Journal of National Fisheries University 72（1）1-9（2023）

Full Paper

Effect of hypoxia on the oxygen and acid–base status ofhemolymph in Akoya pearl oyster,
Pinctada fucata martensii
Takeshi Handa 1 † and Akira Araki 1
Abstract : We investigated the hemolymph oxygen and acid–base status of Akoya pearl oysters, Pinctada fucata
martensii, exposed to hypoxic seawater to elucidate the acid–base balance. Akoya pearl oysters cannulated to theanterior aorta for hemolymph collection from the submerged animals showed oxygen and acid–base disturbance ofthe hemolymph during environmental hypoxia for 24 h (O2 partial pressure in seawater, Pwo2 8 torr). The
hemolymph O2 partial pressure (Po2) decreased from 72.2 torr to 13.6 torr, pH decreased from 7.581 to 7.129, andCO2 partial pressure (Pco2) increased from 0.86 torr to 3.31 torr during hypoxia. The hemolymph total CO2
concentration (Tco2) and bicarbonate ion concentration ([HCO3–]) were 1.93–1.95 mM/L and 1.80–1.91 mM/L,
respectively, and there was no statistically significant change between pre-hypoxia and hypoxia for 24 h. Whennormoxic seawater was resumed after the hypoxia, the hemolymph Po2, pH, and Pco2 returned to their initial levels
for about 3 h, and hemolymph Tco2 and [HCO3–] gradually increased. These results showed that Akoya pearl
oysters undergo hypoxemia and respiratory acidosis in the hypoxic environments for 24 h (Pwo2 8 torr). In posthypoxia, most of the disturbances disappeared within 3–24 h, and the increase in hemolymph [HCO3–] which was asecondary change compensated for respiratory disturbance.
Key words : Pinctada fucata martensii, hemolymph acid–base balance, Pco2, CO2 dynamic phase, environmental
hypoxia, respiratory physiology

Introduction

environmental hypoxia 3). The hemolymph acid–base

The Akoya pearl oyster, Pinctada fucata martensii, is a

the resting condition7), prolonged air exposure8), short-filibranchial bivalve classified in the Pteriidae1). Akoya

term air exposure9), and post-cannulation to the adductor

pearl oysters are used to produce Akoya pearls, and themuscle10). Akoya pearl oysters in normoxic seawater atprocess of pearl production is directly related to

20–28°C, have a hemolymph pH of 7.330–7.586; total CO2

metabolism. The metabolism of the Akoya pearl oyster

concentration (Tco 2) of 1.90–2.25 mM/L; CO 2 partial

has been studied in terms of the regulation of oxygenpressure (Pco 2) of 0.92–2.2 torr; and bicarbonate ion

uptake, gill ventilation volume, and ciliary movement

concentration ([HCO 3–]) of 1.72–2.21 mM/L. In the air

under normoxic and hypoxic conditions2-6). The pearl

exposure, the animals showed hypoxemia and respiratory

oyster, P. fucata martensii, was found to maintain the level

acidosis in short-term exposure (4 h)9), and accompaniedof oxygen uptake (water temperature, 20.5°C), even if themetabolic acidosis in prolonged exposure (24 h)8). There

oxygen partial pressure of seawater (Pwo2) was reduced

are, however, few reports on the effect of hypoxia on

3)

to about 30 torr . However, below 22 torr, oxygen uptake
3)balance of Akoya pearl oysters has been studied under

Akoya pearl oysters from the viewpoint of the CO2

decreased . A further reduction in Pwo2 below 10.3 torr

dynamic phase and acid–base balance of the hemolymph.

induced frequent closing of shell valves and mantle lobe,

In pearl production, Akoya pearl oysters are cultured indecreasing gill ventilation. Ciliary movement enabledinner bays, and are often exposed to hypoxic seawater

maintenance of the initial level without stopping under(oxygen-deficient water mass). We examined the effect ofReceived date: 5 June 2023. Accepted date: 22 August 2023
1 Department of Applied Aquabiology, National Fisheries University, 2-7-1 Nagata-honmachi, Shimonoseki, Yamaguchi, Japan†
Corresponding author: handat@fish-u.ac.jp (T. HANDA)

