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The preferred temperature (PT) of Poecilia sphenops (Pisces, Poeciliidae) (n = 240; 1.13 ± 0.06 g wet weight; 3.26 ± 0.88 cm standard length) from Presa Piedra Azul, Oaxaca, Mexico, was measured at acclimation temperature of 20 °C, 23 °C, 26 °C, 29 °C, 32 °C and 35 °C. The thermal preference of males was 25.5 °C in summer and 29.6 °C in winter; whereas females preferred 29 °C in both seasons. Fish held at 20 °C and 23 °C preferred temperatures 36% higher than their acclimation temperature (AT), and those held at 35 °C and 32 °C preferred 30% and 20% lower temperatures than their AT. In intermediate temperatures, no significant differences between PT and AT were found. The interval of thermal selection based on the higher (HAT) and lower (LAT) avoidance temperatures was of 10–14 °C and not different between sexes in both seasons. Knowledge of the thermal biology of P. sphenops based on the preferred and avoidance temperatures was very important in order to understand the capacity of the species to adapt to the thermal changes occurring in its habitat. Based on the results, we can recommend the use of P. sphenops in aquaculture owing to its thermal plasticity.

A study was made of the effect of salinities of 0, 3, 6, 9, 12, 17, 22 and 27%o on the oxygen consumption and osmoregulation of Macrobrachium tenellum. The metabolism salinity curve is "U" shaped although in freshwater the metabolic rate reduces by 38%. The oxygen consumption observed over this range of salinities corresponds to Type II of Kinne (1971). In M. tenellum, osmotic concentration of haemolymph remains constant around 531 mmol Kg -1 which means that this species is a strong hyper-hypo osmoregulator. The isosmotic point was 533 mmol Kg -1 which suggests that this species has only recently invaded the freshwater environment.

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