K .B. Brokordt et al. J. Exp. Mar. Biol. Ecol. 251 2000 205 –225
211
Preliminary studies indicated that glutamate, malate, pyruvate and succinate at final
21
concentrations of 30, 6, 0.9 and 24 mmol l , respectively, gave maximal rates. To attain
21
maximal rates of pyruvate oxidation, ‘sparking’ levels of malate 0.6 mmol l , which
by themselves did not support significant rates of oxygen uptake, were required. To measure maximal rates of respiration we added ADP at a final concentration of 0.6
21
mmol l . The respiratory control ratio RCR was calculated from the ratio of the state
3 rate in presence of ADP to state 4 rate when all ADP had been phosphorylated Estabrook, 1967.
To estimate the maximal aerobic capacity of the phasic adductor muscle, we measured the level of CS in the mitochondrial suspensions and, using the specific activity of CS
determined for the same muscle, carried out the following conversion:
21 21
21 21
21
nmol O min ? CS U
CS U g 5 nmol O g
min
2 2
where the first factor is mitochondrial oxygen uptake expressed per unit CS in the mitochondrial preparation and the second factor is the CS activity in the muscle
homogenates.
2.6.2. Protein concentrations Mitochondrial protein concentrations were determined using the bicinchoninic acid
method of Smith et al. 1985 with BSA as the standard. The concentration of BSA in the resuspension medium was subtracted to establish the concentration of mitochondrial
protein.
2.7. Chemicals All biochemicals were from Boehringer Mannheim Co. Montreal, Canada or Sigma
Chemical Co. St. Louis, MO. All other chemicals were analytical grade. 2.8. Statistical analysis
Data were analyzed using a one-way ANOVA to test the null hypotheses of no differences between reproductive stages Sokal and Rohlf, 1981. Normality was tested
using a Shapiro–Wilk’s test SAS, 1991 and homogeneity of variances using a Levene test Snedecor and Cochran, 1989. Multiple pairwise comparisons Tukey were used to
test for specific differences when the ANOVAs indicated significant differences SAS, 1991. Comparisons between the proportions of scallops, for each reproductive stage,
that responded to restimulation 2 min after opening their valves were made using
2
x -tests Sokal and Rohlf, 1981.
3. Results
3.1. Reproductive stages Animals sampled in mid-June 1996 were immature or maturing, as indicated by a
low gonadal index and significantly lower gonadal lipid and protein content compared
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.B. Brokordt et al. J. Exp. Mar. Biol. Ecol. 251 2000 205 –225
with animals sampled in mid-July and early August Fig. 1. The scallops sampled in mid-July and early August did not differ in their gonadal indices or in the lipid and
protein contents of the gonads. As the latter sample was taken only one week before spawning began, we identified the groups sampled in mid-July and those sampled in
early August as mature and prespawned scallops, respectively. The scallops sampled in mid-August showed a marked decrease of the gonadal index and of the lipid and protein
contents in their gonads, indicating that animals had spawned Fig. 1. Our classification of these reproductive stages was supported by histological observations of gonadal
sections from the scallops sampled in the four periods.
3.2. Environmental conditions Phytoplankton abundance fluctuated considerably during the study. In general, during
21
maturation scallops had more phytoplankton available |0.5 mg l of chlorophyll a
than before and after spawning, where phytoplankton availability decreased |0.3
21
mg l , Fig. 2.
During gonadal maturation, bottom temperature was higher and more variable than during spawning Fig. 2. During July and until early August, temperature fluctuated
around 68C. During spawning, it dropped to |48C and remained low until the end of August.
3.3. Escape response When Chlamys islandica were stimulated with the seastar Leptasterias polaris, they
responded with a series of |26 claps in a period of |2 min. There was no difference in the mean number of claps performed by the scallops at different reproductive stages
21
Table 1. The clapping rate of 13 claps min remained relatively stable, except for
21
prespawning scallops for which the rate was 17 claps min Table 1.
The clapping response was followed by a period of |30 min where the animals remained closed and refractory to further stimulation. This period was similar between
reproductive stages, with the exception of mature animals which spent less time closed Table 1. To evaluate the extent of recovery during valve closure, scallops were
restimulated 2 min after they reopened their valves, to measure the proportion of initial claps performed during restimulation and the proportion of animals that responded to
stimulation. Spawned scallops could only do 11 of the initial claps, compared to the other reproductive stages which responded with 25–35 of their initial claps Table 2.
Interestingly, all immature animals responded after restimulation with the seastar, whereas after gonadal maturation only 62 of mature and 82 of prespawning animals
responded Fig. 3. Spawned animals markedly decreased their responsiveness as only 38 reacted 2 min after reopening their valves.
Even after the scallops reopened their valves, the time required for full recovery of their initial clapping capacity varied markedly with reproductive stage Table 2.
Immature scallops needed only 4 h for complete recovery. In contrast, after gonadal maturation 18 h were needed to regain initial capacity; 12 h were required before
spawning and 18 h after spawning. Therefore, during gonadal maturation and spawning,
K .B. Brokordt et al. J. Exp. Mar. Biol. Ecol. 251 2000 205 –225
213
Fig. 1. Gonadosomatic index and total content of lipid and protein in gonads of female and male Chlamys islandica at different reproductive stages. Values represent means6S.E. n 520 for GSI; n 58 for lipid and
protein content. Means sharing the same letters were not significantly different P ,0.01 as determined by Tukey multiple comparisons.
