206 K
.B. Brokordt et al. J. Exp. Mar. Biol. Ecol. 251 2000 205 –225
1. Introduction
Gametogenesis represents a period of high energy demand, particularly for organisms with broadcast spawning, and when external food supplies are limited, gamete
production occurs at the expense of biochemical components in somatic tissues Calow, 1985. In bivalves and fish, muscle is one of the tissues most affected during gonadal
maturation, with both protein and glycogen levels decreasing Shevechenko, 1972; reviewed by Barber and Blake, 1991; Von der Decken, 1992. Such mobilization of
macromolecules could decrease muscle metabolic capacities.
In scallops the adductor muscle is one of the largest soft tissues and while its primary role is the movement of the valves during escape responses or swimming, it also serves
as a major site of reserve deposition. In the bay scallop Argopecten irradians concentricus, 63–99 of total energy gain by the gonad during gametogenesis is
thought to come from the adductor muscle Epp et al., 1988. In this species, gametogenesis occurs mainly at the expense of muscle protein, however, in other
scallops the substrate most mobilized is glycogen reviewed by Barber and Blake, 1991;
´ ´
Martınez, 1991; Martınez and Mettifogo, 1998. How far the reproductive cycle affects biochemical reserves may depend on the
timing of gonadal proliferation in relation to food availability Shafee, 1981. In the black scallop Chlamys varia, during the spring reproduction, when food is abundant,
only carbohydrate reserves are used for gonadal development, while during the autumn reproduction, when food is less abundant, all reserves protein, lipid and glycogen
decrease with gonadal development Shafee, 1981. In fishes, such as the salmon, Oncorhynchus nerka and Salmo salar, not only are lipid reserves from muscle mobilized
during gonadal maturation and their non-feeding spawning migration, but the levels of enzymes and contractile proteins are also affected Mommsen et al., 1980; Olin and von
der Decken, 1987. In the roach Rutilus rutilis, swimming activity is markedly reduced during gonadal maturation Koch and Wieser, 1983, either due to the metabolic costs of
gonad maturation or to decreases in muscle metabolic capacities. A decrease of muscle metabolic capacities after gonadal maturation and spawning could reduce an animal’s
locomotor ability and thereby the capacity to escape predators.
Among bivalves, scallops are well known for their swimming ability Brand, 1991. They swim by jet propulsion using a succession of claps consisting of alternate
adduction and abduction of the valves Olson and Marsh, 1993. Swimming is powered by the adductor muscle, principally by the large phasic portion and to a lesser extent by
the smaller tonic catch muscle de Zwaan et al., 1980.
The physiological and biochemical aspects of the escape response valve clapping and valve closure are best understood in the giant scallop Placopecten magellanicus
Thompson et al., 1980; de Zwaan et al., 1980; Livingstone et al., 1981 and in the bay scallop Argopecten irradians concentricus Chih and Ellington, 1983, 1986. In
Placopecten magellanicus, the main source of ATP during valve clapping is arginine phosphate, and anaerobic glycolysis, supported by octopine accumulation, only makes a
small contribution de Zwaan et al., 1980. Following intense valve clapping, partial recuperation of arginine phosphate occurs during valve closure with further accumula-
tion of octopine.
K .B. Brokordt et al. J. Exp. Mar. Biol. Ecol. 251 2000 205 –225
207
Once the valves have opened, the full restoration of arginine phosphate pools and elimination of octopine is achieved aerobically Livingstone et al., 1981. Similarly, in
Argopecten irradians concentricus intense valve clapping is initially supported by arginine phosphate and only toward the end do anaerobic glycolysis and octopine
accumulation intervene Chih and Ellington, 1983, 1986. Mitochondria isolated from the adductor muscle of the tropical scallop, Euvola ziczac, seem adapted for a role in
recovery metabolism given their increased affinity for pyruvate at the pH values likely to occur in muscle fibers after intense clapping Guderley et al., 1995. Oxidative
capacities and respiratory control ratios of mitochondria isolated from the adductor muscle are lower during the first of two spawnings than during other periods Boadas et
al., 1997, suggesting an impact of reproduction.
We reasoned that changes in muscle metabolic capacities or in the levels of energetic reserves during the reproductive cycle of adult scallops could modify the escape
response or recuperation from an exhausting escape response. We predicted that before gonadal maturation, scallops would show a stronger escape response and would
recuperate faster from exhausting burst exercise given the higher muscle metabolic capacities and energy reserves the scallops should have at this time.
To evaluate these hypotheses, we compared the escape response and capacity for recuperation from exhausting exercise for adult Iceland scallops, Chlamys islandica,
sampled at different stages in the annual reproductive cycle immature, mature, before and after spawning. In parallel, we determined the effect of these reproductive stages on
the energetic reserves in the phasic adductor muscle arginine phosphate, glycogen and proteins. Given that in fish muscle, starvation leads to more extensive mobilization of
sarcoplasmic than structural insoluble proteins Beaulieu and Guderley, 1998, we quantified both fractions. To assess the impact of the reproductive cycle on muscle
metabolic capacities, we measured muscle levels of the glycolytic enzymes, glycogen phosphorylase GP, pyruvate kinase PK, phosphofructokinase PFK, octopine
dehydrogenase ODH and arginine kinase AK, as well as the mitochondrial enzyme, citrate synthase CS. We measured the oxidative capacities, substrate preferences,
respiratory control ratios and CS levels of mitochondria isolated from the phasic adductor muscle at different reproductive stages. By profiling these enzyme activities
and mitochondrial capacities we sought to assess the capacity of enzymes likely to supply ATP for contraction AK and glycolytic enzymes GP, PFK, PK and ODH,
recuperation during valve closure glycolytic enzymes and recuperation after reopening of the valves CS and mitochondrial capacities.
2. Materials and methods
