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First published online January 12, 2004
Journal of Experimental Biology 207, 683-696 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00812

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The dichotomous oxyregulatory behaviour of the planktonic crustacean Daphnia magna

R. Pirow^* and I. Buchen

Institut für Zoophysiologie, Westfälische Wilhelms-Universität, Hindenburgplatz 55, 48143 Münster, Germany

View larger version (13K): [in a new window]   Fig. 1. Effect of increasing food concentrations on (A) appendage beating rate (fA) and (B) heart rate (fH) of D. magna (2.68±0.22 mm long) at normoxic conditions. Data are given as means ± S.D. (N=4 except for 5.6x104 cells ml-1, where N=2). A repeated-measures ANOVA was performed for all food levels except 5.6x104 cells ml-1. Neither the mean fA (F=49.9, groups d.f.=6, remainder d.f.=18, P<0.001) nor the mean fH (F=28.8, groups d.f.=6, remainder d.f.=18, P<0.001) were the same in animals on all seven food levels. The results of multiple comparisons among pairs of means are shown in Table 1.

View larger version (59K): [in a new window]   Fig. 2. (A) Dorsal view of the microcrustacean Daphnia magna showing the medium flow pattern (white arrows) and the circulatory pattern (black arrows). A dorsal piece of the left carapace valve (chequered area) was removed (for details see Pirow et al., 1999b). (B) Conceptual model for oxygen transport in D. magna based on a cylinder-within-a-tube arrangement. Medium flows through the space between the carapace and the trunk in a posterior direction (open arrows) while oxygen is released both into the carapace lacuna and the peripheral tissue layer of the trunk. This tissue layer is supplied with oxygen from the medium and from a truncal haemolymph space by diffusion (broken arrows). Oxygenated haemolymph leaves the double-walled carapace and then enters the truncal haemolymph space (solid arrows). While flowing in an anterior direction, oxygen diffuses from this haemolymph space both into the coaxial tissue cylinder and the cortical tissue layer (broken arrows). Pin, Pex, inspiratory and expiratory oxygen partial pressures, respectively; Pa and Pv, oxygen partial pressures of the haemolymph entering and leaving the trunk, respectively.

View larger version (34K): [in a new window]   Fig. 3. Subdivision of the cylindrical model of radius r0 and height h0 for numerical analysis. This subdivision yields coaxial cylindrical and hollow-cylindrical volume elements of height (h) with radial extensions being multiples of 0.5r. The numerical analysis aims to determine the oxygen partial pressures for the discrete set of grid points (filled circles) representing the volume elements. The axial and radial coordinates of the points are (j+0.5)h and ir, where the indices j and i are integers with j=0,..., Nax-1 and i=0,..., Nrad. The oxygen partial pressure (Pj,i) of a representative volume element (white rectangle) is affected by diffusive (broken arrows) and convective (solid arrows) exchange processes with the adjacent volume elements. The hatched areas represent those fractions of the respective volume elements that are shifted to the left by convection during the time interval t. The white line on the left exemplifies the radial position of a compartment interface that has to be rounded to a multiple of r.

View larger version (34K): [in a new window]   Fig. 4. Oxygen transfer across a compartment interface with additional diffusion barrier (cuticle). The grid point referenced by the indices i and j is located on an infinitesimal thin diffusion barrier separating the tissue from the medium compartment. This grid point is characterized by two variables, and , which represent the oxygen partial pressure at the inner and the outer side of the circular diffusion barrier, respectively. To calculate the temporal changes in and (see equations 4, 5), the following geometrical parameters are required: the cylindrical wall areas Bi-0.5, Bi and Bi+0.5, the areas of the hollow-cylindrical bases and , and the hollow-cylindrical volumes and . Three example equations for calculating these parameters are given. vM, flow velocity in the medium compartment; Pj,i-1, Pj,i+1, , oxygen partial pressures of the neighbouring grid points; h and r, distance between two grid points in axial and radial direction.

View larger version (16K): [in a new window]   Fig. 5. Responses in (A,C) heart rate (fH) and (B,D) appendage beating rate (fA) to decreasing ambient oxygen tensions at food-free conditions (left; N=5) and food-rich conditions (right; 105 algal cells ml-1; N=11 except for 0.8 kPa, where N=4). Data are given as means ± S.D. A repeated-measures ANOVA was performed for all data. At food-free conditions, the mean fA (F=25.6, groups d.f.=9, remainder d.f.=36, P<0.001) and the mean fH (F=48.0, groups d.f.=6, remainder d.f.=36, P<0.001) were not the same in animals on all 10 oxygen levels. At food-rich conditions, the mean fA (F=3.1, groups d.f.=9, remainder d.f.=36, P=0.025) and the mean fH (F=15.4, groups d.f.=9, remainder d.f.=36, P<0.001) were not the same in animals on all 10 oxygen levels. The results of multiple comparisons among pairs of means are shown in Tables 3, 4.

View larger version (21K): [in a new window]   Fig. 6. (A) Individual responses (N=11) in appendage beating rate (fA; solid and broken lines) to decreasing ambient oxygen tensions at high food concentration (105 algal cells ml-1). The profiles were shifted vertically for clarity. Eight of 11 animals showed a hyperventilatory response (solid lines). The dotted lines indicate the two reference points, the initial normoxic level at 21 kPa and the hypoxic level of 3 kPa, which were used to assess the hypoxia-induced changes in fA. (B) Magnitude of hypoxia-induced changes in fA in relation to the initial fA prevailing before the start of the hypoxic exposure. The dotted line demarcates positive (open circles) from negative responses (filled circles).

View larger version (23K): [in a new window]   Fig. 7. Model predictions revealing the efficiency of ventilatory and circulatory adjustments. Solid curves show the predicted dependencies of the oxygen consumption rate (100%24 nmol h-1) upon ambient oxygen tension for the fasting state/food-free conditions (A) and fed state/food-rich conditions (B). Both states differ from each other in the volume-specific oxygen consumption rate (a0), perfusion rate (H) and ventilation rate (M). H and M represent normoxic values. Vertical lines mark the critical ambient oxygen tensions (PO2crit) at which the rates of oxygen consumption decreased to 99% of the maximum. Below PO2crit, the central tissue cylinder experiences an inadequate supply with oxygen. The overproportional decline in oxygen consumption rate in A and B below 4 kPa and 6 kPa (bold arrows), respectively, indicates the incipient impediment of oxygen provision to the peripheral tissue layer. Horizontal arrows indicate the reductions in PO2crit by hypothetically doubling either H or M. The grey shaded areas reflect the amounts of oxygen transported by the circulatory system; the remaining white areas below the solid curves are those amounts diffusing from the respiratory medium directly into the peripheral tissue layer.

View larger version (95K): [in a new window]   Fig. 8. Oxygen partial pressure distribution in the median plane of the radially symmetrical model at the critical ambient oxygen tension (PO2crit) of 10.1 kPa (see Fig. 7A). The upper-case letters along the radial axis mark the different compartments, and the vertical lines indicate the compartment interfaces (A, central tissue cylinder; B, truncal haemolymph space; C, peripheral tissue layer; D, medium lacuna; E, carapace lacuna). Note the formation of the anoxic corner in the central tissue cylinder.

View larger version (19K): [in a new window]   Fig. 9. Sensitivity analysis showing the effect of individual parameter changes on the critical ambient oxygen tension (PO2crit). The initial state refers to the fasting state (Table 2) with a PO2crit of 10.1 kPa (100%). The value of each parameter (listed by its symbol along the horizontal axis) was decreased to 50% (white) and increased to 200% (grey) of its initial value (Table 2) while keeping all other parameters unchanged.