Ecological traits of two sympatric species of Hyalella Smith, 1874 (Crustacea, Amphipoda, Dogielinotidae) from southern Brazil

This work was performed with the aim to test theoretical predictions regarding that the sympatric species Hyalella pleoacuta and H. castroi might show distinct population biology to facilitate its coexistence. The specimens were collected monthly with nets from August 2003 through July 2004 in two trout ponds at Sítio Vale das Trutas locality, São José dos Ausentes County, southern Brazil. In the laboratory, the specimens were measured as cephalothorax length (mm), being the sex and ovigerous conditions checked. The species H. pleoacuta was 2.94 times more frequent than H. castroi. Males were significantly greater in size than females (H. pleoacuta—males: 0.74 ± 0.002 mm and females: 0.66 ± 0.001 mm; H. castroi—males: 0.84 ± 0.00 mm and females: 0.72 ± 0.003 mm). Males and females of H. castroi showed a greater mean body size than H. pleoacuta. Sexual maturity was attained at 0.53 mm in males and 0.48 mm in females of H. pleoacuta, and 0.72 mm in males and 0.67 mm in females of H. castroi. The frequency distribution in size classes was usually bimodal in H. pleoacuta and polymodal in H. castroi throughout the year. Sex ratio was female-biased in either species of Hyalella. Ovigerous females (carrying eggs or juveniles in the marsupium) were collected throughout the year in both Hyalella species, but H. pleoacuta and H. castroi were found with more frequency during the winter and fall, respectively. Recruitment occurred in all months of sampling, the juvenile frequency being more than 50% of the amphipods collected in almost all months in both species. The biological differences (especially body size, size at sexual maturity, number of specimens collected and reproductive peak) and microhabitat specialization can be facilitating factors in the coexistence of H. pleoacuta and H. castroi in artificial ponds raising trout.