Effect of Morphometric Maturity and Size on Enzyme Activities and Nucleic Acid Ratios in the Snow Crab Chionoecetes Opilio

Abstract

Indicators of metabolic capacity and the potential for protein synthesis in male snow crabs, Chionoecetes opilio, were measured to estimate how these factors are affected by morphometric maturity and body size. The metabolic capacity of muscle, as depicted by cytochrome C oxidase (CCO), citrate synthase (CS), and lactate dehydrogenase (LDH) activity, was greater in adults than in adolescents, while that of the digestive gland was not affected by maturity. These differences may reflect the different locomotory habits of adults and adolescents. Indicators of the potential for protein synthesis, the RNA:DNA and protein:RNA ratios, did not differ between adult and adolescent crabs, presumably because the animals were in late intermolt stage and flesh growth was already completed. The total mass of muscle, as estimated by the flesh content of the merus, and the total capacity of muscle CCO scaled isometrically with body mass, while that of CS scaled with an exponent <1. The mass of the digestive gland, as well as the total capacity of both mitochondrial enzymes in this organ, increased more slowly than body size. These results support the hypothesis that the negative allometry generally observed for aerobic metabolism may be caused by the decreasing size of the metabolically most active tissues with an increase of body mass. In contrast with the positive allometry of the anaerobic capacity of fast-swimming fish with body mass, we noted an isometric relationship between these two variables in the rather slow-moving C. opilio. This supports the hypothesis that an enhanced anaerobic capacity is necessary for the largest individuals of a fast-moving species in order to maintain their burst swimming speed despite the increased friction generated by increased speed and body size. The effects of body size on the aerobic capacity of vertebrates have been thoroughly investigated during the last two decades (reviewed by Robinson et al., 1983; SchmidtNielsen, 1984; Goolish, 1991). Far fewer allometric studies have been concerned with invertebrates, on the one hand, or with glycolytic capacity, on the other hand. Given the differences in locomotory strategies and dependency upon aerobic and anaerobic metabolism, the relationships between body size and metabolic capacities may differ between mammals, fish, and invertebrates. Growth not only leads to increases in body size, but may also be associated with changes in maturity, morphology, and behavior. For example, immature males of many crustacean species, including crabs and shrimps, undergo molt cycles until they reach the adult stage. Whether the molt leading to morphometrical maturity is terminal or not in majid male crabs is still subject to controversy (Hartnoll, 1963; Conan and Comeau, 1986; Dawe et al., 1991; Sainte-Marie and Hazel, 1992), although the general opinion favors the terminal molt hypothesis. Adult males have larger gonads as well as a higher chela height to carapace width ratio than adolescent males, and th y display mating behaviors more frequently (Donal son an Adams, 1989; Claxton, 1992; Cassier et al., 1997). Adult males of various crab species also migrate over longer distances than immature males (SainteMarie and Hazel, 1992; Hines et al., 1995). Maturity is thus likely to affect muscle metabolic capacities and should be considered in allometric studies. Whether an adjustment in enzyme activities with maturity or body size is strictly due to locomotory habits can be inferred by comparison with enzymatic responses in nonlocomotory tissues such as the digestive gland. To our knowledge, the effect of maturity on the relationship between metabolic capacity and body mass in invertebrates has not been investigated. The equation describing the relationship between metabolic rate (Y) and body mass (M) is Y = aMb, where a is a constant and b the scaling factor. Numerous studies have demonstrated that the oxygen consumption of a given endothermic species is proportional to the body mass to the power 0.67. Interspecifically, an exponent of 0.75 is often reported for ectothermic animals, although