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21
ˆ Fig. 2. Variations in temperature 8C and chlorophyll a mg l
1 m above the Chlamys islandica bed, at Ile ˆ
du Fantome Mingan Islands from May to August 1996. Values represent means6S.E. Arrows indicates the times when scallops were sampled. P-S5prespawned.
C . islandica did not change its clapping capacity, but markedly slowed its rate of
recuperation from exhausting escape responses. Both the recuperation during valve closure and that occurring once valves reopened were diminished.
Table 1 Mean number S.E. of claps, clapping rate and time spent with the valves closed after exhaustive exercise for
the scallop Chlamys islandica at different reproductive stages Reproductive
Number Clapping rate
Time spent n
21
stage of claps
claps min closed min
Immature 26 1
13 1 33.2 2.7
40 Mature
25 1 13 1
23.2 1.7 48
Pre-spawned 30 2
17 1 34.1 3.4
29 Spawned
24 1 13 1
28.8 3.3 48
P ,0.01.
K .B. Brokordt et al. J. Exp. Mar. Biol. Ecol. 251 2000 205 –225
215 Table 2
Mean S.E. proportion of initial claps recovered at different time intervals after exhausting escape response for the scallop Chlamys islandica at different stages in gonadal development
Recuperation time Immature
Mature Prespawned
Spawned with valves open
a
2 min 34.8 2.7
24.6 3.6 28.1 6.7
10.6 5.8 2 h
73.3 5.5 54.2 7.1
72.7 3.4 69.5 11.5
4 h 95.2 6.3
60.5 6.3 60.9 9.8
70.1 8.9 6 h
103.0 4.1 63.0 7.0
78.1 6.2 70.8 4.8
12 h 50.0 10.7
102.0 3.9 81.9 5.5
18 h 100.6 4.7
105.2 8.8
a
Recovery efficiency during valve closure. Comparisons between reproductive stages for 2 min P ,0.05; Statistically different from the earlier
test P ,0.05, indicate that recovery was completed.
3.4. Biochemical composition of muscle at different reproductive stages The concentration of glycogen in the phasic muscle decreased markedly as the gonad
matured and remained low until after spawning Fig. 4. The decrease in muscle glycogen coincided with a strong increase of total lipid and protein in the gonads of
mature and prespawning scallops Fig. 1. Total protein concentration in the phasic muscle remained stable during the course of gonadal maturation and spawning Fig. 4.
Fig. 3. Proportion of Chlamys islandica at different reproductive stages that responded to restimulation with the seastar, 2 min after the end of glycolytic recuperation closed valves n 58. Bars sharing the same letters
2
were not significantly different P ,0.05 as determined by a x -test.
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21
Fig. 4. Concentration of glycogen mmol glucosyl U g wet mass, total, structural and sarcoplasmic proteins
21 21
mg g wet mass and of arginine phosphate mmol g
wet mass in the phasic adductor muscle of the scallop Chlamys islandica at different reproductive stages. Values represent means6S.E. n 59–14. Means
sharing the same letters were not significantly different P ,0.01 as determined by Tukey multiple comparisons. In muscle protein, different kind of lettering indicate different comparisons.
K .B. Brokordt et al. J. Exp. Mar. Biol. Ecol. 251 2000 205 –225
217
However, structural protein oscillated and the level of sarcoplasmic protein fell slightly before spawning. Muscle arginine phosphate gradually increased during gonadal
maturation and was highest before and after spawning Fig. 4.
3.5. Muscle enzyme levels at different reproductive stages With the exception of AK, the enzymes showed similar fluctuations in activity during
gonadal maturation and spawning Fig. 5. The glycolytic enzymes, GP, PFK, PK and ODH, and the mitochondrial enzyme, CS, showed their highest levels in immature
scallops and levels declined with gonadal maturation. PK recovered its initial levels before spawning, but showed its lowest level after spawning. Much like the levels of
arginine phosphate, AK activity increased during the course of gonadal maturation, reaching a maximum before spawning Fig. 5.
Thus, the levels of enzymes that participate in the recovery of the adductor muscle from exhausting escape responses decreased during gonadal maturation and spawning. In
contrast, the enzyme which generates most of the ATP required for the escape response gradually increased its activity during gonadal maturation and spawning.
3.6. Oxidative capacities of mitochondria and adductor muscle during the reproductive cycle
Rates of mitochondrial oxygen uptake and respiratory coupling varied with substrate and reproductive stage Fig. 6. Glutamate and succinate gave highest oxidation rates at
most times. Rates of glutamate and malate oxidation changed little during the reproductive cycle. In contrast, mitochondrial rates of pyruvate and succinate oxidation
decreased considerably after gonadal maturation and remained low after spawning Fig. 6. For all substrates, the respiratory control ratio RCR decreased during the
reproductive cycle Fig. 6. The RCR values for glutamate and pyruvate oxidation were lower in mitochondria from mature and spawned scallops, whereas those for the
oxidation of malate and succinate declined only after spawning.
Total muscle oxidative capacities were calculated from mitochondrial rates of substrate oxidation, mitochondrial CS levels and muscle CS levels. Reproductive status
markedly affected the muscle’s capacity for oxidizing pyruvate and succinate, with decreases occurring with maturation and spawning Fig. 6. As pyruvate generated from
octopine is metabolized during recuperation, the total capacity for pyruvate oxidation is most pertinent to evaluation of the capacity for recuperation from exhausting escape
responses. Overall, the mitochondrial oxidative capacities, total muscle aerobic capacity and activities of enzymes involved in glycolytic and aerobic phases of recovery followed
virtually the same pattern, a decrease in capacity with maturation and spawning.
4. Discussion
