AM. ZOOLOGIST, 8:471-479 (1968).
Transport of Oxygen by the Blood of the Land Crab,
Gecarcinus lateralis
JAMES R. REDMOND
Department of Zoology and Entomology, Iowa State University,
Ames, Iowa 50010
SYNOPSIS. Parameters relating to transport of oxygen were measured in the pericardial
blood and venous outflow from the first walking leg of Gecarcinus lateralis.
O2-equilibrium curves of the hemocyanin of G. lateralis were found to be sigmoid and,
at 27°C and pH 7.45, to have a half-saturation pressure of about 17 mm Hg
oxygen. Average partial pressures of oxygen as measured by O2-electrode were 32 mm
Hg in pericardial blood and 9 mm Hg in the venous samples. Analysis of the
O2-content in corresponding samples by the Van Slyke technique revealed an average of
2.17 volumes % O2-capacity for whole blood, 1.45 volumes % for pericardial blood,
and 0.61 volumes % for venous blood. Estimates based on the Van Slyke analyses
indicated an average pO2 of 29 and 14 mm Hg in pericardial and venous samples, respectively. These figures agree fairly well with those obtained by means of Cyelectrodes.
Of the oxygen carried to the tissues, about 94% is carried as oxyhemocyanin and
about 6% is carried in physical solution. As the blood passes through the gills, the
hemocyanin, on an average, becomes 80-85% saturated with oxygen and returned from
the tissues 18-45% saturated with oxygen. These results indicate that the blood of
G. lateralis has a higher O2-capacity than the blood of most other decapod crustaceans
for which similar information is available. In addition, the blood of G. lateralis
transports more oxygen to the tissues per unit volume than do other crustacean
bloods.
The role of hemocyanin for transporting oxygen in the blood of Crustacea
appears well established. It has been
shown in six species of crabs and lobsters
that oxygen entering the gills is transported to the tissues largely in the form of
oxyhemocyanin (Redmond, 1955, 1962;
Spoek, 1962). There are, however, many
questions still unanswered concerning the
specific details of the functioning of hemocyanins in the respiratory physiology
of crustaceans. The exact partial pressures of oxygen in the blood before and
after it has passed through the tissues are
uncertain. The O2-tensions reported in
the above studies were obtained by indirect means, whereas a direct measureThis investigation was supported in part by a
PHS research grant, GM 11199, from the Division
of General Medical Sciences, U. S. Public Health
Service.
Contribution N'o. 435 from the Bermuda Biological Station for Research, Inc. The author wishes to
express his gratitude to Dr. William H. Sutcliffe,
Jr., and the staff of the Biological Station for their
frequent assistance during the couise of this study,
and to Mr. Charles Sivanson for his help with
analytical procedures.
ment of circulatory pOL»'s would be much
more desirable.
The hemocyanins of different species
vary somewhat in their specific properties. It is not yet known to what extent
these variations in properties represent
adaptations to habitat or physiology of a
particular species (Redmond, 1964, 1968).
The present study was undertaken to help
answer these questions and to determine
the properties and respiratory functions of
hemocyanin from a crustacean that, as an
adult, leads a highly terrestrial existence.
The land crab, Gecarcinus lateralis
(Freminville), was chosen as an experimental subject for two reasons. Since this
crab is among the most highly terrestrial
of the marine decapod Crustacea, it serves
as a contrast to typical marine forms and
as a comparison with Cardisoma guanliumi, a related but less terrestrial crab
previously studied (Redmond, 1962).
PROCEDURES
Oxygen-equilibrium curves were established for G. lateralis hemocyanin and
471
472
JAMES R. REDMOND
measurements were made of the pH, pO2,
and O2-content of the blood. All determinations were made upon freshly drawn
samples. With the exception of the Van
Slyke O2-analyses, these studies were conducted at the Bermuda Biological Station
for Research. For the Van Slyke analyses,
crabs were shipped from Bermuda to
Iowa State University. All crabs used were
in the "hard" intermolt stages.
For O2-equilibrium curves, blood was
withdrawn from the base of a walking leg
and prepared as in previous studies. Two
or three ml of blood were diluted 1:1
with a 0.1 M tris-maleate buffer to bring
it to the desired pH, and the mixture was
placed in a tonometer-cuvette. The
blood was equilibrated with air of known
pressures and the corresponding optical
densities of the mixture were measured at
575rn.fiin a Spectronic 20 colorimeter. The
pH of the blood was measured at the
end of the run. Temperatures varied from
27°-28°C. For a more detailed description
of the procedure, see Jones (1955) and
Redmond (1955, 1962).
