Recent studies on Daphnia magna have revealed that the feeding current is important for uptake of oxygen from the ambient medium. Respiratory gas exchange should therefore mainly occur within the filtering chamber, whose boundaries are formed by the trunk and the extended carapace shell valves. The precise site of gas exchange in the genus Daphnia is, however, a matter of conjecture. We have developed a method of imaging the haemoglobin oxygen-saturation in the circulatory system of transparent animals, which provides an opportunity to localize oxygen uptake from the environment and oxygen release to the tissues. Experiments were carried out at 20 degrees C on 2.8-3.0 mm long parthenogenetic females maintained in hypoxic culturing conditions, which had resulted in an increased haemoglobin content in the haemolymph. In lateral views of D. magna, the highest values of haemoglobin oxygen-saturation occurred near the posterior margin of the carapace and, surprisingly, in the rostral part of the head. The ambient oxygen partial pressures at which haemoglobin was half-oxygenated were 15 mmHg (2.0 kPa) for the posterior carapace region and 6 mmHg (0.8 kPa) for the rostrum. Although not all parts of the circulatory system could be analyzed using this technique, the data obtained from the accessible regions suggest that the inner wall of the carapace is a major site of respiratory gas exchange. Taking the circulatory pattern and the flow pattern of the medium in the filtering chamber into consideration, it becomes clear that the haemolymph, after passing from the limbs to the carapace lacuna, becomes oxygenated while flowing through the ventral part of the double-walled carapace in a posterior direction. The laterally flattened rostral region, where sensory and central nervous system structures are located, seems to have direct diffusive access to ambient oxygen, which could be especially advantageous during severe hypoxia when the convective transport systems fail to supply enough oxygen to that region.

REFERENCES

Croghan
P. C.
(
1958
).
The osmotic and ionic regulation in Artemia salina (L.)
.
J. Exp. Biol
35
,
219
–.
Fox
H. M.
(
1945
).
The oxygen affinities of certain invertebrate haemoglobins
.
J. Exp. Biol
21
,
161
–.
Fox
H. M.
(
1948
).
The haemoglobin of Daphnia
.
Proc. R. Soc. Lond. B
135
,
195
–.
Fox
H. M.
(
1952
).
Anal and oral intake of water by Crustacea
.
J. Exp. Biol
29
,
583
–.
Fryer
G.
(
1991
).
Functional morphology and the adaptive radiation of the Daphniidae (Branchiopoda: Anomopoda)
.
Phil. Trans. R. Soc. Lond. B
331
,
1
–.
Fryer
G.
(
1996
).
The carapace of the branchiopod Crustacea
.
Phil. Trans. R. Soc. Lond. B
351
,
1703
–.
Gicklhorn
J.
,
Keller
R.
(
1925
).
Über elektive Vitalfärbungen der Kiemensäckchen von Daphnia magna Muller als Beispiel organ-und zellspezifischer Differenzierung
.
Z. Zellforsch
2
,
515
–.
Gicklhorn
J.
,
Sullman
H.
(
1931
).
Die Permeabilität der Kiemensäckchen von Daphnia magna M
.
Protoplasma
13
,
617
–.
Graham
J. B.
(
1988
).
Ecological and evolutionary aspects of integumentary respiration: body size, diffusion and the Invertebrata
.
Am. Zool
28
,
1031
–.
Green
J.
(
1956
).
Variation in the haemoglobin content of Daphnia
.
Proc. R. Soc. Lond. B
145
,
214
–.
Green
J.
(
1957
).
Carotenoids in Daphnia
.
Proc. R. Soc. Lond. B
147
,
392
–.
Halcrow
K.
(
1976
).
The fine structure of the carapace integument of Daphnia magna Straus (Crustacea Branchiopoda)
.
Cell. Tissue Res
169
,
267
–.
Halcrow
K.
(
1982
).
Some ultrastructural features of the nuchal organ of Daphnia magna Straus (Crustacea: Branchiopoda)
.
Can. J. Zool
60
,
1257
–.
Herring
P. J.
(
1968
).
The carotenoid pigments of Daphnia magna Straus. I. The pigments of animals fed Chlorella pyrenoidosa and pure carotinoids
.
Comp. Biochem. Physiol
24
,
187
–.
Kikuchi
S.
(
1983
).
The fine structure of the gill epithelium of a freshwater flea, Daphnia magna (Crustacea: Phyllopoda) and changes associated with acclimation to various salinities. I. Normal fine structure
.
Cell. Tissue Res
229
,
253
–.
Kobayashi
M.
,
Gonoi
H.
(
1985
).
Horizontal movement of pale and red Daphnia magna in low oxygen concentration
.
Comp. Biochem. Physiol
58
,
190
–.
Kobayashi
M.
,
Hoshi
T.
(
1982
).
Relationship between the haemoglobin concentration of Daphnia magna and the ambient oxygen concentration
.
Comp. Biochem. Physiol
72
,
247
–.
Kobayashi
M.
,
Nezu
T.
,
Tanaka
Y.
(
1990
).
Hypoxic induction of hemoglobin synthesis in Daphnia magna
.
Comp. Biochem. Physiol
97
,
513
–.
Kobayashi
M.
,
Takahashi
Y.
(
1994
).
In vivo oxygenation of hemoglobin in early embryos of Daphnia magna
.
Comp. Biochem. Physiol
107
,
127
–.
Kobayashi
M.
,
Tanaka
Y.
(
1991
).
Oxygen-transporting function of hemoglobin in Daphnia magna
.
Can. J. Zool
69
,
2968
–.
Lo
L.-W.
,
Koch
C. J.
,
Wilson
D. F.
(
1996
).
Calibration of oxygen-dependent quenching of the phosphorescence of Pd-meso-tetra (4-carboxyphenyl) porphine: A phosphor with general application for measuring oxygen concentration in biological systems
.
Analyt. Biochem
236
,
153
–.
Paul
R. J.
,
Colmorgen
M.
,
Pirow
R.
,
Chen
Y.-H.
,
Tsai
M.-C.
(
1998
).
Systemic and metabolic responses in Daphnia magna to anoxia
.
Comp. Biochem. Physiol
120
,
519
–.
Pirow
R.
,
Wollinger
F.
,
Paul
R. J.
(
1999
).
Importance of the feeding current for oxygen uptake in the water flea Daphnia magna
.
J. Exp. Biol
202
,
553
–.
Potts
W. T. W.
,
Durning
C. T.
(
1980
).
Physiological evolution in the branchiopods
.
Comp. Biochem. Physiol
68
,
475
–.
Rombough
P. J.
(
1998
).
Partitioning of oxygen uptake between gills and skin in fish larvae: a novel method for estimating cutaneous oxygen uptake
.
J. Exp. Biol
201
,
1763
–.
Seiyama
A.
,
Chen
S.-S.
,
Kosaka
H.
,
Shiga
T.
(
1994
).
Microspectroscopic measurement of the optical properties of rat liver in the visible region
.
J. Microsc
175
,
84
–.
Sugano
H.
,
Hoshi
T.
(
1971
).
Purification and properties ofblood hemoglobin from fresh-water cladocera, Moina macropa and Daphnia magna
.
Biochim. Biophys. Acta
229
,
349
–.
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