Variation in the distribution of Corbicula species (Mollusca: Bivalvia: Corbiculidae) after 25 years of its introduction in the Río de la Plata, Argentina

In 1981, Corbicula fluminea (Müller, 1774) and C. largillierti (Philippi, 1844) were recorded at the Río de la Plata River. During the 1980’s, C. largillierti was found on a continuous fringe in the shore of that river, while C. fluminea was only found North of Buenos Aires’ harbor. Ten years later, C. fluminea spread to Punta Indio, while C. largillierti remained restricted to tributary streams. The density and size frequency distribution of both species were compared with previous data from samples performed in the same area in 1985–1989. The aim of this research was to determine whether the densities of both species decreased; whether C. fluminea is still more abundant than C. largillierti; and whether the size frequency distributions indicate different population structures from those observed in the end of the 1980’s. Nine localities from the river shore were sampled, and each individual was identified and measured. The size of C. fluminea varied between 2 and 39 mm, showing the highest frequency of individuals between 20 and 25 mm; furthermore the density decreased 90% in 2015/2016. No specimens of C. largillierti were found. In conclusion, the populations of Corbicula inhabiting the Río de la Plata River are retracting.


INTRODUCTION
The Asiatic clam, Corbicula fluminea (Müller, 1774) naturally inhabits freshwater bodies from Southeast Asia, Africa, India, Pacific Islands and Australia (McMahon 1983(McMahon , 2000)).This species arrived in North America in 1920 (McMahon 2000) causing important environmental and economic problems, which resulted from its ability to reproduce quickly, and its capacity to adapt to different ecological conditions (Britton andMorton 1979, McMahon 1983).The first report of the presence of C. fluminea in Europe was in 1981, based on findings on the shores of the Dordogne River in France and the Tajo River in Portugal (Mouthon 1981).
In 1979, Ituarte (1981) mentioned the genus Corbicula for the first time in the Río de la Plata River shore and described that Corbicula spp.occupied a continuous fringe along its margins, from headwaters (34°29'S, 58°13'W) to Magdalena's beach (35°1.813'S,57°30.317'W),being absent from its tributary streams.That distribution pertained C. largillierti, while C. fluminea was only found at the north of the Buenos Aires harbor (34°29'S, 58°13'W) (Ituarte 1981(Ituarte , 1985)).During 1982, C. fluminea settled in Atalaya, near the southern limit of the species in the river's shore (Ituarte 1985).In 1985, speciemens of Corbicula were found for the first time in tributary streams of Río de la Plata River (Darrigran 1992a).At the end of the 1980's, almost a decade after the first record, the distribution of the genus changed (Fig. 1): C. fluminea inhabited a continuous fringe on the shore, from the headwaters to Punta Indio, where it was sporadically found; whereas C. largillierti was only found from La Balandra to Magdalena (Darrigran 1992b).
Corbicula fluminea reached high densities when compared to the native bivalves, approximately 1,200 ind./m 2 in the Río de la Plata River (Darrigran 1991), or between 4,000 and 5,000 ind./m 2 in Río Grande do Sul, Brazil (Santos et al. 2012).Despite this, there were only a few reports of the species' economic impact in South America during 1998, at a hydroelectric power station located in Rio Grande do Sul (macrofouling in heat exchangers) (Santos et al. 2012), contrasting with the impact that species of Corbicula cause in North America (see McMahon 1983McMahon , 2000)).
The aim of the present work was to ascertain the distribution of Corbicula in the Argentinian shore of the Río de la Plata River.The following hypotheses were tested: 1) the densities of both species have decreased; 2) C. fluminea is found in higher frequencies than C. largillierti; 3) the size frequency distributions indicate different population structures than those found in the end of the 1980's.