Direct measurements of partial pressures of oxygen in the blood were obtained using a Beckman macro O2-electrode and the Beckman Physiological Gas
Analyzer, Model 160. Samples of blood
were taken from the base of the first walking leg (cheliped) between the basipodite
and coxopodite, and from the pericardium. The pericardium contains blood that
has just passed through the gills, while
the blood from the leg was taken as an
example of pre-branchial blood.
In order to minimize the effects that
handling the animal might have on circulation of the blood and, consequently, on
O2-content of the blood, attempts were
made to implant small polyethylene tubing (1.5 mm O.D., Intramedic PE 100)
into the ventral thoracic sinus and into
the pericardium. The blood could then
be sampled while the animal was allowed
to move about freely. This proved successful in the case of the pericardium but
not for the ventral thoracic sinus. A hole,
slightly smaller in diameter than the tub-
ing, was drilled into the carapace directly over the heart, and the tubing, filled
with sea-water, was pushed into it for a
distance of about 2 mm. The free end of
the tubing was stoppered with a small
glass plug. The animal was placed in a
large box with opaque sides and the free
end of the tubing was hung over the side
of the box.
After 30-45 minutes a blood sample was
taken by means of a 2 ml syringe connected to the tubing. The first 0.5 ml of mixed
blood and sea-water was discarded and
another milliliter of blood was drawn into
the syringe. This second sample was
immediately transferred from the syringe
to a small glass tube containing the O2electrode. The upper end of the tube was
slightly larger in diameter than the O2electrode that was inserted into it. The
lower end of the tube tapered to an opening of about 4 mm. A small length of
rubber tubing was slipped over this end
and was used to connect the 2 ml syringe
to the tube containing the O2-electrode.
The blood sample was then pushed up,
completely filling the space between the
syringe and electrode, and flowed around
the electrode to a height of 4 cm. A series
of calibrating tests indicated that readings
stabilized after about 4 minutes, so that
final readings of pO2 were taken at this
time. The electrode was standardized at 0
mm Hg O2 and at atmospheric pressure
before and after each blood sample, and a
calibration curve was established using a
series of water samples equilibrated with
known pressures of oxygen.
Tubes were not implanted successfully
into the ventral thoracic sinus, nor could
a syringe be used quickly enough to sample this site without the possibility of
handling having some effect on the blood
parameters. Therefore, although the sample might not represent as good an average of pre-branchial blood as would that
of the thoracic sinus, it was decided to use
blood from the base of the cheliped.
This could be obtained quickly and easily
with a 2 ml hypodermic syringe. In all
cases involving the measurement of inter-
473
OXYGEN-TRANSPORT IN Gecarcinns
i Hg Oiygan
FIG. 1. Oxygen-equilibrium curves of the hemocyanin of G. lateralis. These curves illustrate three
representative determinations at three different
pH's, 27-28«C.
nal pO2, pH, or O2-content, only those
samples were used which were obtained
quickly and without air bubbles.
Samples obtained by the preceeding
two methods were placed in 5ml beakers
and their pH was measured with a Beckman Zeromatic pH meter. The pH of the
blood in the beaker was very stable and
did not change for several minutes.
Samples of blood for Van Slyke analysis
were taken by the following procedure.
A hole slightly smaller in diameter than
that of a number 20 syringe needle was
bored through the carapace over the
heart, and immediately a number 20 needle on a 2 ml syringe was inserted about 2
mm into the hole. A sample of 1.2 ml of
blood was withdrawn and the syringe was
placed in an ice bath. A similar sample
was then taken from the base of the cheliped and placed in the ice bath. A third
sample of 1.5-2.0 ml, for measurement of
O2-capacity, was taken from other legs
and placed in a stoppered vial. Because of
the low O2-capacities of hemocyanin
bloods, a minimum of 1 ml was required
for each Van Slyke analysis. One ml each
of the two samples in the ice bath was
immediately analyzed for O2-content. The
third sample was allowed to clot and was
then expressed through a small piece of
cloth to free the fluid from the clot. This
fluid was kept in a stoppered vial overnight in a refrigerator and its O2-capacity
determined the next day. Because of air
bubbles in the sample or the small size of
the animal, it was often not possible to
obtain all three samples.
