Preference, tolerance and resistance responses of Poecilia sphenops Valenciennes, 1846 (Pisces: Poeciliidae) to thermal fluctuations

Abstract

Poecilia sphenops was acclimated to two thermal fluctuations, each following a symmetrical and asymmetrical cycle. The critical maximum temperatures of the fish were significantly different (P ≤ 0.001) between the thermal fluctuations: 40°C for the 20-29oC thermal cycle and exceeding 42°C for the 26- 35oC cycle. Fish acclimated to the 20-29oC thermal cycle were more resistant to cold, with a critical thermal minimum of 10oC, unlike specimens of the 26-35oC regime, with a critical minimum of 11.7°C. In both cycles, the upper incipient lethal temperature interval was 38.8 to 39.5oC and the lower incipient lethal temperature interval was 10.8 to 11.8°C. Females preferred temperatures between 30 and 31°C, in all but the 26-35oC symmetric cycle. Males preferred a temperature between 23.8 and 24.2°C (symmetrical cycle). The high and low avoidance temperatures of male and female fish acclimated to the 20-29oC cycle were, respectively, 14 and 16oC, compared to the 26-35°C treatment, in which case these values were 8.4 and 11.4oC. The results indicate that P. sphenops is perfectly adapted to the marked seasons of rain and drought of their habitat.