Verónica Williner

Littoral Communities. Macrocrustaceans

The littoral community of lotic and lentic environments of the Middle Parana River is complex and dynamic, providing shelter to a high biological diversity and abundant populations. A numerically important group and with active participation in the community structure is that of macrocrustaceans, and, specifically, the order Decapoda, superorder Eucarida Calman 1904, subphylum Crustacea Brunnich 1772. The taxonomic unit Decapoda records more than 8,500 species, most of them restricted to marine areas (Brusca and Brusca 1990); however, some of them have successfully conquered freshwater and brackish environments. Among the latter, mangrove swamps and lenitic environments of large river floodplains provide the highest diversity and density of individuals due to their environmental heterogeneity (Bliss 1989).

Biogeography of the Freshwater Decapods in the La Plata Basin, South America

Abstract The distributions of freshwater fauna in the La Plata Basin, including decapods, are dynamic; their actual distributions depend on not only their movements but also on environmental fluctuations. The La Plata system has a diverse range of environments, which are colonized by crustaceans with complex evolutionary histories. Our aim was to elucidate the distribution patterns and relationships of current freshwater decapod fauna in terms of the different geo-climatic processes at work in the La Plata system. We recognized 13 zones based on their unique environmental characteristics. Species lists were assembled through field samples, examination of museum collections, and literature analysis. Faunistic similarity was evaluated using the Jaccard index and cluster analysis. A parsimony analysis of endemicity (PAE) was applied to determine the most parsimonious of all possible tree topologies. There were 69 species freshwater decapods analyzed in the La Plata Basin. The Mata Atlántica (MA), Paraná Superior (PS), and Uruguay Superior (US) zones are distinguished by their high species richness values, whereas the western and southern basin regions exhibit the lowest species richness values. The Paraguay-Pantanal (PP) and Paraguay-Pilcomayo-Bermejo (PPB) zones share some prawn taxa with the Amazon Basin. The eastern zones of the La Plata Basin were grouped by the presence of several species of the genera Parastacus, Aegla, and Trichodactylus. The northern, central, and western zones were characterized by species of the genera Dilocarcinus, Sylviocarcinus, and Zilchiopsis. According to the PAE analysis, two major zones were recognized in relation to the presence of freshwater decapods (eastern and western-central zones). In the western-central zones, three sub-areas were identified; one shares species with the Amazon basin the others have a relationship with the eastern and northern regions and represents a region that is strongly influenced by a temperate climate and therefore possesses diminished species richness values.

Size Selective Predation on an Invasive Bivalve, Limnoperna Fortunei (Mytilidae), by a Freshwater Crab, Zilchiopsis collastinensis (Trichodactylidae)

ABSTRACT Limnoperna fortunei, or the golden mussel, invaded South America through the Río de la Plata estuary in 1991. The size-selective predation on this bivalve by freshwater decapods crustaceans could be an important part of selective trophic behavior because shell sizes are correlated with the flesh contents and their resistance to being broken. The aim of the paper was analyze the size selective predation by the freshwater crab Zilchiopsis collastinensis Pretzmann, 1968 on different sizes of the invasive bivalve L. fortunei. We considered three combinations (AC, AD and BD) of equal numbers of three different shell lengths of mussels: A (small) = 7 to 10.99 mm, B (medium) = 11 to 14.99 mm and C= 15 to 18.99 mm and D= 19 to 22.99 mm (large). These combinations were offered to each adult female intermolt crab individually, and the number of mussels remaining was registered after one, two and 24 hours to evaluate prey-size selection. After this trial, ten female crabs were recorded preying on two different sizes of mussels (A and C) in combination (AC), and we registered the selective feeding behavior (feeding methods and time of predation) with each size of mussel. The results indicated that female crabs ate different sizes of golden mussels, showing an increased consumption of large mollusks after 24 hours, although these sizes demanded higher predation times. Large mussels were encountered at the first time (one and two hours) by the crabs, and these mussels were consumed successfully in the combination AD. These female crabs showed several alternative strategies to access the flesh and were efficient in handling large mussels, presenting an advantageous degree of plasticity for their predatory responses. L. fortunei could be a new food resource for adult Z. collastinensis females, and it could be responsible for a new interaction “native predator-invasive prey” in the system.

Feeding spectra and activity of the freshwater crab Trichodactylus kensleyi (Decapoda: Brachyura: Trichodactylidae) at La Plata basin

BackgroundIn inland water systems, it is important to characterize the trophic links in order to identify the ‘trophic species’ and, from the studies of functional diversity, understand the dynamics of matter and energy in these environments. The aim of this study is to analyze the natural diet of Trichodactylus kensleyi of subtropical rainforest streams and corroborate the temporal variation in the trophic activity during day hours.ResultsA total of 15 major taxonomic groups were recognized in gut contents. The index of relative importance identified the following main prey items in decreasing order of importance: vegetal remains, oligochaetes, chironomid larvae, and algae. A significant difference was found in the amount of full stomachs during day hours showing a less trophic activity at midday and afternoon. The index of relative importance values evidenced the consumption of different prey according to day moments. Results of the gut content indicate that T. kensleyi is an omnivorous crab like other trichodactylid species. Opportunistic behavior is revealed by the ingestion of organisms abundant in streams such as oligochaetes and chironomid larvae. The consumption of allochthonous plant debris shows the importance of this crab as shredder in subtropical streams. However, the effective assimilation of plant matter is yet unknown in trichodactylid crabs.ConclusionsThis research provides knowledge that complements previous studies about trophic relationships of trichodactylid crabs and supported the importance of T. kensleyi in the transference of energy and matter from benthic community and riparian sources to superior trophic levels using both macro- and microfauna.