The procedure for the Van Slyke analysis was modified slightly from that used in
the past (Redmond, 1955). It was found
that slightly larger amounts of oxygen
were obtained by omitting the 5% sodium
cyanide previously used to poison the hemocyanin. Simple evacuation, followed by
adding 0.5 ml of 5% sodium hydroxide, was
adequate to release the oxygen held by
the blood. In addition to absorbing any
carbon dioxide released by the blood, the
sodium hydroxide denatured the hemocyanin and prevented it from recombining
with oxygen.
RESULTS
Representative O2-equilibrium curves
are shown in Figure 1. With one exception the pH of all blood samples fell
within the range 7.30-7.60 (see Table 1).
At normal blood pH and the temperatures of these determinations (26-27°C)
the hemocyanin becomes half-saturated at
16-17 mm Hg O2-pressure. Daytime temperatures at which the animals were living ranged from 25-30°C. The curves in
Figure 1 indicate that the O2-affinity of
the hemocyanin of G. lateralis is similar
to that of other crustacean hemocyanins.
The curves are distinctly sigmoid and,
like all other crustacean hemocyanins examined, shift to the right as the pH
drops. Figure 2 illustrates the extent to
which the half-saturation pressure changes
with pH. The Phi value for the pH
range 7.2-7.7 is -.33, where Phi = A log
p50/A pH (Wyman, 1948). The magnitude of this shift seems similar to that of
the few other hemocyanins similarly
tested (Redmond, 1962), although the
scatter of points about the curve renders
the exact Phi value uncertain.
Oxygen-tensions in the pericardium and
venous return from the cheliped were
measured by means of an O2-electrode
(Table 1). Each pO2 listed represents a
single animal with the temperature and
474
JAMES R. REDMOND
I"
pH
FIG. 2. Effect of p H on the O 2 -pressure at which
the heinocyanin of G. laleralis becomes halfsaturated with oxygen. Blood p H of 33 specimens
Kinged from 7.20 to 7.60.
pH of the blood sample as indicated. The
%-saturation of the heinocyanin at each
pO2 was estimated with the use of the
Oo-equilibrium curves of Figure 3. These
curves are extrapolated to the pH range
observed and are based on the curves of
Figures 1 and 2. According to the averages given in Table 1, the hemocyanin
becomes approximately 87% saturated
with oxygen in the gills and returns from
the tissues about 18% saturated, while the
pO2 changes from 32 to 9 mm Hg. As
might be expected, there is considerable
variation in the per cent to which the
blood coming from both gills and tissues
is saturated with oxygen.
Table 2 presents the results of O,-analyses of pericardial and venous blood by
the Van Slyke method. Gaps in the table
indicate where samples could not be obtained. Because of the problem of obtaining adequate blood, the pH of these samples was not measured.
Partial pressures of oxygen in the blood
were estimated as follows. For pericardial samples, an O2-equilibrium curve extrapolated to pH 7.45 was drawn. The vertical axis of the graph was subdivided to
represent volumes % oxygen instead of the
more usual %-saturation. Because the
O2-capacity of the blood varied among
the individual crabs, the vertical scale of
the graph had to be adjusted for each
individual animal on the basis of the
O2-capacity of the hemocyanin of its
blood. To estimate the O2-capacity of the
hemocyanin, a constant 0.45 ml of oxygen
was subtracted from the total O2-capacity
of a blood sample. This 0.45 ml represents
the oxygen in physical solution, the remainder of the oxygen being present as
oxyhemocyanin.
On the same graph, the O2-solubility in
the blood was plotted. This took the
form of a straight line passing from the
origin through the point, 0.45 ml and 150
mm Hg oxygen.
The partial pressure of oxygen in a
TABLE 1. Oxygen-electrode measurements of blood
samples from the pericardium and venous outflow
of the cheliped of G. lateralis. Per cent oxyhemocyanin values were estimated from
O,-equilibrium
curves drawn to the proper pH (see Fig. 5).
