Ecologia de populações e biologia reprodutiva em Hyalella (Crustacea, Amphipoda, Hyalellidae)

Abstract

The genus Hyalella has been widely studied in the last years, especially their taxonomy. Little is known about the biology and ecology of this genus, particularly in respect to Brazilian species. Thus, we aimed to elucidate in the present work the population ecology and reproductive biology of H. longistila and H. carstica. Both are found on the state of Minas Gerais, Southeastern Brazil, in distincti streams, and H. carstica inhabits na intermittent waterbody. In both streams we collected macrophytes or the substrate above them during twenty minutes, and the animals were thoroughly separated from the sediment in the field. All individuals were preserved on etanol 70% and ovigeorus females and couples were individualized in microtubules. In the laboratory we classified such crustaceans in males, females, ovigerous females or juveniles and measured each individual in relation to the cephalothorax length (CL), total body length (TL) and in the case of ovigerous females the marsupium width (MW) also was measured. Adults predominated in most of the months on the H. longistila population, while juveniles of H. carstica predominated before the dry period of the stream (September to November) and adults were more abundant after it. The mean ratio CL/TL of juveniles was superior to the adults in both species, thus suggesting that such ration may be used as an indicative of the adult phase together with other parameters. The mean total sex ratio of both populations was 1 male : 0.75 females, and this ratio favored females of H. longistila on summer months and males in the rest of the year. In the H. carstica population males predominated along the year. The reproduction of H. longistila was continuous and H carstica reproduced more intensively after the dry, when we suggest that occur the local recolonization by adults from upstream centers of population abundance. In relation to the environmental factors, a positive and independent effect of temperature over abundance and dissolved oxygen over the frequency of ovigerous females was observed on the H. longistila population, while the frequency of ovigerous females of H. carstica was influenced by temperature and conductivity. In general paired adults presented larger CL than non-paired adults in both sexes and species. The mean fecundity of H. longistila was 12.88 ± 2.00 eggs/female and 12.60 ± 7,20 eggs/female for H. carstica. The stage and number of eggs was negatively correlated on females of H. longistila but not in H. carstica. The mean volume of eggs on the first stage did not differ between species, but had differed on stages 2 and 3. A linear relation was observed between the number of eggs on stage 1 and the size of the respective ovigerous female in both species. The reproductive success of H. carstica was higher on intermediate size classes, while for H. longistila there was a progressive increase in the success as the female size increased. Differences were observed between the population dynamics and reproductive biology of the two species. The distinct conditions (especially of the hydric regime) under which H. longistila and H. carstica were found may have caused such differences, however the peculiarities of the biology of each species also may have influenced the results. Furthermore, collections were made in distinct periods, what also may be a causal factor for the observed differences. The information generated by this work are important to subsidize future works with such amphipods, as well as for conservation practices for them and the ecosystem in which they are inserted.