Spatial partitioning between juveniles and adults of the freshwater anomuran crab Aegla parana (Crustacea Aeglidae) from southern Brazil

ABSTRACT In order to understand the role of lower order tributaries in the population structuring of the freshwater anomuran crab Aegla parana Schmitt, 1942, a study on the distribution of demographic categories was carried out along Negro River and in its tributary Totó River that belong to Upper Iguaçu Basin, southern Brazil. Two collection points were established along the main channel of Negro River, in a 7.00 km long stretch, and three points in the entire creek Totó River with 6.61 km long. The animals were captured with Surber net and baited traps and had the carapace length (CL) measured. A total 146 crabs were obtained from Negro River and 361 from Totó River, performing 507 analyzed aeglids. From the point 1 of Negro River, males measured 27.68 ± 12.68 mm CL and females 18.98 ± 5.88 mm CL; from point 2 of the same river, the values were 24.45 ± 13.70 mm CL and 27 ± 8.50 mm CL, respectively. There was a positive and direct relationship between the mean CL of the aeglids from Totó River and the distance of the collection points from its source. Males: point 1 (source of the creek): 7.90 ± 1.23 mm CL; point 2: 9.55 ± 3.06 mm CL; point 3: 9.91 ± 2.62 mm CL. Females: point 1 (source): 6.68 ± 1.70 mm CL; point 2: 9.01 ± 1.94 mm CL; point 3: 10.49 ± 2.57 mm CL. The sexually immature aeglids are distributed mainly in the lower order tributary (= Totó River), and the mature ones in the main channel of Negro River, indicating that this population perform ontogenetic migrations from tributary to the main river. The spatial partition can be explained under three aspects that are quite contrasting between Negro River and Totó River: 1. substrate architecture; 2. depth of the water column and 3. quality of available food.