°c
PH
Pericardial blood
27.6
28.7
27.0
28.9
29.0
29.0
29.6
29.6
28.0
28.0
28.8
27.3
27.2
28.8
28.9
28.4
Av. 28.4
7.55
7.35
7.50
7.31
7.41
7.52
7.48
7.57
7.30
7.40
7.47
7.42
7.60
7.45
7.45
7.39
7.45
Venous blood
27.2
27.0
26.8
27.2
27.3
27.2
27.3
27.8
27.9
27.8
28.0
27.9
27.5
27.2
27.2
Av. 27.4
7.32
7.55
7.36
7.35
7.30
7.47
7.42
7.44
7.42
7.35
7.45
7.42
7.20
7.44
7.30
7.39
pO 3
mmHg
Estimated
% OxyHcy
49
31
30
31
35
24
30
38
30
30
19
34
35
98
87
90
86
22
34
42
32
12
11
10
12
4
8
10
14
14
9
8
6
3
2
8
9
92
78
88
97
84
87
56
92
96
67
93
96
87
24
28
18
25
7
15
20
36
36
14
15
9
2
3
11
18
OXYGEN-TRANSPORT IN
•175
Gecarcinns
DISCUSSION
FIG. 3. Theoretical O.-equilibrium curves used to
estimate the %-saturation o£ hemocyanin values
given in Table 1 and the O3-pressures given in
Table 2. The shape of the curves was determined
by the shape of the curves in Figure 1, with the
half-saturation pressures adjusted to agree with the
curve in Figure 2. The pH of each curve,
reading from left to right is 7.60, 7.50, 7.40, 7.30
and 7.20. 27-28°C.
sample of blood was estimated by finding
a point on the horizontal axis of the
graph at which the corresponding volumes % of the Oo-equilibrium curve and
the O2-solubility curve would sum to
equal the amount found in the sample by
Van Slyke analysis. This point on the
horizontal axis indicates the partial pressure of oxygen in the blood.
The partial pressure of oxygen in samples of venous blood was estimated in the
same way except that an Oo-equilibrium
curve at pH 7.39 was used.
In pericardial blood, the average pO2
was 29 mm Hg and the hemocyanin was
81% oxygenated. For venous blood from
the leg, the corresponding figures were 14
mm Hg and 36%. This series of measurements agrees fairly well with those obtained by use of the O2-electrode. The
greatest discrepancy between these two
sets of figures lies in the %-saturation of
blood returning from the tissues of the
leg, 36% by Van Slyke analysis and 18% by
oxygen electrode. Table 2 also indicates
that an average of 0.80 ml of oxygen is
delivered to the tissues by 100 ml of
blood. Note in addition that the O2-capacity of the blood of G. lateralis is high for
a crustacean.
The gaseous exchange between leg and
gills by the blood of G. lateralis is summarized in Figure 4 which presents graphically certain of the data from Tables 1
and 2. Since these data were obtained
from blood at various pH's, only the partial pressures of oxygen at the average
%-saturations are indicated. The curve
represents the O2-equilibrium of the hemocyanin at pH 7.40 and 27°C.
The ranges of pOo's and %-saturations
are fairly large, but the general results are
clear. As with other crustaceans examined, oxygen transported by the blood is
largely in the form of oxyhemocyanin.
The quantity of oxygen in physical solution at any O2-pressure may be read
from the sloping line of Figure 4. At the
higher blood pO2's this is about 0.1 vol. %,
and at venous pO2's about 0.03 vol. %.
FIG. 4. Summary of O3-exchange by the blood of C.
lateralis. The curve indicates the O:-cquilibrium of G. lateralis' hemocyanin at pH 7.40 and
27°C. The vertical bars crossing the curve show the
%-saturation of the hemocyanin and the volumes % oxygen found in the blood by Van Slyke
analysis (V.S.) and by O2-electrode (E). The two
bars to the right represent pericardial blood; the
two bars to the left, the venous outflow of the
cheliped. The bars show the range, standard deviation, and average for each set of measurements.
Since conditions of pH and temperature were not
constant during these measurements, the O»-pressure corresponding to the % saturations may not
be read from the pH 7.40 Os-equilibrium curve.
The bars are drawn over the average partial pressures of oxygen found in the pericardial and cheliped blood. The O2-solubility line at the bottom of
the graph shows the approximate volumes %
oxygen that will be dissolved in the blood as the
Oj-pressure varies.
l
E 2. Oxygen content of the blood of G. lateralis as measured by Van Slyke analysis. Each horizontal line indicates data from a single specimen.
gen-pressures were estimated by using O,-equilibrium curves at pH 7.45 and 739 for pericardial and venous blood samples, respectively. These
measurements were made on animals living at 26-27'C.