2

Takeshi Handa and Akira Araki

hypoxia on hemolymph oxygen and acid–base status,

near the dorsal margin was resected in a rectangle (4 x

evaluating acid–base balance and CO2 dynamics. These

10 mm) with a router and a scalpel. Through theresults would contribute to the fundamental

rectangular window, the cannula (polyethylene tubing,understanding of Akoya pearl oysters and assist

0.97 mm outer diameter, Clay Adams PE50, Becton

environmental assessments of sea areas in which they

Dickinson Co., USA) was inserted into the anterior aorta

are cultured.

with a stylet and advanced toward to the heart direction

The direct measurement of Pco2 is difficult when therefor 5 mm. The stylet was then removed, and the outside

is only a small quantity of hemolymph sampled, becauseof the cannula was sealed. The cannula was gently fixed

the Pco 2 of the bivalves is very low under normal

to the left shell valve, and the window was securely

conditions. Estimation of the CO2 partial pressure byclosed with dental adhesive (Kobayashi Pharmaceuticalapplication of the Henderson–Hasselbalch equation isCo., Japan) and surgical superglue (Aron alpha A, Sankyo

practiced in studies on the acid–base balance owing toCo., Japan). The cannulated animals were returned to the11)

its relative ease and accuracy . In the Henderson–

respiratory chamber and allowed to rest overnight under

Hasselbalch equation, the CO2 solubility coefficient (αco2)

a normoxic condition.

and apparent dissociation constant (pKapp) of carbonicacid in the hemolymph are required. The hemolymph
αco2 and pKapp in Akoya pearl oysters at 20°C were
8)

Experimental protocol and Hemolymph collection
The cannulated animals were induced to hypoxia.

previously reported , and we used the results to

Pwo2 was adjusted at 8 torr, because the pearl oyster

calculate the hemolymph CO 2 partial pressure anddecreased gill ventilation volume under Pwo2 10.3 torr3).

bicarbonate concentration in this study.

The hypoxic seawater, which was gassed continuouslywith N2 in a counter-current gas exchange column, wasMaterials and Methods

flowed into the respiratory chamber for 24 h. After
hypoxia for 24 h, the supply of hypoxic seawater was
stopped, and the inflow of normoxic seawater was

Experimental animals
Akoya pearl oysters (n=60; shell length, 59.8 ± 3.2 mm

resumed into the respiratory chamber. The animals(mean±SD); shell height, 64.9± 3.1 mm; total wet weight,

experienced normoxia for 24 h after the hypoxia. As a

35.9 ± 6.1 g) were obtained from a marine farm incontrol group, the cannulated animals did not induce toTsushima, Nagasaki Prefecture, Japan. After cleaning thethe hypoxia and were examined at the same time withshell valves, they were reared for one month at 20ºC in

the hypoxic group.

aerated seawater containing added cultivated12)

Hemolymph collection was performed on different

phytoplanktons . The Akoya pearl oysters wereanimals each time. In the hypoxic group, an initialtransferred to a respiratory chamber with a flow ofhemolymph sample was collected through the cannula

particle-free (>0.45 µm) seawater before the experiment.

before the induction of hypoxia (pre-hypoxia, PreHy).

All experiments were conducted in seawater with a

When the experimental animals experienced the hypoxia

salinity of 31, water temperature (WT) of 20°C, O 2

for 24 h, the hemolymph samples were similarly collectedsaturation (Swo2) of 97%, pH of 8.08, and total CO2 content(Hy24). After the hypoxia was finished, the hemolymphof 1.9 mM/L.

samples were collected at 3 h, 12 h, or 24 h (post-hypoxia,
PostHy3, 12, 24). In the control group, hemolymph
samples were collected from the cannulated animals at

Animal Surgery
We operated on the shell valve and anterior aorta of

the same time as those in the hypoxic experiment. All

the Akoya pearl oysters for hemolymph collection fromhemolymph samples were anaerobically collected

the submerged animals

7,13). Part of the left shell valve

through the cannula which was connected to a gas-tightAcid–base balance of hypoxic akoya pearl oysters

microsyringe (Model 1750LTN, Hamilton Co., USA). The
volume of each hemolymph sample was approximately
0.3 mL.

