An introduced polychaete in South America: ecologic affinities of Manayunkia speciosa (Polychaeta, Sabellidae) and the oligochaetes of Uruguay River, Argentina

ARMENDARIZ, L.; SPACCESI, F. & RODRIGUES CAPÍTULO, A.

Diversity of Ecosystems

In Tech

Año: 2012; p. 73 - 94

South America is characterized by the vast extensions of its river basins, with Amazonas, Orinoco and Paraná rivers being the most important. Due to the geologic structure of the continent, the rivers with the largest basins carrying the greatest flow are those which drain in the Atlantic Ocean, being generally long, abundant and torrential. Uruguay River and its alluvial valley stand out forming an ecosystem with relatively important and extensive wetlands. The gallery forest is connected to the drainage system forming a narrow but densely populated strip with fairly tall species of “ibirapita”, “lapacho rosado”, “azota caballo” and “pitanga”. Towards the lower Uruguay there is an almost absolute predominance of the “sarandí blanco” in the riparian zone of Argentina and in the islands. This riverside vegetation plays an important role in the intake of thick particulate allochthonous material which is then used by the benthic organisms associated to the riverside, determining a fauna characteristic of these environments. The features of the physical habitat, the water quality, and the availability and quality of the food determine the presence and the abundance of the benthic organisms. Therefore, it is possible to distinguish a coastal benthos – with habitat heterogeneity, greater diversity, thick particulate organic matter – and a deep benthos – with a lesser quantity of available habitats, moderate diversity and thin particulate organic matter. In fluvial environments the current and the type of substratum such as gravel, sand, lime and clay, together with the riverside vegetation also determine the benthos structure. The introduction of species is one of the processes that produce major alterations in the biodiversity of ecosystems. Even though the biologic invasions are not a phenomenon caused only by human activity, the number of species involved and the frequency of their relocation has grown enormously as a result of commercial activity and transportation. The continental aquatic environments are highly susceptible to the accidental or deliberate introduction of exotic species. The transfer of these exotic species occurs through a wide variety of ways and means, for example, in maritime transportation, in the ballast water of ships. Among the benthic organisms, the polychaeta constitute an important group with several invasive species recorded, many of which have enlarged their geographic distribution due to human activity. Manayunkia speciosa Leidy (Sabellidae, Fabriciinae) has been recently collected in Uruguay River. This small polychaete, strictly from freshwater, was firstly recorded in Schuylkill River (Pennsylvania) in 1858. Some decades later its distribution extended through the whole Nearctic Region, from the Great Lakes area to California and Oregon, and also to Alaska and Mississippi, covering the oriental and occidental lothic systems of North America and of the Mexican Gulf Coast as well. A hundred and fifty years after its first record, it is found for the first time in the Neotropical Region, enlarging its distribution to Argentina, South America. Although we still cannot determine whether its introduction has been relatively recent, we can assume that M. speciosa has entered Del Plata Basin through the ballast water of ships reaching Uruguay River through their habitual navigation route. In recent years the studies on M. speciosa have been focused in aspects of its population dynamics, reproductive biology and its role as intermediate host of Myxozoa parasites of salmonids. In relation to the zoobenthos of the Uruguay River, and due to the construction of the Salto Grande Dam in the 70´, many authors have studied its fauna as well as the physicochemical characteristics of the water and the bottom features. However, most of the information available is referred specially to the knowledge in the upper and middle Uruguay River. Due to the lack of information on the ecological requirements of M. speciosa together with the scarce knowledge on the annelid fauna of the Lower Uruguay River, we propose, in this chapter, to analyze the main ecological aspects which enabled to determine the establishment of M. speciosa with the rest of the annelids, oligochaeta and polychaeta forming the benthic ensemble of the Lower Uruguay River. The work methodology included the extraction of samples with a van Veen dredge (470 cm2). Seasonal samples, from November 2007 to March 2009, were extracted in 8 sites located between 33º 05´ S 58º 12´ W, 33º 05´ S 58º 25´ W, distinguishing the main basin and the bullrushes from the coast. Six replicas by site and by station were collected. Material fixation was realized in situ with formaldehyde 5%. The following physicochemical parameters were measured: temperature, pH, dissolved oxygen, conductivity, turbidity, depth and vegetal cover for the environment characterization. In the laboratory, the samples were washed and separated from the sediment with a 500 µm mesh sieve. They were then dyed with erythrosine B in order to enable the observation of the organisms under stereoscopic microscope when manually separated. Determinations of annelids were carried out under optical microscope according to keys and specific bibliography. Final preservation of the organisms was performed in alcohol 70%. Indirect measurements of the content of organic matter (OM) in the sample sediment collected were realized using the loss of organic matter by ignition method muffling at 500° C during 4 hrs, with a previous drying of 48 hrs at 60° C. The annelid ensemble was evaluated through the diversity index (Shannon), the evenness and the species richness. The relationships between the percentage organic matter of the sediment versus the mean density of M. speciosa and versus the mean density of the oligochaeta were analyzed. The relationship between the sampled sites and the physicochemical and environmental variables recorded was explored through the Principal Component Analysis (PCA). The relationships between the annelid abundance and the environmental variables was explored by a Canonical Correspondence Analysis (CCA) with a previous Detrended Correspondence Analysis (DCA) in order to determine if the species responded linearly to gradients or passed through some environmental optimum