Estimated
pO 2>
mm H g
Estimated
pO*
Vol. % O 2
% OxyHcy
mm Hg
20
0.52
30
0.51
0.57
0.38
0.92
0.99
0.85
0.28
0.53
0.76
0.38
37
57
44
25
12
le blood
Hey
Vol. % O a
% OxyHcy
1.72
1.26
1.27
0.81
0.81
0.81
59
89
1.61
1.16
1.19
93
1.80
2.44
2.04
2.74
2.54
2.61
1.87
1.47
3.19
2.69
1.65
2.96
1.35
1.99
1.59
2.29
2.09
2.16
1.42
1.02
2.74
2.24
1.20
2.51
1.33
1.73
1.28
1.72
93
83
76
72
1.85
2.05
84
91
36
34
27
24
23
28
2.17
1.72
0.93
2.15
1.58
1.02
1.62
1.45
Pericardial-venous difference
Venous blood
Pericardial blood
apacity, Vol. %
32
72
82
23
27
81
29
43
58
35
12
23
49
33
Estimated
pO,
change
Vol. % O3
% OxyHcy
15
0.29
17
17
0.30
0.62
0.95
0.81
0.29
0.87
1.57
1.52
22
32
40
18
37
68
4
9
20
21
0.80
45
14
16
0.93
0.45
32
19
20
14
8
11
18
14
14
10
0.61
36
14
5
49
13
19
68
22
72
11
o
a
477
OXYGEN-TRANSPORT IN Gecarcinus
TABLE 3. Volumes % oxygen transported as oxyhemocyanin and in solution by the blood of
G. lateralis. These are average values obtained from the nine complete sets of data presented in
Table 2.
Volumes % Oxygen
Pericardial blood
Venous blood
Oxygen delivered to tissues
% Oxygen
Total
As OxyHcy
Dissolved
1.42
0.62
0.80
1.33
0.58
0.75
0.09
0.04
0.05
The relative and absolute amounts of
oxygen present and delivered to the tissues are seen in Table 3, the data of
which are averages of the complete sets of
Van Slyke analyses (first nine animals of
Table 2). The data obtained indirectly by
O2-electrode indicate similar findings.
However, since the latter data show a
greater loading of oxygen in the gills and
greater unloading of oxygen in the tissues
than the direct data show, the result of
measurements with the O2-electrode would
be to show a greater quantity of oxygen
delivered to the tissues.
Although the results of the Van Slyke
analyses and the measurements by O2electrode agree fairly well, the exact cause
of the difference between them should be
considered. Presumably, since they are direct measurements, the pO 2 values obtained by the electrode and the total
amounts of oxygen in the blood as determined by the Van Slyke analyses are the
most reliable. It seems likely that most
discrepancies are due to the lack of pH
values for the blood samples used in Van
Slyke analyses and to the calculations that
determine other values indirectly. These
calculations require accurate O2-equilibrium curves and estimations of the physical
solubility of oxygen in the blood.
The scatter of points in Figure 2 shows
that there is some variation in the O2equilibrium curves. Some of this may be
attributed to the lack of precise control of
temperature during these measurements.
The possibility also exists that there is
some actual variation in the (^-equilibrium curve among individuals of the species. Slight shifts of O2-equilibrium curve
could make the two sets of values converge or diverge.
As OxyHcy Dissolved
94
94
94
6
6
6
The assumption was made that the solubility of oxygen in the blood of G. lateralis is about 90% that of sea-water at corresponding temperatures. This was the
approximate solubility found by Redfield,
et al. (1926) for the blood of Limulus,
Cancer, Busycon, and Callinectes. If this
figure is reduced slightly, the two sets of
values will approach one another even
more closely. Since the amount of protein
in the blood is variable, the solubility of
oxygen will vary slightly from one individual to the next, but the error introduced by this variation should be very
small. Another possibility is that greater
handling of the animals necessary for taking blood samples for the Van Slyke analyses may have had an adverse effect
upon circulatory rate or O2-exchange or
both.
The general pattern of transport of oxygen by the blood of G. lateralis is similar
to that found in previous studies
(Redmond, 1955, 1962; Spoek, 1962), but
there are some interesting differences.
Studies on the spiny lobster, Panulirus
interruptus, the American lobster, Homarus americanus, the sheep crab, Loxorhynchus grandis, and the land crab,
Cardisoma guanhumi, indicate very low
pO2's in the pericardium (5-8 mm Hg)
and in venous blood (2-4 mm Hg), calculated indirectly. The present study indicates that in G. lateralis blood pO2's are
noticeably higher, 28-32 mm Hg and 9-14
mm Hg, respectively. The values found for
G. lateralis agree most closely with those reported by Spoek (1962) for the spider crab,
Maja squinado, and the lobster, Homarus
gammarus.
