On the Hemoglobin in Daphnia (Crustaces: Cladocera)

The concentration of Hb in Daphnia magna hemolymph is inversely related to the O2 concentration of its aquatic habitat. Under low oxygen conditions, immature animals are most capable of Hb synthesis. Hb-rich animals can swim a longer distance even in very low O2 concentration in which Hb-poor animals are able to swim only a short distance. Daphnia magna is an O2 regulator, and Hb-rich animals have a lower critical O2 concentration than that of Hb-poor animals. The fifty per cent of in vivo oxygenation of Hb is found at 15 and 35 torr for Hb-rich and Hb-poor animals, respectively. The purified Hb from Hb-rich animals has a higher O2 affinity than that from Hbpoor animals. Hemoglobin is separated into at least six components by isoelectric focusing. Hemoglobin solution from Hb-rich animals has a different proportion of the Hb component from that of Hb-poor animals, and the increase in O2 affinity is concomitant with an increase in the relative amounts of Hb components of high isoelectric point. This suggests that Daphnia magna may adapt to a wide range of environmental O2 conditions by means of multiple Hb.