3

Statistical analysis
The data are expressed as means ± standard deviation.
Kruskal–Wallis test was performed for changes in thehemolymph properties over the experimental timecourse. The multiple comparison of all pairs used the

Hemolymph analysis and calculation
The hemolymph oxygen partial pressure (Po2, torr),

Steel–Dwass test. Differences between the control grouppH, and total CO 2 concentration (Tco 2, mM/L) wereand hypoxic group were compared using the Mann–measured immediately after each collection. Po2 was

Whitney U test. Statistically significant differences were

measured using a blood gas meter (BGM200, Cameron

set at P < 0.05. All analyses were performed with theInstruments Co., USA) and Po2 electrode (E101, Cameronstatistical software Kyplot v. 5.0 and 6.0 (KyensLab Inc.,

Instruments Co., USA). The pH was measured using a

Japan).

blood gas meter with pH glass and reference electrodes(E301, E351, Cameron Instruments Co., USA). The Po2
and pH electrodes were installed in a water jacket

Results

maintained at 20°C. Tco2 was measured using a total CO2

Akoya pearl oyster in the control group did not show

analyzer (Capnicon 5, Cameron Instruments Co., USA).

significant changes in hemolymph properties. (P > 0.05,

The hemolymph CO2 partial pressure (Pco2, torr) and

Figs. 1–6). In the hypoxic group, the experimental–

bicarbonate concentration ([HCO 3 ], mM/L) wereanimals were exposed to the hypoxic seawater for 24 h

calculated by rearranging the Henderson–Hasselbalch(Pwo 2 , 8 torr), and hemolymph properties were

equation

11,14). In the equation, the CO2 solubility coefficient

statistically significantly changed. The mean values of(αco2, µM/L/torr) and apparent dissociation constant ofhemolymph Po2 decreased from 72.2 torr to 13.6 torr

carbonic acid (pKapp) of the Akoya pearl oyster are

during hypoxia (P < 0.05, Fig. 1). The hemolymph pH

required. Handa and Araki (2021) described thedecreased from 7.581 to 7.129 during hypoxia (P < 0.05,

hemolymph αco2 (40 µM/L/torr), and pKapp, Pco2, andFig. 2). The hemolymph Tco 2 was 1.93–1.95 mM/L

8)–

[HCO3 ] were calculated using the following equations :between PreHy and Hy24, and there was no significant
change (P > 0.05, Fig. 3). The calculated hemolymph Pco223

pKapp =183.939 – 77.811 • pH + 11.340 • pH – 0.5508• pH

and [HCO3–] at PreHy were 0.86 torr and 1.91 mM/L,

Pco2 = Tco2 • [0.040 • (1+10 (pH-pKapp))] –1

respectively. The hemolymph Pco 2 increased during–[HCO3 ] = Tco2 − 0.040 • Pco2

hypoxia, reaching 3.31 torr at Hy24 (P < 0.05, Fig. 4). The
hemolymph [HCO3–] was 1.80–1.91 mM/L between PreHy

where the units of the parameters are torr for Pco2,–

and Hy24, and there was no significant change (P > 0.05,
Fig. 5). After the period of hypoxic condition, the inflow

and mM/L for Tco2 and [HCO3 ].

of normoxic seawater into the respiratory chamber was
For assessment of the relationship between–

resumed, and hemolymph Po2 and pH increased to theirhemolymph pH and [HCO3 ] of the experimental animals,

initial levels for 3–24 h (P < 0.05, Figs. 1–2). The

the non-bicarbonate buffer value (ꞵ NB , the slope ofhemolymph Pco2 decreased to the initial level and was

