Previous field studies suggest that Jenynsia multidentata Jenyns, 1842 populations reach larger sizes in the estuarine region of the Patos Lagoon when compared to the adjacent freshwater environments. A laboratorial experiment was carried out in order to test possible differences in growth rate, total length and average weight of individuals kept in two different salinities (zero and 16). Right after birth, four offspring of different females were split into separate aquariums of four liters each, being that half of the individuals of each offspring was submitted to salinity zero and the another half to salinity 16. Four aquariums were kept at the constant temperature of 28°C, whereas the other ones suffered a change in temperature (from 28 to 24°C) that occur 18 days from the beginning of the experiment. After thirty-eight days, all the individuals were measured and weighed. Individuals kept in constant temperature and salinity zero showed a significantly lower total length (TL16 = 23.5mm; TL0 = 20.4mm; p < 0.05) and weight (W16 = 0.151 g; W0 = 0.086 g; p < 0,05), when compared with the ones kept in salinity 16. Those individuals kept in aquariums that suffered variation in temperature also had significantly differences in both response variables (TL16 = 21.1mm; TL0 = 18.2 mm; p < 0.05 and W16 = 0.104 g; W0 = 0.058 g; p < 0,05). In both experiments, there was a higher mortality rate in those individuals kept in salinity zero when compared with those ones from salinity 16. Another experiment was set up to assess differences in growth rate between salinity zero and 16. A fifth offspring was split into aquariums with salinities 0 and 16. During 70 days, individuals were measured to the nearest mm at regular intervals of five days. Individuals significantly had higher growth rate in salinity 16 than zero. Our laboratorial results seemed to corroborate the field observations showing that the one-sided livebearer Jenynsia multidentata reaches greater total length and weight in brackish, estuarine waters when compared to freshwater.
Anablepidae; growth; one-sided livebearer; salinity