MATERIAL AND METHODS
Samples were taken from nine localities along the Argentinian shore of Río de la Plata River, between June 2015 and June 2016, during low tides (Table 1, Fig. 1).In each locality, a sampling area was delimited and the clams were manually collected.Depending on the type of environment, the area was delimited by a cylindrical sampler of 0.05 m 2 , which was pushed into the sediment up to 0.10 m depth; or by a square of 1 m 2 .From each site, 2 to 4 samples were taken (Table 1).The sediment was sieved through a mesh of 1mm of pore diameter in the field and the specimens were separated.In the laboratory, each individual was identified and measured (maximum length) using a digital caliper, following Darrigran and Maroñas (1989).In the present work, size frequency distributions in the Argentinian shore of the Río de la Plata River were compared between the late 1980's (Darrigran and Maroñas 1989) and this study.Density (D = number of individuals/m 2 ) and size frequency distribution were plotted and analyzed for both species.Frequency percentages were calculated by: F = m.100/M,where m is the number of samples with presence of specimens, and M is the total number of samples collected.Due to the lack of the assumptions of the Student t test, a non-parametric statistical analysis of Mann-Whitney U test was performed to compare the densities values obtained between 1985-1989 for the same localities (Darrigran 1991(Darrigran , 1992b)).

RESULTS
Corbicula fluminea was present in seven sampling sites (Table 1).Its mean population density varied between 0 and 52.23 ind/m 2 .Figure 2 shows the variation between the density values.The frequency value obtained for C. fluminea was 52%.No live specimens of C. largillierti were found during the sampling.
Through a non-parametric statistical analysis Mann-Whitney U test (U = 1097, p = 0.000039), significant differences were found when comparing the densities obtained during this study with those previously registered for the same localities (Darrigran 1992b).
Figure 3 compares the size distribution values published by Darrigran and Maroñas (1989) with those from this work (2015/2016).The size frequency for C. fluminea varied between 2 and 39 mm, showing the highest frequency of individuals between 20 and 25 mm in Magdalena's beach.