8)relational expression) used 0.46 Slykes . The hemolymph

maintained for 3–24 h (P < 0.05, Fig. 4). The hemolymph2+calcium concentrations ([Ca ], mM/L) were determined

Tco 2 and [HCO 3–] gradually increased after hypoxia,

with a test kit (Calcium E-test, Wako Pure Chemical Co.,

reaching 2.39 mM/L and 2.34 mM/L at PostHy24,

Japan) and a spectrophotometer (Spectronic 20A,

respectively (P < 0.05, Figs. 3, 5). Hemolymph Tco2 andShimadzu Co., Japan).

[HCO 3–] of the hypoxic group between PostHy3 and
PostHy24 were higher than that of the control group (P4Takeshi Handa and Akira Araki< 0.05, Figs. 3, 5). Hemolymph [Ca 2+ ] did not show

hemolymph [HCO3–] did not change significantly, and thesignificant changes during each phase (P > 0.05, Fig. 6).

point at Hy24 followed along the non-bicarbonate buffer

The progress of change in the acid–base balance of theline. After hypoxia, the hemolymph [HCO3–] at PostHy24

experimental animals is summarized in a pH-[HCO 3–]

was higher than the initial value, and this point was

diagram (Fig. 7). The hemolymph pH of the hypoxic

located above the non-bicarbonate buffer line (Fig. 7).

animals (Hy24) decreased with increasing Pco 2, but

80
60

7.800
a
a

a

a

a

a

Hemolymph pH

Hemolymph Po2 (torr)

100

a

a
a

40

7.400

a

a

a

a

a
a

b†

b†
PreHy

Hy24

PostHy3

7.000

PostHy12 PostHy24

Phase
Fig. 1 42％

a

a

7.200

20
0

a

7.600

PreHy

Hy24

PostHy3

PostHy12 PostHy24

Phase

Fig. 1 Effect of hypoxia (Pwo2 8 torr) on the hemolymph
Fig. 2 Effect of hypoxia (Pwo2 8 torr) on the hemolymph
O2 partial pressure (Po2, torr) in the Akoya pearl
pH in the Akoya pearl oyster, Pinctada fucata
Fig. 2 42％
martensii. Hemolymph from the anterior aorta wasoyster, Pinctada fucata martensii. Hemolymph of thecollected via cannula from each experimental
anterior aorta was collected via cannula from eachanimal. Solid circle: hypoxic group; open circle:
experimental animal. Solid circle: hypoxic group;
control group (no induced hypoxia). PreHy:
open circle: control group (no induced hypoxia).
pre-hypoxia (Pwo 2 155 torr); Hy24: exposed to
PreHy: pre-hypoxia (Pwo 2 155 torr); Hy24:
exposure to hypoxic seawater for 24 h (Pwo2 8
hypoxic seawater for 24 h (Pwo2 8 torr);torr); PostHy3–24: post-hypoxia for 3 h, 12 h, or 24
PostHy3–24: post-hypoxia for 3 h, 12 h, or 24 hh (resumption of exposure to normoxic seawater,(resumption of exposure to normoxic seawater,
Pwo2 155 torr). The values are means ± SD (n = 6
Pwo2 155 torr). The values are means ± SD (n = 6
in each plot). Different lowercase letters indicate
in each plot). Different lowercase letters indicate
significant differences (P < 0.05, Steel–Dwass
significant differences (P < 0.05, Steel–Dwass
multiple comparison test). The dagger indicates amultiple comparison test). The dagger indicates asignificant difference from the control group andsignificant difference from the control group andhypoxic group (P < 0.05, Mann–Whitney U test).
hypoxic group (P < 0.05, Mann–Whitney U test).

