Metabolic responses to temperature change in a tropical freshwater copepod (Mesocyclops brasilianus) and their adaptive significance.

Metabolic rates of Mesocyclops brasilianus from Lake Valencia, Venezuela, were determined at several temperatures spanning the environmental range (22-28° C). The QO's (oxygen consumption per unit weight) of all Mesocyclops stages from Lake Valencia are higher than most but not all QO's from temperate copepod species that have been studied. The QO is essentially static through naupliar development and shows a sudden jump between N6 and CI, which probably results from the major change in morphology and behavior at this point in the life history. QO declines steadily between CI and adult stages. Acclimated copepodite and adult Mesocyclops show a decreasing metabolic rate with increasing temperature (i.e. Q< 1.0) over the temperature range 26-28° C. This is the range of temperatures normally encountered during the daily vertical migration when the lake is thermally stratified (April-November). Since vertical migration would result in a compromise between a fully acclimated and an acute response, a nearly constant metabolic rate or a slight decline in metabolic rate in the warmer water would be expected in field populations. The results thus show that the metabolic rate of Mesocyclops is not reduced when it moves into deeper (cooler) water, as would be predicted by certain energy-based hypotheses that have been used to explain vertical migration. In contrast to the low Q's between 26 and 28° C, copepodites and adults have very high Q values in the range 22-26° C. This indicates an adaptive decrease in metabolic rate which is thermally programmed to coincide with the cooler temperatures that are encountered during the mixing season (December-March), when a drastic change in ecological conditions occurs in the lake.Nauplii show evidence of the same seasonal response but without the superimposed plateau at high temperatures, which they would not need because they are weak migrators. Nauplii show a plateau at the lowest temperatures, however, which suggests that a fixed metabolic reduction occurs at the onset of mixing and metabolism is not altered thereafter with declining temperature.The change in QO with temperature generally supports the hypothesis that all Mesocyclops stages are adapted to hold a high, constant metabolic rate through the diel cycle but experience a seasonal reduction in metabolic rate in response to major ecological changes in the lake at the time of seasonal mixing.