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. Mayrand et al. J. Exp. Mar. Biol. Ecol. 255 2000 37 –49
1. Introduction
In crustaceans, moulting leads to carapace growth. In contrast to the rapid shift in carapace size, flesh growth takes place gradually during the interval between successive
moults Passano, 1960; Stevenson, 1972. Anger and Darwis 1981 noted that the rate of tissue growth is affected by the feeding status of Hyas araneus, while the length of
the moult cycle is independent of food ration once the animals have attained a certain threshold of reserves content. In the snow crab Chionoecetes opilio, muscle growth rate
is strongly affected by food ration and by the time elapsed since moulting Mayrand et al., 2000. Crustaceans of similar size and moult stage can exhibit different degrees of
carapace ‘filling’, according to the conditions to which they have been previously exposed. Starved crabs have a lower muscle dry mass per merus, a higher tissular water
content and fewer muscle cells per merus than well-fed ones Mayrand et al., 2000. Since muscle content in the walking legs is likely to alter the animals’ locomotory
capacity and thus their ability to forage and migrate, it is likely to affect their fitness. Estimates of muscle growth are important when studying the performance of intermoult
crustaceans under field conditions.
Muscle growth rate cannot be estimated by temporal changes in crustacean body mass, since growth is constrained by the carapace and because tissue water is gradually
replaced by proteins during growth Passano, 1960 with no appreciable change in density. Biochemical indicators reflecting muscle growth could be an interesting
alternative to the classical methods of evaluating growth rates. An advantage of such indicators is that they do not require repeated measures on the same animal. The
relationships between somatic growth and biochemical variables have been studied in fish and crustaceans. Promising biochemical indicators are the activity of metabolic
enzymes and the RNA:DNA ratio. The rationale of testing these potential indicators is that growth depends on ATP production, which in turn depends on the activity of
glycolytic and mitochondrial enzymes. Growth also requires the cellular machinery for protein synthesis, i.e. the ribosomal RNA, which represents 85–94 of cellular RNA
McMillan and Houlihan, 1988. Growth rate is positively correlated with the specific activity of glycolytic and mitochondrial enzymes in muscle and gastrointestinal tract of
saithe Mathers et al., 1992, cod Pelletier et al., 1993a,b, 1995; Dutil et al., 1998; Lemieux et al., 1999, largemouth bass Goolish and Adelman, 1987 and threespine
stickleback Guderley et al., 1994. The RNA:DNA ratio has been shown to be a good indicator of growth in fish e.g. Bulow, 1970; Kearns and Atchison, 1979; Goolish et al.,
1983; Buckley, 1984; Barron and Adelman, 1984; Mathers et al., 1992. Nevertheless, the correlation between somatic growth and the RNA:DNA ratio does not hold in all fish
¨ species Dagg and Littlepage, 1972; Jurss et al., 1987; Mathers et al., 1994; Pelletier et
al., 1994; Dutil et al., 1998. In comparison with fish, crustaceans have received little attention. Positive relationships between the RNA:DNA ratio and growth rate or feeding
level have been reported for the blue crab Callinectes sapidus Wang and Stickle, 1986, 1988 and postlarval American lobster Juinio and Cobb, 1994. While the these authors
worked with whole animals, Houlihan et al. 1990 and Edsman et al. 1994 examined changes in muscle and found that the RNA concentration in this tissue matches the short
E . Mayrand et al. J. Exp. Mar. Biol. Ecol. 255 2000 37 –49
39
term changes in the nutritional status of Carcinus maenas and Pacifastacus leniusculus. On the other hand, Anger and Hirche 1990 did not detect a significant correlation
between growth and the RNA:DNA ratio measured in whole early juvenile spider crabs. To our knowledge, the present study is the first to investigate the relationship between
growth rate, on one hand, and the RNA:DNA ratio and metabolic enzyme levels in crustacean muscle, on the other.
The energetic status of an animal can be assessed by indices such as the condition factor and the liver somatic index Lambert and Dutil, 1997. Somatic growth and
condition indices often covary Dutil et al., 1998, although this depends on the exact nutritional history. For example, a positive growth rate in muscle of snow crab can be
accompanied by a low or high value of digestive gland dry mass, depending on the food ration which the animals were fed Mayrand et al., 2000. It is thus important to
discriminate the effect of growth rate from that of the nutritional status on the variates which are tested as potential indicators of growth.
The aims of this study were to examine how the metabolic capacity of muscle in snow crab changes during growth, and to assess the usefulness of glycolytic and mitochondrial
enzyme activity and of the RNA:DNA ratio as biochemical indicators of muscle growth. From a mechanical point of view, it is interesting to examine cell content and the
metabolic capacity of muscle in the whole merus. Indeed, the degree of merus ‘filling’ and the metabolic capacity of the whole merus muscles are likely to affect the locomotor
ability of the animal. In order to correct for the slight differences among the mean merus volume calculated for the various experimental groups, the DNA content mg and the
enzyme activity international units: mmole of substrate converted to product per minute were expressed per ml of merus. The DNA content per ml of merus is related to the
density of muscle cells in the merus, without discriminating between differentiated cells which are fused into multinuclear fibers and, on the other hand, undifferentiated and
unfused myosatellite cells. Muscle cell size was evaluated by the protein:DNA ratio which has been shown to reflect the ratio of sarcoplasm to nucleus volume in fish muscle
Koumans et al., 1993.
2. Materials and methods