Hemolymph Tco 2 (mM/L)

3.00

2.00

a

ab

a

a

PreHy

Hy24

ab †

a

a

b†

a

1.00

0.00

PostHy3

Phase
Fig. 3 42％

ab †PostHy12 PostHy24

Fig. 3 Effect of hypoxia (Pwo2 8 torr) on the hemolymph
total CO2 concentration (Tco2, mM/L) in the Akoya
pearl oyster, Pinctada fucata martensii. Hemolymph
from the anterior aorta was collected via cannulafrom each experimental animal. Solid circle:
hypoxic group; open circle: control group (noinduced hypoxia). PreHy: pre-hypoxia (Pwo2 155
torr); Hy24: exposed to hypoxic seawater for 24 h
(Pwo2 8 torr); PostHy3–24: post-hypoxia for 3 h, 12h, or 24 h (resumption of exposure to normoxic
seawater, Pwo2 155 torr). The values are means ±SD (n = 6 in each plot). Different lowercase letters
indicate significant differences (P < 0.05, Steel–
Dwass multiple comparison test). The daggers
indicate a significant difference from the control
group and hypoxic group (P < 0.05, Mann–
Whitney U test).

Hemolymph Pco 2 (torr)

5.00
4.00
b†
3.00
2.00
1.00
0.00

a

a

a
a

a
PreHy

Hy24

PostHy3aaaaHemolymph [HCO 3-] (mM/L)

Acid–base balance of hypoxic akoya pearl oysters
3.00

a

ab

a

a

PreHy

Hy24

2.00

a

a

a

a

a
a

a

5.0

0.0

PreHy

Hy24

PostHy3

Phase

PostHy12 PostHy24

Hemolymph [HCO 3-] (mM/L)

Hemolymph [Ca2+E] (mM/L)

3.00

a

a

a

PostHy12 PostHy24

Fig. 5 E ffect of hypoxia (Pwo 2 8 torr) on the hemolymph–
42％ bicarbonate concentration ([HCO3 ], mM/L) in the Akoya
Pinctada
fucatamartensii.
pearl oyster,
Hemolymph fromthe anterior aorta was collected via cannula from eachexperimental animal. Solid circle: hypoxic group; open
circle: control group (no induced hypoxia). PreHy: prehypoxia (Pwo2 155 torr); Hy24: exposing to the hypoxic
seawater for 24 h (Pwo2 8 torr); PostHy3–24: post-hypoxia
for 3 h, 12 h, or 24 h (resumption of exposure tonormoxic seawater, Pwo 2 155 torr). The values aremeans ± SD (n = 6 in each plot). Different lowercase
letters indicate significant differences (P < 0.05, Steel–
Dwass multiple comparison test). The daggers indicate asignificant difference from the control group and hypoxic
group (P < 0.05, Mann–Whitney U test).

15.0

10.0

a

b†

Phase

Fig. 4 E ffect of hypoxia (Pwo2 8 torr) on the hemolymph CO2
Fig. 4 42％ partial pressure (Pco2, torr) in the Akoya pearl oyster,Fig. 5
Pinctada fucata martensii. Hemolymph from the anterioraorta was collected via cannula from each experimental
animal. Solid circle: hypoxic group; open circle: control
group (no induced hypoxia). PreHy: pre-hypoxia (Pwo2
155 torr); Hy24: exposure to hypoxic seawater for 24 h
(Pwo2 8 torr); PostHy3–24: post-hypoxia for 3 h, 12 h, or
24 h (resumption of exposure to normoxic seawater,
Pwo2 155 torr). The values are means ± SD (n = 6 ineach plot). Different lowercase letters indicate significant
differences (P < 0.05, Steel–Dwass multiple comparisontest). The dagger indicates a significant difference fromthe control group and hypoxic group (P < 0.05, Mann–
Whitney U test).

a

ab †

PostHy3

Phase

a

ab †

1.00

0.00

PostHy12 PostHy24

5

5

10

1.0

0.5

Pco2
(torr)

