Federico Giri

SIZE AND SHAPE VARIATION OF TWO FRESHWATER CRABS IN ARGENTINEAN PATAGONIA: THE INFLUENCE OF SEXUAL DIMORPHISM, HABITAT, AND SPECIES INTERACTIONS

Abstract The morphological variation of Aegla neuquensis and A. riolimayana was studied using a geometric morphometric. Specimens included both allopatric and sympatric populations from rivers and lakes of the Argentinean Patagonia. Cephalothorax shape was analyzed throughout 16 landmarks representing the half configurations of all specimens after symmetrization process. Ordination analysis of the weight matrix revealed a clear distinction between the two species. Multivariate test run on the weight matrix for A. neuquensis and A. riolimayana was highly significant (P < 0.0001). Females and males of the two species differ significantly (P < 0.0001). Females of A. neuquensis and A. riolimayana present a wider posterior region, probably to carry eggs and juveniles. The first relative warp clearly indicates a difference in the shape of crabs inhabiting rivers vs. those living in lakes (P < 0.0001). Lake specimens show a more compact rostrum than river individuals. This is probably associated with the difference in the dynamics of both systems. The shape of A. neuquensis in allopatric populations appears to be similar to the shape of A. riolimayana, while sympatric populations appear much closer to the “typical” A. neuquensis shape. Thus, allopatric populations appear in some way to expand their shape toward A. riolimayana.

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.

A geometric morphometric analysis of two sympatric species of the family Aeglidae (Crustacea, Decapoda, Anomura) from the La Plata basin

Abstract The crustaceans of the family Aeglidae are endemic to the southern regions of South America. Geometric morphometrics was used to assess differences in size and shape between two sympatric species of the family, A. uruguayana and A. platensis. Eleven landmarks on the dorsal region of the cephalothorax were recorded on 57 adult specimens. Interspecific and intraspecific differences in size and shape were analyzed through univariate and multivariate statistics performed on the generalized pro‐crustes analysis aligned coordinates. Shape differences between A. uruguayana and A. platensis, and between males and females, were readily identifiable along the first and the second relative warp. MANOVA showed these differences to be significant. Intraspecific comparisons also revealed a significant sexual dimorphism in cephalothorax shape. Two‐way ANOVA on centroid size did not show any significant difference between species nor between sexual.

Spatial variability of chlorophyll-a and abiotic variables in a river–floodplain system during different hydrological phases

Chlorophyll-a (Chl-a) and abiotic variables were measured in the main channel and floodplain waterbodies of the Middle Paraná River to analyse the system dynamics and to assess their spatial variability during different hydrological phases, including an extreme flood. We wanted to test that the flood does not always have a homogenising effect in a river–floodplain system. An explanatory model for Chl-a was performed according to Akaike’s Information Criterion (AIC), and the relation of water level with the coefficient of variation (CV) among sites for each variable was explored. The model explained 64% of Chl-a variability. Water level, depth:euphotic zone ratio (Zd:Zeu) (inverse correlation) and conductivity (direct correlation) were the significant explicative variables. The CV of Chl-a decreased with flood from the main channel to the floodplain, but for turbidity, Zd:Zeu, pH, dissolved oxygen, soluble reactive phosphorus and Chl-a:pheophytin-a ratio, it increased. However, within the floodplain, CV of turbidity, Zd:Zeu and pH decreased during flood. These suggest that the homogenising effect frequently observed during inundation cannot be generalised and that the floodplain may maintain its identity even during flood. The extreme flood and its overlap with the warm season and sedimentological pulse probably contributed to the heterogeneity in the spatial gradient.

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.

Unravelling the role of determinism and stochasticity in structuring the phytoplanktonic metacommunity of the Paraná River floodplain

One of the ongoing debates around metacommunity ecology is to what extent stochastic and deterministic processes act on community assembly. We explored the influence of both determinism, mediated by environmental filters, and stochasticity, mediated by dispersal and ecological drift, on phytoplankton assembly in a floodplain river. A probabilistic co-occurrence model revealed the presence of 94.1% random and 5.9% non-random species pairwise associations. The latter were higher at both hydrologically isolated (4.42%) and connected environments (2.2%). Variation partitioning analysis showed similar significant explanations by the unique environmental (7.7%, Secchi, conductivity, vegetation, phosphorous) and spatial (7.2%, watercourse distance, longitude) components. Temporal variability was poorly represented (2.4%) because we only considered two low-water periods. Species co-occurrence patterns showed that most taxa coexist randomly. The environmental explanation is in line with niche-assembly models (species sorting), but the similar proportion explained by spatial organisation related to random dispersal guides the evidence to both deterministic and stochastic processes. The higher percentage of random co-occurrence and the larger assemblage variability observed in isolated environments suggests that random dispersal, ecological drift, and priority effects could promote stochasticity. We concluded that both processes affect the structure of phytoplankton metacommunities in a floodplain system and suggest the preponderance of stochastic organisation.

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.

Evaluación de palaemonetes argentinus (Decapoda, Natantia) en el control biológico de larvas de Culex pipiens (Diptera, Culicidae) en condiciones de laboratorio

The predator-prey relationship of Palaemonetes argentinus Nobili, 1901 and Culex pipiens s.l. larvae was studied under laboratory conditions. The prawns were separated in two groups, isolated and grouped ones. Mosquitoe larvae were offered to prawns in two forms, limited and unlimited offer to both groups. In the first analysis, values of predation did not differ significantly between males and females of P. argentinus. Predation in 24 h was 14,9 ± 4,2 larvae/prawn. A reduction of predation was observed with unlimited offer for the same hour, during all the experiment. Predation with unlimited offer was higher on the first day than on the second, but returned to high values on the third day, for both groups. In the grouped experiment, predation with limited offer was total on the first days, decreasing to the last day. As a result, P. argentinus was considered an efficient predator of C. pipiens s.l. larvae under laboratory conditions.

Nutrient Dynamics in Wetlands of the Middle Paraná River Subjected to Rotational Cattle Management

The progressive degradation of wetlands has stressed the need of developing sustainable management strategies for maintaining their ecological character within the context of sustainable development. Rotational cattle grazing is a common grazing strategy in aquatic systems but its effect on nutrient dynamics in lakes is scarcely known. The objectives of this study were to evaluate if cattle produces a significant increase of TN and TP in wetlands, and if the value of these nutrients decrease after cattle are removed. Samples of water, sediment and macrophytes were collected for nutrient, organic matter and other chemical analyses. Results showed a significant increase in water nutrients during the presence of cattle, decreasing during their absence. These changes occurred surprisingly rapid, suggesting a high resilience of these systems to this impact. The presence of submerged vegetation in lakes ameliorated the effect of cattle on water nutrients, suggesting an important role of these plants in ecosystem management. Rotational management would minimize the increase of nutrients in water, maintaining the ecological integrity of wetlands.