Débora de Azevedo Carvalho

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.

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.

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.

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.

The mark–recapture method applied to population estimates of a freshwater crab on an alluvial plain

Mark–recapture methods are a useful population estimation tool, although with many assumptions that cannot always be satisfied for all types of organisms and environments. In the present study, three mark–recapture methods (Petersen, Schnabel and Schumacher–Eschmeyer) were applied in a preliminary trial to estimate the population size of the crab Trichodactylus borellianus and to gain information that would support the use of the methods in the field. The accuracy of these estimates was verified by analysing the percentage of bias, the width of the confidence intervals, and by a chi-square test. The assumptions of equal catchability and closed population were verified, along with assumptions related to the efficiency of marking. The adjusted methodology was applied in a short-term study of a pond on the Parana floodplain. The results showed that the assumptions were satisfied for both the experimental and field studies. The Schnabel was the most accurate method evaluated in both studies. Although the Schumacher–Eschmeyer method also provided accurate results in the field study, it needed large samples to give reliable estimates. The applicability of these methods depends on the stage of the hydrological cycle. The choice of a short-term research design will ensure that the assumption of a closed population is valid for research of this type on an alluvial plain.

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.

Behavioural responses of freshwater zooplankton vary according to the different alarm signals of their invertebrate predators

Single behavioural interactions between two freshwater planktonic crustaceans and invertebrate predators from different zones of the common environment were studied. The planktonic prey organisms were the cladoceran Ceriodaphnia dubia and the copepod Notodiaptomus conifer. The three invertebrate predators were the shrimp Macrobrachium borellii, the crab Trichodactylus borellianus and the larva of the dipteran midge Chaoborus. Feeding experiments were first performed to document the effects of exposure time and prey density on predation. All the selected predators fed on C. dubia and N. conifer, and predation rates were dependent on prey density and exposure time. The ability of microcrustaceans to modify their swimming and avoidance behaviour when faced with cues from each predator was then analysed. The cladoceran was more vulnerable to decapod predation, whereas the prey microcrustaceans were taken equally by the dipteran larvae. The analysis showed that the microcrustaceans detected the presence of at least one invertebrate predator through the predator alarm signals. The cladoceran responded to chemical signals from the three predators but copepods modified their behaviour only in the presence of infochemicals of M. borellii. The different outcomes suggest that macrocrustaceans have different vulnerabilities when faced with the same predator and so have evolved specific and different strategies to reduce invertebrate predation.

Functional morphology of comminuting feeding structures of Trichodactylus borellianus (Brachyura, Decapoda, Trichodactylidae), an omnivorous freshwater crab

Crustaceans exhibit great diversity of feeding structures with morphological traits that are useful to infer the general trophic habits of species. In this study, we analyzed the functional morphology of comminuting feeding structures (mandibles, chelipeds, gastric mill) of the freshwater crab Trichodactylus borellianus directly related with the food fragmentation. The heterochely and mechanical advantage (MA) of the chelae were also studied. In both analyses, we considered the relationship between morphology and the natural diet. We expected to find a consistent relation between feeding habits and morphological traits. In general, we found simple structures armed with uniform setal systems and feeding appendages without pronounced teeth or spines. Mandibles have primarily cutting functions, helping with the food anchoring and fragmentation with mandibular palps armed with pappose setae. Chelipeds were covered with spines and simple setae. Adult males exhibited right-handedness with high MA of the major chelae. The ingested, relatively large pieces of food are finally chewed by a gastric mill equipped with sharp cusps characteristic of decapods with low ingestion of crude fiber material. The morphology of the feeding apparatus revealed that it is well adapted to an omnivorous diet, being able to cope with dietary changes.

The diel feeding rhythm of the freshwater crab Trichodactylus borellianus (Decapoda: Brachyura) in mesocosm and natural conditions

Biorhythm studies have rarely used feeding rhythms of freshwater crustaceans to assess the internal clock. Even less often, they have compared the diel rhythms of juveniles and adults. In this study, the 24-h cyclic feeding behavior of the freshwater crab Trichodactylus borellianus was investigated in mesocosm and natural field conditions during summer. Sampling occurred throughout the day and included the dawn and dusk periods. The fullness of the stomach was used to characterize the feeding cycle and was analyzed with MESOR and a three-day time series plot. The rhythms of juveniles and adults were analyzed in the field. A bimodal feeding rhythm with midday and midnight peaks was observed in adults in the field, while juveniles had an asynchronous behavior. Mesocosm animals also showed no cyclic behavior. Rhythmic responses to nonphotic cues may result from a trade-off between foraging at an optimum time and shifting the diel rhythm to avoid competition and predation risk. Juvenile feeding asynchrony could be a strategy that allows them to be active in the same habitat as adults.

A comparative study of the gastric ossicles of Trichodactylidae crabs (Brachyura: Decapoda) with comments on the role of diet and phylogeny in shaping morphological traits

The gastric armature of decapod foregut is a feeding structure that sparks controversial debates about the role dietary and historical components have in shaping its morphological traits. Having previous information about the natural diet is an interesting way to gather evidence on this issue. For the present study, we analyzed the morphological traits of gastric ossicles involved directly in the maceration of food in nine species of freshwater crabs of the family Trichodactylidae (Brachyura: Decapoda) representing five genera, three tribes and two subfamilies. The analyzed gastric ossicles were quite consistent among closely related species, suggesting that the observed traits had a clear phylogenetic component. However, it was also noted that the morphological traits of the gastric teeth of trichodactylid crabs match well with the natural diet and presented likeness with general features of other species with a similar trophic habit. We discuss the influence of phylogeny and function on the design of morphological traits and propose to quantify the role of phylogeny and function in shaping morphological traits through the analysis of phylogenetic signals.