PostHy24

2.50

PostHy12
PostHy3

2.00
1.50

2.5

0.25

PreHy

Hy24

1.00
0.50
0.00
6.80

7.00

7.20

7.40

7.60

7.80

Hemolymph pH

Fig. 6 E ffect of hypoxia (Pwo 2 8 torr) on the hemolymph
Fig. 7 Hemolymph pH-[HCO3–] diagram of Akoya pearl oysters,
calcium ion concentration ([Ca2+], mM/L) in the Akoya
Pinctada fucata martensii. Hemolymph from the anteriorFig. 6 42％
pearl oyster, Pinctada fucata martensii. Hemolymph fromFig. 7 36％
aorta was collected via cannula from each experimental
the anterior aorta was collected via cannula from eachanimal. PreHy: pre-hypoxia (Pwo2 155 torr, white circle);experimental animal. Solid circle: hypoxic group; open
Hy24: hypoxia for 24 h (Pwo 2 8 torr, black circles);circle: control group (no induced hypoxia). PreHy: prePostHy3–24: post-hypoxia for 3, 12, or 24 h (Pwo2 155 torr,
hypoxia (Pwo 2 155 torr); Hy24: exposure to hypoxic
grey circle). The values are means ± SD (n = 6 in each).
seawater for 24 h (Pwo 2 8 torr); PostHy3–24: postThe Pco 2 isopleths are derived from rearranging thehypoxia for 3 h, 12 h, or 24 h (resumption of exposure to
Henderson–Hasselbalch equation. The dashed line is thenormoxic seawater, Pwo2 155 torr). The values are means
nonbicarbonate buffer line: [HCO3–] = 5.77 – 0.463 · pH. The± SD (n = 4-6 in each plot). Different lowercase lettersnon-bicarbonate buffer value (ꞵNB, 0.46 slykes), which is theindicate significant differences (P < 0.05, Steel–Dwass
slope of the relational expression, was described in a
multiple comparison test). There were no significant
previous study 8).
differences between the control and hypoxic groups.

6

Takeshi Handa and Akira Araki

Discussion

was accumulated gradually in the hemolymph during

We examined the hemolymph oxygen and acid–basec a r b o n i c a c i d , a n d c a rb o n i c a c i d d i s so c i a t es t ostatus of Akoya pearl oysters under hypoxic

bicarbonate and hydrogen ions. The concentration ofenvironment to elucidate the acid–base balance and CO2hydrogen ions gradually increased in the hemolymph,

dynamics. The experimental animals in this study were

and the hemolymph pH continued to decrease during

cannulated to the anterior aorta for hemolymph collection

hypoxia for 24 h. In the case of air exposer, Akoya pearl

from submerged animals. Hemolymph properties of theoysters showed mixed acidosis (respiratory and metaboliccannulated control group animals did not significantly

acidosis) with the increase in the hemolymph [HCO3–] andchange. In the hypoxic group, the cannulated animals[Ca2+] under prolonged exposure8). Because acidic end-

were exposed to hypoxic seawater, and the hemolymph

products, which were increased by anaerobic metabolism,

oxygen and acid–base status showed significant changes.

dissolved the shell valves (CaCO3 crystal), the bicarbonate

These changes were caused by the hypoxic exposure,and calcium ions were mobilized to the hemolymph8,17). In

and not due to surgery.

this study, the hemolymph [HCO 3–] and [Ca 2+] of thehypoxia for 24 h. The accumulated CO 2 hydrates to

Akoya pearl oysters decreased Po2 from 72.2 torr to

Akoya pearl oysters did not change significantly during

13.6 torr during hypoxia for 24 h, and showed ahypoxia for 24 h. Therefore, the shell valves of thehypoxemia in this study. In some marine bivalves, theexperimental animals were not dissolved by the acidic

hemolymph showed reductions in the oxygen partial

end-products, and anaerobic metabolism hardly

pressure during environmental hypoxia or air exposure.

progressed. The hemolymph acidosis in this study should

The hemolymph Po2 decreased from 108 torr to 8 torr

be derived from the excessive CO2 accumulation.

during environmental hypoxia for 8 h in the blue mussel,
15)

When the inflow of normoxic seawater was resumed

Mytilus edulis . The hemolymph Po2 decreased during air

to the respiratory chamber, Akoya pearl oysters showed

exposure for 2 h from 118 torr to 57 torr in the kingincreases in the hemolymph Po2 and pH, and a decrease

