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First published online November 19, 2004
Journal of Experimental Biology 207, i-a (2004)
Copyright © 2004 The Company of Biologists Limited
doi: 10.1242/jeb.01365
Inside JEB |
DAPHNIA BREATHES EASY
yfke{at}biologists.com
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Understanding how millimetre-sized animals function poses unique problems for biologists, because processes that work in large animals often cannot be scaled down simply. But as far as Ralph Pirow is concerned, the tiny crustacean Daphnia is the perfect model system to study oxygen transport processes, precisely because of its minute size. Oxygen is transported in animals by diffusion or convection, depending on the animal's size. While diffusion takes care of oxygen delivery in something as small and simple as a fish egg, circulatory convection takes over when things are scaled up in larger creatures. But intermediate sized animals like Daphnia might use a mixture of the two. Pirow and Rüdiger Paul at the University of Münster wondered how oxygen is transported in the little crustaceans: do they rely on diffusion or convection? They were surprised to find that it all depends on the oxygen levels in the environment (p. 4393).
Pirow explains that Daphnia masterfully adapt to changing oxygen levels. When the little creatures find themselves starved of oxygen they produce haemoglobin, an oxygen transport molecule, to help them cope. So, do Daphnia with different levels of haemoglobin transport oxygen differently? To find out, Pirow, Paul and Christopher Bäumer decided to compare oxygen profiles inside the bodies of normal-oxygen-adapted and low-oxygen-adapted Daphnia.
They reared one population of the transparent minicrustaceans at normal oxygen levels and a second population in low oxygen conditions. The low-oxygen-population went from transparent to a bright red colour as the animals pumped up production of haemoglobin. While the low-oxygen-adapted population became haemoglobin-rich, the population reared in normal oxygen conditions remained haemoglobin-poor. The team was now ready to visualise oxygen levels inside animals from the two populations. They injected an oxygen-sensitive phosphorescence probe (which emits more light as oxygen levels decrease) into the crustaceans' circulatory system. The light intensity images provided by the probe allowed the team to see two-dimensional oxygen profiles in Daphnia's haemolymph circulation.
Examining a cross-section through the middle of the animal's body under a microscope, Pirow saw steep oxygen gradients in haemoglobin-poor Daphnia but fairly flat oxygen gradients in haemoglobin-rich animals. He explains that the steep slopes of the oxygen gradients in the bodies of the haemoglobin-poor animals indicate diffusion-based oxygen transport, as `you can imagine the oxygen rolling down the slopes from regions of high oxygen to regions with less oxygen'. The presence of haemoglobin, acting as a buffer that stabilizes the release of oxygen from the haemolymph to body tissues, restricts the haemolymph oxygen concentration to a narrow range, smoothing out the oxygen gradients in the body. The gentle oxygen gradients in the haemoglobin-rich Daphnia indicate that, as haemoglobin levels increase, the animals switch from a diffusion-dominated to a convection-dominated oxygen transport system. `Oxygen can be transported at much flatter internal oxygen gradients by convection than by diffusion' says Pirow, `so animals with high haemoglobin levels have the advantage that they can cope much better with oxygen-deficient habitats'.
If having lots of haemoglobin is so useful, why aren't all Daphnia bright red? Many aquatic animals, like Daphnia, are transparent in a bid to escape unwanted attention from predators. Having high levels of haemoglobin may help the creatures survive in low oxygen habitats, but turning bright red also makes them much easier to spot by a hungry fish. To avoid becoming lunch, Daphnia may have to pay the price of struggling for air in oxygen-starved water.
References
Pirow, R., Bäumer, C. and Paul, R. J.
(2004). Crater landscape: two-dimensional oxygen gradients in the
circulatory system of the microcrustacean Daphnia magna. J. Exp.
Biol. 207,4393
-4405.
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