Haemoglobin synthesis in Daphnia magna Straus (Crustacea: Cladocera): ecological differentiation between neighbouring populations

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Abstract
SUMMARY. 1. Variations in the haemoglobin index of two neighbouring populations of Daphnia magna were recorded over a range of dissolved oxygen concentrations (0.5–4.0ml O2 1−1, 20°C). Reciprocal transfer experiments between habitats compared haemoglobin synthesis in situ.2. An inverse relationship was found between the oxygen content of pond water and the haemoglobin indices of laboratory and natural populations.3. Significant genetic differences in the synthesis of haemoglobin were found between the two populations. Animals from the poorly oxygenated habitat (0.8±0.18ml O2 1−1) had consistently higher haemoglobin contents (maximum HI, 87.7±4.5) at all experimental and in situ oxygen levels. D. magna from the well oxygenated pond (4.3±0.59 ml O21−1) had a lowered physiological ability to synthesize haemoglobin (maximum HI, 48.3±4.2). The process of ecological differentiation in Daphnia populations is discussed.