16)

scallop, Pecten maximus ; and from 88 torr to 29 torr for 1
9)

in Pco2 within 3–24 h. The experimental animals would

h in the pearl oyster, P. fucata martensii . In this study,

accelerate O2 uptake and discharge excessive

hypoxemia would be caused in the early time of theaccumulated CO 2 from the gills (the respiratoryenvironmental hypoxia, and Akoya pearl oysters

compensation action) under a normoxic condition after

experienced the hypoxemia for about 24 h. The

hypoxia. The animals could exchange hemolymph gases

respiratory responses were reported in the pearl oyster,by diffusion from the surface of the soft body. Changes in

3)

P. fucata martensii, under hypoxic conditions . When Pwo2

the hemolymph Po2, pH, and Pco2 at PostHy3–24 were

was less than 10.3 torr at 20.5°C, the movements of thenot significant between the hypoxic and control group.

mantle lobe and shell valves increased, and the gill

Therefore, the effect of hypoxia on the hemolymph Po2,

3)

ventilation volume decreased . The oxygen uptake

pH, and Pco 2 almost disappeared for 3–24 h under adecreased to below 0.75-fold of the initial level under

normoxic condition. On the other hand, the hemolymph3)

Pwo 2 10.3 torr . Although the Akoya pearl oysters

Tco 2 and [HCO 3–] of the hypoxic group increased atshowed hypoxemia in this study, the animals could show

PostHy24 from the PreHy values, and were higher thanslight gill ventilation and oxygen uptake even if thethe control group. The hemolymph Tco2 increased with3)

environmental oxygen tension decreased .

the elevation of [HCO3–] at PostHy24. The increase in

The hemolymph pH decreased from 7.581 to 7.129, and[HCO 3 – ] was a secondary change and a metabolic

Pco2 increased from 0.86 torr to 3.31 torr during hypoxia

compensation response. The experimental animals mightfor 24 h. The hypoxic Akoya pearl oysters showedreabsorb bicarbonate at the renal region or absorb it

acidosis with an elevation of Pco2 in the hemolymph. CO2

from the seawater, though there was no result of ion

Acid–base balance of hypoxic akoya pearl oysters

7

CO 2 and by the increment of [HCO 3 – ] which was atransport evaluation in this study.
A c c o r d i n g t o t h e p H - [ H C O 3–] d i a g r a m o f t h esecondary change caused by respiratory disturbance.

hemolymph (Fig. 7), the Akoya pearl oysters had a

Therefore, the respiratory disturbance was classified as

reduced pH with the elevation of Pco2 during hypoxia forcompensated respiratory acidosis. Yamamoto et al. (1999)–

reported the oxygen uptake of the pearl oyster at

diagram from the blood . If a decrease in pH is due

different water temperatures under normoxic andsolely to a change in Pco2, the blood would be simply

hypoxic conditions. The pearl oyster showed an oxygen

titrated along the non-bicarbonate buffer line, and theuptake per wet weight of 0.43 mL/min/kg at 20.5°C and24 h. Wood et al. (1977) expounded the pH-[HCO 3 ]
18)

18)

point of the pH value moves on this line . The decrease

0.46 mL/min/kg at 27.2°C, and there was no significant

in pH is determined by simple respiratory acidosis. In

difference3). The pearl oyster maintained the oxygen

metabolic acidosis, a decrease in pH is due solely to an

uptake till 22 torr at 20.5°C, but decreased it with theincrease in non-volatile acid, and the blood will belowering of Pwo2 at 27.2°C3). Thus, the effect of hypoxia

titrated along a constant Pco2 isopleth and decreasedon the hemolymph acid–base balance of Akoya pearl– 18)[HCO3 ] . The decrease in pH is determined by simple

oysters would be greater at higher water temperatures.