Feeding of Acetes paraguayensis (Nobili) (Decapoda: Sergestidae) from the Parana River, Argentina

Feeding habits of Acetes paraguayensis (Hansen) were studied by examining the stomach content of specimens from two lakes. A. paraguayensis is omnivorous, feeding mainly on members of littoral-benthic and lotic communities. Algae, rotifers and microcrustaceans (copepods and cladocerans) are the more frequent items in the bulk of the stomach content. Oligochaeta and Diptera larvae were an alternative food source when available. Shrimp feeding habits difference between the two lakes may be due to the resource availability. A. paraguayensis are trophic linkers between limnetic and lotic habitats with an important role in the transportation of energy from the floodplain to the principal channel in the Paraná Rivers System.

Relative growth and morphological sexual maturity size of the freshwater crab Trichodactylusborellianus (Crustacea, Decapoda, Trichodactylidae) in the Middle Paraná River, Argentina.

Abstract The relative growth of a number of morphological dimensions of the South American freshwater crab Trichodactylus borellianus (Trichodactylidae) were compared and related to sexual dimorphism. Crabs were collected from ponds in the Middle Paraná River in Argentina. A regression model with segmented relationship was used to test for relative growth between these measurements where breakpoints infer the body size at which crabs reach sexual maturity. In both sexes the carapace width and the length, height, and thickness of the right and left chelae were measured, as well as the male pleopod length and the female abdomen width. All of these measurements were found to show positive allometry with the exception of the male pleopod length and the left chelae, which did not show a breakpoint. In females the breakpoint for the abdomen width inferred a morphological sexual maturity at carapace width 6.9 mm. In males the break point for the pleopod length was at carapace width 6.6 mm, with that for the chelae measurements was between carapace widths 6.4 and 6.9 mm. The relative growth pattern in Trichodactylus borellianus was found to be similar to that recorded for other species of the family Trichodactylidae.

Daily Rhythm of Feeding Activity of the Freshwater Crab Dilocarcinus Pagei Pagei in the Rio Pilcomayo National Park, Formosa, Argentina

Dilocarcinus pagei pagei (Simpson) is one the several Argentinean freshwater crabs; its geographical distribution extends from northeastern Argentina to Buenos Aires (Morrone and Lopretto 1995, Magalhaes and Turkay 1996). This crab is an important component of the littoral fauna in the lagoons of the Rio Pilcomayo National Park, in the province of Formosa; however, there is little information in the literature about its biology and ecology (Lopretto 1981, Lopretto 1995, Lopretto 1998, Magalhaes and Turkay 1996).

Sexual maturity and ontogenetic shape variations in the freshwater anomuran crab, Aegla uruguayana (Decapoda, Aeglidae)

Summary Sexual maturity is marked by morphological changes in freshwater anomuran crabs. Specific modifications to the dorsal and ventral forms of the cephalothorax and the coxa of the fifth pereiopod in males indicate reproductive maturity. Here, we describe changes in cephalothorax and coxa that develop during sexual maturity in the freshwater anomuran, Aegla uruguayana. To evaluate shape, the dorsal and ventral cephalothorax and the coxa of the fifth pereiopod of this species were digitally photographed. With specific landmarks as reference points, the crab shape was analyzed for each of the three body parts. Relative warps analysis were performed revealing shape differences among the size forms and sexes. The dorsal cephalothorax and the coxa of the fifth pereiopod are different between juveniles, males and females. Individuals of 12–16 mm cephalothorax length have a shape closest to the “consensus configuration” in the population, which is correlated with the onset of sexual maturity in males. Additionally, there are size and shape differences among juveniles and adult individuals of the population. The shape differences were observed mainly in the coxa of the fifth pereiopod of males, as the length of the sexual tube increases during sexual development. Moreover, sexual maturity occurred simultaneously with cephalothorax and coxa development. These results show that multiple cohorts of A. uruguayana coexist in the population and that individuals can have a different reproductive capacity within the active sexual population.

First record of the non-indigenous mussel Limnoperna fortunei (Bivalvia, Mytilidae) as an epibiont of the crab Trichodactylus borellianus (Decapoda, Trichodactylidae)

In the area of Middle Parana River, Argentina, in the pleuston community associated with the roots of Eichhornia crassipes (Mart.) Solms-Laubach, 1883, an epibiosis of the non-indigenous mussel Limnoperna fortunei (Dunker, 1857) on the native crab Trichodactylus borellianus Nobili, 1896 was occasionally recorded. The organisms were measured, weighed, and photographed. Attached molluscs were larger and heavier than their host crabs. This new interaction in the system was analyzed with regard to several biological and ecological characteristics of both species.

Quantitative food webs and invertebrate assemblages of a large River: a spatiotemporal approach in floodplain shallow lakes

To test the hypothesis that the hydrological regime of large Rivers affects the structure of invertebrate communities and food webs in floodplain lakes, we studied invertebrate assemblages and stomach contents of fish and decapods in two shallow lakes in the Parana River floodplain, Argentina, with different connectivity (indirect and permanent, IPC; direct and temporal, DTC) to the fluvial system over three hydroperiods (flooding, transition, drought). Invertebrate assemblages exhibited temporal variation, with higher dissimilarity during the drought phase. However, zooplanktonic and pleustonic attributes varied spatiotemporally, and were better explained by other environmental variables. The food webs in the two lakes differed, with higher connectance and fewer predators in the DTC Lake. In general, the use of trophic resources by fish and decapods reflected the local and abundant resources in the system. The persistence of communities in floodplain lakes with temporary connectivity to the river may relate to the foraging decisions of consumers to buffer environmental fluctuations. The consumption of abundant resources highlighted the importance of autochthonous inputs into floodplain lakes. However, the variation in food supply as a key factor governing food-web structure and stability should be further investigated.