The similarities or differences among species of the same family or among different families suggest a relationship with their feeding habits or a preference for the various types of food. The aim of this study was to identify the presence of digestive enzymes lipases, total protease, amylases and cellulose, and the effects of sex and size in these activities in two decapod freshwater crustaceans. Specimens of different sizes and sexes of Aegla uruguayana and Macrobrachium borellii were collected in the natural environment. In laboratory, enzymatic preparation consisted in homogenising the digestive gland in a buffer Tris‐HCl. The homogenates were centrifuged and the lipid layer was removed. Enzyme activities were determined using standard protocols. There were no statistically significant differences between sexes in any of the species under study. However, there were changes in the activity of amylases during the ontogeny of A. uruguayana, indicating that in juveniles the values of said activity are significantly higher than in the rest of the groups. M. borellii showed ontogenetic differences in total proteinase activity: organisms of range 2 (intermediate sizes) presented enzymatic activities much smaller than organisms of range 4 (larger sizes). The comparisons of enzymatic activities between both species yielded significant differences, being higher the values in the species A. uruguayana with respect to M. borellii. The results obtained in this work indicate that although both species are omnivorous and consume similar items, the activity of these digestive enzymes could be an evidence of different strategies of utilisation of the resource.

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ABSTRACT.Crustacean growth studies typically use modal analysis rather than focusing on the growth of individuals. In the present work, we use geometric morphometrics to determine how organism shape and size varies during the life of the freshwater crab, Aegla uruguayana Schmitt, 1942. A total of 66 individuals from diverse life cycle stages were examined daily and each exuvia was recorded. Digital images of the dorsal region of the cephalothorax were obtained for each exuvia and were subsequently used to record landmark configurations. Moult increment and intermoult period were estimated for each crab. Differences in shape between crabs of different sizes (allometry) and sexes (sexual dimorphism; SD) were observed. Allometry was registered among specimens; however, SD was not statistically significant between crabs of a given size. The intermoult period increased as size increased, but the moult frequency was similar between the sexes. Regarding ontogeny, juveniles had short and blunt rostrum, robust forehead region, and narrow cephalothorax. Unlike juveniles crabs, adults presented a well-defined anterior and posterior cephalothorax region. The rostrum was long and stylised and the forehead narrow. Geometric morphometric methods were highly effective for the analysis of aeglid-individual-growth and avoided excessive handling of individuals through exuvia analysis. KEYWORDS.Crustacea, cephalothorax, ontogeny, intermoult, morphometrics. RESUMEN. Variaciones de forma y tamaño deAegla uruguayana (Anomura, Aeglidae): Una aproximación desde la morfometría geométrica al crecimiento. Los estudios de crecimiento en crustáceos típicamente utilizan análisis modal en lugar de focalizarse en el crecimiento individual de los organismos. En el presente trabajo, utilizamos morfometría geométrica para determinar cómo varia la forma y el tamaño a lo largo de la vida del cangrejo de agua dulce, Aegla uruguayana Schmitt, 1942. Un total de 66 individuos, en diferentes etapas del ciclo de vida, se examinaron diariamente, registrándose la presencia de exuvias. Imágenes digitales de cada muda fueron obtenidas de la región dorsal del cefalotórax y se utilizaron para registrar las configuraciones de landmarks. El incremento por muda y el período de intermuda se estimaron para cada cangrejo. Diferencias de forma entre tamaños (alometría) y sexos (dimorfismo sexual; SD) se observaron. Se registró la presencia de alometría entre los especímenes; sin embargo, el SD no fue estadísticamente significativo respecto del tamaño. El período intermuda aumentó de manera directamente proporcional al tamaño, pero la frecuencia de muda fue similar entre los sexos. Durante la ontogenia, los juveniles presentaron rostro corto y romo, frente robusta, y ancho del cefalotórax estrecho. Los adultos presentaron la región anterior y posterior del cefalotórax bien definido en relación con los juveniles. El rostro fue largo y estilizado y la frente estrecha. Los métodos de morfometría geométrica fueron muy efectivos para el análisis del crecimiento individual e...

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The presence, density, and distribution pattern of a species in a particular space and time result from evolutionary and ecological processes. We evaluated the density and spatial distribution pattern of the freshwater anomuran crab, Aegla uruguayanaSchmitt, 1942, in relation to temporal and spatial scales. The temporal scale followed a seasonal analysis, and the space scale was evaluated from the study of three streams located in different provinces of Argentina. Specimens were collected in four sampling campaigns during spring, summer, autumn, and winter in three temperate freshwater environments of the La Plata Basin, Argentina, using 1 m2 sampling units. The density varied according to the studied populations and was not by seasonality. The distribution pattern was not influenced by the evaluated scales. The A. uruguayana populations presented a clumped pattern showing a negative binomial distribution. A possible cause of such a pattern is the strong environmental heterogeneity of the three studied sites. The species presented gregarious behavior that could be associated with the spatial arrangement of the shelters available in the particular habitats as well as ecological strategies for optimal and efficient use of resources in these topographies. Our study is the first on the dynamics of the distribution in terms of temporal and spatial scopes of a species of AeglaLeach, 1820, an endemic freshwater anomuran.

The first instance of a pair of conjoined embryos in Aegla uruguayanaSchmitt, 1942 is reported. Collected in a stream in Entre Ríos province, Argentina, the embryos were detected among 550 eggs containing embryos with normal morphological development, and constitute the second observed case of twins in AeglaLeach, 1820. The twins were joined together by their dorsum and lived for a few days in the laboratory; however, they did not hatch, dying inside the egg envelope.