ABSTRACT
Bivalve molluscs have a highly plastic feeding and growth physiology. The increasing availability of families artificially selected for faster growth has enabled physiological experiments to investigate the genetic basis for variable rates of growth. Fast growth is achieved by a combination of increased rates of feeding, reduced metabolic rates and lower metabolic costs of growth. In at least one species there is a trade-off between growth in protein and the storage of lipids that are utilized in gametogenesis. Energy requirements for maintenance are also higher in slow-growing individuals. Reduced costs of growth are due in part to increased efficiencies of protein turnover. Nevertheless, high protein turnover (and therefore high metabolic cost) may benefit fitness in the later stages of gametogenesis. Faster feeding rates do not impair flexibility in feeding behavior which compensates for changes in the food environment. Both inter- and intra-species differences in feeding behavior are evident and suggest possible constraints imposed by faster feeding on the efficiency of selection between food particles of different nutritional value.
ABSTRACT
Two divergent taxa in the marine mussel genus Mytilus are largely isolated geographically and are routinely exposed to distinctly different thermal environments. We tested the hypothesis that the two taxa are physiologically differentiated with respect to temperature and examined the evolved adaptations allowing one of the taxa to exploit habitats where warm-temperate conditions prevail for prolonged periods. We first analyzed the physiological response to high temperature of mussels collected from a hybrid population containing members of both pure taxa, F, hybrids, and a variety of introgressed genotypes. The experimental temperature of 23°C was chosen to be permissive to the taxon that occurs in warm-temperate regions (Mytilus galloprovincialis) and restrictive to the cold-water taxon (Mytilus edulis). The results show that the two taxa are physiologically differentiated. Under the experimental conditions, M. galloprovincialis exhibited a threefold higher feeding rate and a slightly elevated metabolic rate compared with M. edulis. These differences did not result in a significant difference in net energy balance between the two taxa, probably because of an interaction between physiological response and food availability. However, M. galloprovincialis grew significantly faster in the field, indicating that the physiological differences observed in the laboratory also occur in nature. Numerous introgressed genotypes provided the opportunity to test for cosegregation between the physiological differences and four highly differentiated genetic markers. Two of the markers (esterase and octopine dehydrogenase) cosegregate with variation in feeding rate and shell growth and explained most of the physiological differences observed between taxa. A strong concordance existed between these two loci, suggesting that they may be linked and may mark segregation of the same linkage group. The results suggest that the physiological differentiation between these taxa may be controlled by a few genes (perhaps only one) each with large effect.