DISCUSSION
The comparison between the density values of the species of Corbicula obtained during this study and those recorded 25 years ago in the same samples sites, leads to the conclusion that these species are retracting from the shores of Río de la Plata River'.While in the 80's C. fluminea and C largillierti showed frequency values of 75% and 37% respectively, the frequency value obtained for C. fluminea in this study was 52%, and no individuals of C. largillierti were found alive.Significant differences between densities recorded in 2015-2016 with those registered in 1990 for the same localities allow us to state that the density of the species has decreased by about 90%.
The decreasing density (e.g. C. fluminea) or the displacement of a species from an environment (e.g. C. largillierti), are frequent phenomena and their causes are very difficult to ascertain.Similar examples are the mass mortality of the native species Amarilladesma mactroides (Reeve, 1854) in South America (Fiori and Cazzaniga 1999) or the invasive species, Limnoperna fortunei (Dunker, 1857), in Embalse Río Tercero 32°13'S, 64°26'W), Argentina (Mariñelarena et al. 2016).Also, the reduction and displacement of the species of Corbicula in the Argentinian shore of Río de la Plata River could be associated  with the tourist activities there.However, tourism in the area has not changed much since the introduction of the species.Tourism and related outdoor recreation involve the frequent congregation of people, vehicles and vessels from geographically diverse areas.These are considered major pathways for the spread of non-native species (Anderson et al. 2015).For this reason, the tourism-related activities in the Argentinian shore of Río de la Plata River are more likely to spread non-native species like C. fluminea and C. largillierti, than they are to control their populations.Apparently, C. largillierti is being displaced to tributary streams, where it lives in sympatry with C. fluminea.
It is, however, found in lower densities than C. fluminea.This preference C. largillierti has for streams and other lotic environments, was detected by Reyna et al. (2013), in the Suquía River basin, Argentina (31°32'S, 64°10'W).Also, it should be noted that there are environmental differences between the Río de la Plata and the tributary streams.Cortelezzi et al. (2015) described the Argentinian pampean region streams as having: low slope (<1%), slow speed of flow, high turbidity, abundant organic matter, and elevated content of suspended solids.Their streambed consists of fine sediments (primarily silt and clay) and few stones or pebbles.Along their course dense macrophyte stands and periphyton growth are observed.According to Darrigran (1999) there are large areas of the Río de la Plata shore with different types of substrate: "soft" substrate: sand beach, stones and aquatic vegetation, "hard" substrate: "caliche" beach.C. fluminea is found in soft sediments and stones.In addition, the margins of the Rio de la Plata are influenced by an unstable and unpredictable hydrological regimen due to the action of daily tides of unequal levels.Also, the wind causes significant disturbances in the daily tides (Darrigran 2002a).
In the Argentinian shore of the Río de la Plata River, the size of C. fluminea varied between 2 and 39 mm.The highest frequency of individuals between 20 and 25 mm was found in Magdalena's beach, which is also the locality with the highest density of individuals in the present work (Fig. 2).The sizes observed in Punta Blanca by Darrigran and Maroñas (1989) varied between 1 and 36 mm, with 2 mm individuals being more abundant (about the 42% of the specimens).The population of C. fluminea in the late 1980's was increasing, right after its introduction into Río de la Plata River, but now, 25 years later, the population is in lower densities and individuals are on average smaller (less than 20 mm length), indicating that the population is shrinking.It is possible that a decrease in population density could be caused by environmental changes such as extreme variations in the water level (Paschoal et al. 2015).However, such changes are temporal and the population usually recovers when conditions are reversed.Two changes have been observed in the Corbicula populations of the Río de la Plata River's shore: 1) the frequency and density of individuals have decreased and 2) C. largillierti has been displaced.
The first change could be the expected outcome of a threephase process of invasion (Darrigran and Damborenea 2009).According to this model, during the first invasion phase, a species establishes in the new environment and its population grows slowly.Then, in the second phase ("panic" phase), the invasive population grows quickly, exceeding the carrying capacity of the environment.In the last phase, the population stops growing exponentially and the density of individuals declines to a level that does not exceed the environment's carrying capacity, reaching a state of "oscillatory equilibrium" around the level mentioned before (Darrigran and Guimarey 2011).
The influence of C. fluminea on benthic species was investigated by Ilarri et al. (2014).C. fluminea often plays the role of ecosystem engineer, generally causing physical disruptions in the ecosystems where the species is established, by changing the structure of the communities of benthic macroinvertebrates (Linares et al. 2017).It is possible that C. fluminea has displaced C. largillierti from the Argentinian shore of Río de la Plata River (Darrigran 2002b), since C. largillierti has not become established there (Darrigran 1992b, Ituarte 1994).Darrigran (1991) analyzed the temporal variation in the densities of both species in two areas of sympatry, in two localities of Río de la Plata River: Punta Blanca (34°56'S, 57°40'W) and Atalaya Beach (35°00'S, 57°33'W).In this result, C. largillierti was rare and was only found in restricted areas of the Río de la Plata River basin (Darrigran 2002b).The same can happen in the Suquía River basin.While C. fluminea is continually expanding geographically, C. largillierti is not (Reyna et al. 2013).About ten years after the ingression and dispersion of the two species of Corbicula to the Río de la Plata River area, other invasive species have arrived in Río de la Plata River.The population of L. fortunei, introduced in the early 1990's, has increased exponentially in density (reaching a density of 150,000 ind/m 2 ), invading the new environment and causing the displacement of native and non-native species (Darrigran andPastorino 1995, Pastorino et al. 1993).Probably the presence of this aggressive invasive species in the same environment as C. fluminea and C. largillierti is a factor that has altered the presence of both species.This is similar to the impact produced by L. fortunei on other benthic species (Darrigran et al. 1998, Darrigran and Ezcurra de Drago 2000, Darrigran 2002b, Darrigran and Damborenea 2011, Sylvester and Sardiña 2015).