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G . Martınez et al. J. Exp. Mar. Biol. Ecol. 247 2000 29 –49
1. Introduction
In most animals, but particularly in broadcast spawners, gonadal maturation is a costly process requiring considerable investment of material and energy. When food availability
is high, the cost of gonadal maturation can be covered entirely by absorbed materials and energy, but when food availability is low, gonadal maturation can require mobilization
of macromolecular components from other tissues for their use for gametogenesis. Scallops are broadcast spawners in which the mature gonad can account for more than
20 of the body mass. Inverse cycles of the biochemical components in the adductor muscle and gonad occur during the reproductive cycle of various scallop species,
supporting the interpretation that materials are mobilized from the muscle to support gonadal maturation. Such cycles have been observed for Chlamys opercularis Taylor
and Venn, 1979, Chlamys varia Shafee, 1981, Argopecten irradians concentricus Barber and Blake, 1981, Argopecten irradians irradians Epp et al., 1988, Placopec-
´ ten magellanicus Couturier and Newkirk, 1991, Argopecten purpuratus Martınez,
´ 1991; Martınez and Mettifogo, 1998, Pecten maximus Pazos et al., 1997 and Euvola
Pecten ziczac Brea, 1986; Boadas et al., 1997. Whereas glycogen and protein are the principal macromolecules which are mobilized from muscle Taylor and Venn, 1979;
Barber and Blake, 1981, 1985; Pazos et al., 1997, lipid levels in the adductor muscle may also decline during gonadal maturation Epp et al., 1988. Clearly, beyond its
obvious contractile role, the adductor muscle fulfills an important role as a site of reserve deposition in scallops.
When muscle macromolecules are mobilized to support the metabolic requirements of other tissues, both the levels of energetic reserves, such as glycogen and lipid, and those
of enzymes or contractile proteins can be affected. For example, during their non- feeding spawning migration, sockeye salmon, Oncorhynchus nerka, first metabolize their
lipid deposits in muscle and then break down muscle enzymes and sarcoplasmic proteins Mommsen et al., 1980. During starvation, cod, Gadus morhua, first mobilise hepatic
lipid, then hepatic and muscle glycogen and finally muscle proteins Black and Love, 1986. During the nutritional stress caused by winter conditions, mussels experience a
high energy loss from body tissues and proteins accounts for 75 of this loss Gabbott and Bayne, 1973. During gametogenesis in Argopecten irradians irradians, digestive
gland lipid reserves are first mobilized followed by muscle glycogen and finally protein reserves Barber and Blake, 1981. The sparing of muscle proteins during starvation
likely reflects the fact that proteins play more roles than simply serving as an energetic reserve. Effectively, the mobilisation of muscle proteins during starvation or reproduc-
tive maturation decreases muscle metabolic capacities Guderley et al., 1994, 1996, particularly the glycolytic capacity of white muscle. Studies of relative utilization of
dietary protein for energy and biosynthesis by the mussel Mytilus edulis have demonstrated a continuous conservation of amino-N over amino-C to satisfy bio-
synthetic demands Kreeger et al., 1995, 1996.
The extensive mobilization of muscle reserves during gametogenesis in scallops suggests that muscle metabolic capacities change during gonadal maturation. However,
little is known of the impact of gonadal maturation upon the metabolic capacities of the adductor muscle. The oxidative capacities and respiratory control ratios of mitochondria
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. Martınez et al. J. Exp. Mar. Biol. Ecol. 247 2000 29 –49 31
isolated from the adductor muscle of Euvola Pecten ziczac during their first spawning were lower than in those isolated at other periods, apparently due to the low food
availability and high temperatures in the preceding period Boadas et al., 1997. When gonadal maturation is induced in recently spawned scallops, muscle metabolic capacities
may first rise and then fall if muscle proteins are mobilized for gonadal maturation Pazos et al., 1997. If muscle enzymes are simply used to provide energy and amino
acids, all enzymes should decrease in parallel during protein mobilization. On the other hand, enzymes which facilitate reserve mobilization or which are restricted to specific
organelles may be spared. For example, as glycogen phosphorylase liberates glucose moieties from glycogen, its levels may be maintained.
Reinitiation of gonadal maturation following spawning is of particular interest for the aquaculture industry, which depends upon a ready availability of high quality larvae.
The extent and quality of this gonadal recuperation depend upon the available food, with certain diets leading to faster gonadal maturation, greater fertilization success and
´ improved larval survival than others Martınez et al., 2000. Thus, in Argopecten
purpuratus, gonadal maturation is faster at 168C than at 208C and the best larval survival is obtained with a microalgal diet supplemented with a lipid emulsion. During
conditioning of A . purpuratus, considerable time is required for gonadal maturation,
´ even with appropriate diets and temperatures Martınez et al., 2000. Thus, recently
spawned individuals are likely to replenish tissue reserves before undertaking gonadal maturation. The levels of biochemical components in muscle would increase before any
improvement in gonadal status is apparent and then decrease during gonadal maturation, particularly if food availability is insufficient to cover the requirements of gonadal
maturation. Under this scenario, muscle metabolic capacities may first rise and then fall during broodstock conditioning, although enzymes which play a role in reserve
mobilisation may be spared.
To examine these possibilities, we evaluated the biochemical composition of muscle and gonad and muscle metabolic capacities in the scallop, A
. purpuratus, during conditioning of recently spawned individuals at 16 and 208C using three diets, one
consisting of microalgae, another of microalgae supplemented with a lipid emulsion and the last of microalgae supplemented with carbohydrate. We measured levels of
carbohydrates, lipids and proteins in muscle and gonad. Since the scallops conditioned at 168C showed more complete gonadal maturation than those at 208C and as we wished to
ascertain the impact of mobilization of muscle reserves in support of gonadal maturation upon muscle metabolic capacities, we determined the activities of glycogen phosphor-
ylase GP, pyruvate kinase PK, octopine dehydrogenase ODH and citrate synthase CS only in the muscles of scallops conditioned at 168C. GP is responsible for the
liberation of glucosyl units from glycogen, PK catalyses an ATP generating reaction in glycolysis, whereas ODH catalyses the reductive condensation of pyruvate and arginine,
thereby regenerating NAD. CS is a component of the Krebs cycle and serves as a marker of mitochondrial abundance. If these enzymes show the same variation during
mobilization of muscle reserves, it would indicate non-selective protein breakdown. Specific changes, in which certain activities were maintained whereas others decline,
would indicate targetted protein mobilization. We predicted that the levels of the glycolytic enzymes, PK and ODH, would decrease whereas GP would be spared during
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G . Martınez et al. J. Exp. Mar. Biol. Ecol. 247 2000 29 –49
mobilization of muscle reserves, given its role in liberating glucose equivalents from glycogen. We reasoned that the localization of CS in the mitochondrial matrix would
protect it during reserve mobilization.
2. Materials and methods
