Behavioural thermoregulation in Daphnia carinata from different depths of a natural water body: influence of environmental oxygen levels and temperature

The influence of environmental parameters (water temperature and dissolved oxygen content) on the haemoglobin content of a naturally occurring population of Daphnia carinata was studied in a population resident in an intermittently flowing, shallow body of water. It was found that the Hb content of the animals was influenced by a combination of both the water temperature and dissolved oxygen profiles of the water body. In Summer, stratification, as indicated by sampling at three depths, constrained thermoregulatory behaviour; Hb-poor animals from the upper levels of the water column chose a lower temperature in hypoxia than their Hb-rich counterparts from greater depth. In Winter, the water column was mixed and remained moderately hypoxic at all times. Due to the low temperature of the water, Hb-contents in winter were much lower than those found in Summer. Overall, it was found that thermoregulatory behaviour in D. carinata is dependent upon Hb-content, which is strongly influenced by the animals' environment.