metabolic acidosis. In this study, the Akoya pearl oyster

Akoya pearl oysters for pearl production are frequently

showed a reduction in pH and elevation in Pco2, and thereared in the culture ground of inner bays, and often

point at Hy24 was slightly below the non-bicarbonate

experience hypoxia (oxygen-deficient water mass). The

buffer line, but it moved along the line (Fig. 7). The

phenomenon of oxygen-deficient water mass includes–

hypoxia and anoxia, and the dissolved oxygen levels (DO)and [Ca ], and did not produce acidic end-products

were proposed in hypoxia between 0.025 mL/L (0.036(anaerobic metabolites) under hypoxemia in this study.

mg/L) and 2.5 mL/L (3.6 mg/L), and in anoxia underDuring the 24 h exposure to hypoxic conditions in this0.025 mL/L (0.036 mg/L)19-22). In this study, Akoya pearl

study, Akoya pearl oyster experienced the respiratory

oysters underwent severe hypoxia for 24 h (Pwo2, 8 torr

acidosis. The points at PostHy3–24 approached PreHy,≈ DO, 0.38 mg/L; WT, 20ºC), and showed hypoxemia andand the respiratory acidosis was alleviated considerably.

respiratory acidosis. Although most hemolymph

Akoya pearl oysters did not increase hemolymph [HCO3 ]2+–

The increase in [HCO3 ] at PostHy3–24 was a secondary

properties recovered from the effect of the hypoxia

change due to respiratory disturbance, and worked onrelatively early in returned to normoxic seawater, thethe respiratory acidosis as a metabolic compensation.

effect of the hypoxia on the animals was prolonged as

The experimental animals recovered the hemolymph pH

hemolymph [HCO3–] needed over 24 h. When hypoxic

sufficiently to the pre-hypoxic level. Hemolymph [HCO3–]

conditions (oxygen-deficient water mass) are repeated or

did not recover to normal levels, despite the recovery ofcontinued over several days, Akoya pearl oysters may–

Po2, pH and Pco2 after 24 h. The hemolymph [HCO 3 ]fall into an irreparable situation because of the prolonged

recovery needed over 24 h. The completion of theinsufficient compensation.

metabolic compensation action would require longer
period than the respiratory compensation action.
In this study, the Akoya pearl oysters showed oxygen

Acknowledgments

and acid–base disturbance (hypoxemia and respiratory

We would like to express our sincere gratitude to Dr.

acidosis) due to environmental hypoxia for 24 h (Pwo2, 8

Ken-ichi Yamamoto, Professor Emeritus, for securing thetorr; WT, 20°C). When the Akoya pearl oysters wereexperimental animals for this study.

returned to normoxic seawater, hypoxemia andrespiratory acidosis disappeared within 24 h. This
acidosis was compensated by discharging the excessive

8

Takeshi Handa and Akira Araki

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Acid–base balance of hypoxic akoya pearl oysters

アコヤガイのヘモリンパ液の酸塩基平衡に及ぼす低酸素の影響
半田岳志・荒木晶

和文要旨 : アコヤガイPinctada fucata martensii を酸素飽和度約5％の低酸素海水に24 時間曝露すると，前大動脈から採取したヘモリンパ液の酸素分圧は72.2 torr（平均値）から13.6 torr，pH は7.581 から7.129 にまで低下し，二酸化炭素分圧は0.86 torrから3.31 torr に増加した．低酸素海水に曝露したアコヤガイを酸素飽和度約100％の正常海水に戻すと，ヘモリンパ液の酸素分圧，pH，二酸化炭素分圧は3～24 時間以内に曝露前の値にまで回復した。カルシウ
ムイオン濃度は，統計的に有意な変動を示さなかった．これらの結果から，アコヤガイは酸素飽和度約5％の低酸素
海水に24 時間曝露されると，低酸素血症と呼吸性アシドーシスを呈することが明らかとなった。酸素飽和度約100％の正常海水にアコヤガイを戻すと，ヘモリンパ液の酸素分圧，pH，二酸化炭素分圧に及ぼす低酸素の影響は24 時間以内に消失することが示された。

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