In the spiny lobster, American lobster,
and the sheep crab, the low pO2's are the
478
JAMES R. REDMOND
result of a relatively low %-saturation of the
hemocyanin as it passes through the gills,
together with a shift of the O2-equilibrium
curve to the left caused by the lower temperatures (about 15°C) at which these species were living.
On the other hand, Cardisoma guanhumi
seems to be a special case in that it has a
hemocyanin with an unusually high O2-alfinity that consequently operates at low
pOo's. Aside from this peculiarity, C. guanhutni is like G. lateralis in that it has a relatively high concentration of hemocyanin in
the blood and, per unit volume, its blood
transports a greater amount of oxygen to
the tissues than does that of the other
species examined.
One reason for initiating the present
study was the finding of the unusually high
Oo-affinity in the hemocyanin of C. guanhumi (p,)0 of 4 mm Hg O2 at pH 7.40 and 25°
C). I suggested that this property of C. gnanliurni hemocyanin might be an adaptation
related to the conservation of water (Redmond, 1962). C. guanhumi is a "land crab"
that spends a great deal of time out of
water, but usually close to it. Compared
with normal aquatic forms, it has a reduced
gill area (Gray, 1957; Bliss, 1963) that helps
to decrease the loss of water from the branchial chamber. For respiratory purposes this
reduction in surface area of the gills could
be partially compensated for by increasing
the Oo-diffusion gradient across the surface
of the gill. This, in turn, could be accomplished by having a hemocyanin with its
Oo-equilibrium curve shifted far to the left,
as is that of Cardisoma.
The question then arose as to whether this
high Oo-affinity is a characteristic of the
hemocyanin of all land crabs. G. lateralis,
which is more terrestrial than C. guanhumi,
was investigated and, as the present study
shows, was found to have a hemocyanin
with a normal Oo-affinity. Preliminary studies on the semi-terrestrial ghost crab, Ocypode quadrata, showed its hemocyanin to
have an even lower Oo-affinity than the hemocyanin of G. lateralis (Redmond, in
press). These findings cast considerable
doubt on the above interpretation of the
low loading pressures of C. gwinhwnis hemocyanin. Relative to body size, G. lateralis has an even greater reduction in gill
surface than does C. guanhumi (Bliss,
1963). If this reduction has created difficulties in gaseous exchange, then it would
seem that the hemocyanin of G. lateralis
should also show a high O2-affinity, which
it does not.
Other factors possibly related to this
difference in O2-affinity may be the type of
burrow inhabited by these two species, or
secondary adaptations concerned with gaseous exchange or water balance. C. guanhumi normally burrows above tide levels,
but in such a situation that the bottom of
the burrow contains water. Such water
could become low in oxygen, in which case
the high O2-affinity of the hemocyanin
would be of advantage. This seems unlikely
to be of importance during periods of normal activity, since C. guanhumi has immediate access to aerial respiration. However,
possibly to molt or to avoid dry or cold
weather, this crab will retreat into a chamber at the bottom of its burrow, seal the
opening to the surface, and remain underground for extended periods of time (Gifford, 1962). The high O,-affinity of the
hemocyanin may well be of adaptive significance under these conditions.
Secondary modifications of the branchial
chamber may be significant. The lining of
the branchial chamber in both C. guanhumi
and G. lateralis is highly vascularized.
These vascular surfaces presumably serve
as additional sites for gaseous exchange between air and blood, although the extent to
which they supplement respiratory exchange has not been determined.
If the vascularization of the branchial
chamber in G. lateralis is significantly more
effective than that of C. guanhumi in aiding gaseous exchange, then it might not be
necessary to increase the rate of Oo-diffusion across the gills by other means, i.e. by
increased O2-affinity of the hemocyanin. It
is even conceivable, though unlikely, that
the high O2-affinity of the hemocyanin of
Cardisoma may be an evolutionary relic
from an aquatic ancestral form that might
OXYGEN-TRANSPORT IN
have lived in regions, such as estuarine
swamps, where oxygen was in limited supply. The significance of the marked difference in O2-affinity of the hemocyanins in
these two species of the same family must,
for the time being, remain uncertain.
The presence of vascularized areas in the
branchial linings brings up another important point. The levels of oxygen in blood
samples in the present study give a good
general picture of O2-transport by the blood
of G. lateralis. However, the picture may
not be complete, since it does not necessarily account for oxygen that may enter
through non-gill surfaces. Similarly, although the venous return from the leg is
indicative of venous levels of oxygen, it may
be somewhat different from the average
O2-content of the total venous return to the
gills. Clarification of these points must wait
for further studies.
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