brevis and H. cubanianus).The extremely low densities of other mollusks in the Tuxpam and Tecolutla rivers are in agreement with statements by Karatayev et al. (2008) who suggested this is one of the major impacts of T. granifera on the indigenous mollusk fauna and is a preliminary stage in the eradication of native species.Pointier et al. (1998) found that the introduction of thiarids to eradicate schistosomiasis in Martinique island affected two species of Biomphalaria, but impacts were negligible on neritids.This is also in agreement with our findings in the Tuxpam and Tecolutla rivers since N. virginea is the dominant species in the estuarine zone where T. granifera has not become established.Pointier et al. (1998) state that the attributes of T. granifera favoring its rapid spread include parthenogenesis, extremely high densities, ability to support faster currents and dislodgement, and use of the entire stream bed.In our study the CCA indicates that T. granifera in the Tuxpam and Tecolutla rivers is able to establish in a wide variety of fine to coarse substrata under nutrient-rich conditions (particularly nitrogen) and with high BOD 5 .These capabilities make T. granifera a successful invasive species and provide an explanation for its pattern of distribution and extremely high densities in both rivers.
The bivalve C. fluminea is native to Asia (China, Korea and southeastern Russia) (McMahon 1999;Mouthon 2001) and colonizes both fresh and brackish waters (salinity <10 PSU) (Sousa et al. 2009).It was possibly introduced for the first time in North America before 1924 in British Columbia, Canada (Naranjo and Olivera-Carrasco 2007) and in 1938 to the US, most probably by Chinese immigrants who use this species as food (McMahon 1999).In Mexico it is known from the northern part of the country on both the Gulf and Pacific slopes.In the south its first record dates back to 1992 in Catemaco, Veracruz (Naranjo and Olivera-Carrasco 2007).In the Tecolutla and Tuxpam rivers, C. fluminea showed different patterns of distribution and abundance, ranking second and fourth in IVI, respectively.It had a more limited distribution in Tuxpam River than in Tecolutla River, and was found in the estuarine zone in the Tecolutla (at Tx7).Sousa et al. (2008) say the attributes of C. fluminea favoring its role as an invasive species include rapid growth, early sexual maturity, short lifespan, high fecundity and its association with diverse human activities.However, Souza et al. (2008), also mention this species was adversely affected in sites with inputs of organic contaminants, while De la Hoz (2008) found it was adversely affected by drops in the water level.Likewise, Sousa et al. (2008) report C. fluminea is successful in environments without considerable water level fluctuations.In our study, the Tuxpam and Tecolutla rivers were found to receive nutrient and organic matter inputs and to fluctuate in water level as a result of floods and torrential flows due to rainfall of northern winds and hurricane seasons, as opposed to the dry season when flow decreases.These factors are perhaps responsible for the low densities and limited distribution of C. fluminea in these rivers, despite reports by other authors of its invasive capability (Mouthon 2001;Mouhton and Parghentanian 2004).Phelps (1994) states that C. fluminea may experience population declines after reaching the lag period of invasion.This species may therefore be undergoing a population decline since records of its presence in the state of Veracruz date back to 1992 (Naranjo and Olivera-Carrasco 2007).This, along with unfavorable abiotic factors, may explain the low densities and the relative limited distribution of this species in the Tuxpam and Tecolutla rivers.Type of substratum is another factor that has been examined in regard to the establishment of C. fluminea.It has been found primarily in fine to coarse sandy substrata (Cordeiro and MacWilliams 1999;Paunović et al. 2007;Sousa et al. 2008), although De la Hoz (2008) reports its occurrence in a muddy substratum.In the Tuxpam and Tecolutla rivers it was present in substrata ranging from coarse sand to fine-grained sediment.
Negative impacts due to high densities of C. fluminea include biofouling (especially in power facilities), occlusion of small diameter pipelines, irrigation canals and domestic water supply systems.It is also able to alter the substratum and affect native species (Paunović et al. 2007).There are no records of biofouling by this species in our study area, but a potential risk exists, particularly in the Tuxpam River in which C. fluminea has colonized the estuarine zone where a seaport and a power plant are located.
It has been reported that when T. granifera is introduced in places previously occupied by M. tuberculata, the former rapidly outnumbers the latter in terms of density (Pointier et al. 1998).Contreras-Arquieta et al. (1995) record presence of M. tuberculata in both the Tuxpam and Tecolutla rivers, but did not report T. granifera.Thus, it appears that in these rivers T. granifera is an alien species that may have recently established in these rivers that are experiencing changes in species invasion processes that are resulting in T. granifera being favored over the two other non-indigenous species.In agreement with Pointier et al. (1998) it appears from our study that T. granifera attains its higher population densities through competitive advantage.
In conclusion, this study lends further support to earlier evidence that T. granifera is a very successful alien species.It has become the dominant mollusk in both the Tuxpam and Tecolutla rivers.C. fluminea, is the other main component in the benthic freshwater zones of both rivers.Environmental conditions prevailing in the Tuxpam and Tecolutla rivers are suitable for T. granifera and C. fluminea species invaders, such as water temperature regimes, substratum availability and high levels of nutrients which increase food availability.Pronounced depletions in density during the rainy and northern winds and hurricanes seasons could indicate that the mollusks are removed by the current during stormy torrential floods.Tarebia granifera has capabilities for recolonization after torrential rains and their populations respond quickly during the dry season when nutrients favor the food availability, and in such situations may be more successful than C. fluminea and M. tuberculata.All three alien species, however, constitute a serious threat to native aquatic mollusks, since they have become the dominant components of the malacological community in both these rivers, where native species have been localized to narrow habitats and occur in relatively low densities.

Figure 1 .
Figure 1.Location of study sites in the Tecolutla and Tuxpam rivers.Tx followed by a number indicates the number of the study site in the Tuxpam river.Tc followed by a number indicates the number of the study site in the Tecolutla river.Geographic coordinates are in Annex 1

Figure 2 .
Figure 2. PCA biplot of Tecolutla River.Vectors represent environmental factors.Codes as in Annex 2. Numbers after Tc= indicate the number of study site and letters after number indicate month of study (Mar= March, Dec= December, Au= August)

Figure 4 .
Figure 4. Mean density of mollusk species in Tecolutla and Tuxpam rivers.The bar represents SD

Figure 5 .
Figure 5. Absolute and relative density of the aquatic mollusk species in Tecolutla River

Figure 7 .
Figure 7. Importance value index of mollusks a) Tecolutla River and b) Tuxpam River

Figure 8 .Figure 9 .
Figure 8. Biplot of CCA of Tecolutla River.a) study sites and species and b) factors and species.Numbers after Tc indicate the number of study site, letters after number indicate month of study (Mar= March, Dec= December, Au= August).See Annex 2 for the code of abbreviations of environmental factors and species

Table 2 .
Records of mollusks species at study sites of Tecolutla (Tc) and Tuxpam (Tx) rivers during study period (for locations of study sites see Figure1and Annex 1) Geographic coordinates of study sites in Tuxpan and Tecolutla riversCode of abbreviations: species, environmental factors and sediment type in the ACP and CCA biplots