Pablo Collins

Feeding ecology of Macrobrachium borelli (Nobili) (Decapoda: Palaemonidae) in the flood valley of the River Paraná, Argentina

The feeding habits and selectivity of Macrobrachium borelliiwere studied by examining the stomach content of specimens from anox-bow lake across one year. M. borellii isomnivorous-carnivorous, feeding largely on members oflittoral-benthic communities. The bulk of the stomach content wasformed by oligochaetes, dipteran larvae, and a variety of organismsranging from algae to palaemonids. Copepods and cladocerans were analternative food source in winter, at low-water, when macrophytes haddeclined. Oligochaetes and dipteran larvae were positively, algae andmicrocrustaceans negatively selected.

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.

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.

A COEXISTENCE MECHANISM FOR TWO FRESHWATER PRAWNS IN THE PARANÁ RIVER FLOODPLAIN, ARGENTINA

Abstract This study examined the circadian feeding cycles of two sympatric decapod prawns, Palaemonetes argentinus (Nobili, 1901) and Macrobrachium borellii (Nobili, 1896), as a coexistence mechanism. The work was done in some shallow lakes of the alluvial valley of the Paraná River (Santa Fe, Argentina). Both prawns consume similar food items, but they differ in their feeding times. Macrobrachium borellii stomach fullness was greater during night than day, whereas P. argentinus foraging activity occurred by day. Trophic niche breadth was wide and similar in both species during the day but was expanded in P. argentinus at night when M. borellii feeding activity increased. Niche overlap fluctuated without a definite pattern. Adaptive modifications of circadian feeding rhythms play an important role in the coexistence of these species.

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.

Predation ability of freshwater crabs: age and prey-specific differences in Trichodactylus borellianus (Brachyura: Trichodactylidae)

Freshwater crabs are rarely represented in food webs and their role in these ecosystems has been largely ignored. Trichodactylus borellianus is an omnivorous crab species that has a diverse natural trophic spectrum. This paper evaluates, in a laboratory assay, the ability of three ages of this crab to prey on three different organisms. The prey selectivity and relative importance of each prey item was also analyzed. Prey items (cladocerans, oligochaetes, and golden mussels) consisted of organisms of different shapes and mobility, representing the natural diet of this crab. Crabs were sorted according to size (juvenile, sub-adult, and adult) and increasing amounts of one prey was offered to each crab after 24 hrs of starvation. In the selectivity trial, all prey types were offered in a fixed set. Crabs in all three age categories were able to prey on all organisms. Cladocerans and oligochaetes were consumed in greater quantities compared to golden mussels when offered separately. However, only the consumption of oligochaetes exhibited a positive correlation with the crabs’ size. When cladocerans and oligochaetes were offered together, more oligochaetes were consumed, both as percentage of volume and occurrence. Age-specific changes in consumption could be related to differences in the stomach capacity of the crab, the digestion time of each prey, and the predator–prey encounter probability. The selection of the most elongated prey with the lowest mobility indicated that the balance of the gain and loss of energy made it an advantageous species to prey on, as it was susceptible to predation. Additionally, the mussel, which is an invasive species, although not positively selected by T. borellianus in the present study, represented a new trophic resource for this crab.

GUT EVACUATION TIME OF MACROBRACHIUM BORELLII (CARIDEA: PALAEMONIDAE) FEEDING ON THREE TYPES OF PREY FROM THE LITTORAL-BENTHIC COMMUNITY

Abstract Gut evacuation time is a very useful parameter that allows us to understand the daily trophic cycle of a predator. Macrobrachium borellii is an inhabitant of the littoral-benthic community of the Paraná River system that preys on various organisms. In this work, three types of prey (cladocerans, dipteran larvae, and oligochaetes), which are different in shape and mobility, were offered to M. borellii to investigate the gut evacuation time of each one. All prey represent the natural diet of this prawn based on previous work. Results showed a faster digestion of cladocerans than mosquito larvae and oligochaete. These last two types of prey are cylindrical and may interfere with the maceration process in a stomach that lacks a gastric mill. In addition, the high mobility of cladocerans and mosquito larvae implies greater energy expenditure in their capture. However, cladocerans are the prey with the lowest caloric value per individual and therefore offers the lowest net energy to the prawn. This implies that a cladoceran diet would require higher daily consumption and more time foraging, and thus potentially exposing the prawn to greater predation risk. All of these factors direct selection toward the more profitable prey (oligochaetes and dipteran larvae), in agreement with the natural diet observation, suggesting that the time of digestion may have little importance in prey selection.

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).