Nature more than nurture affects the growth rate of mussels

Metabolic size scaling reflects growth performance effects on age-size relationships in mussels (Mytilus galloprovincialis)

Body-size scaling of metabolic rate in animals is typically allometric, with mass exponents that vary to reflect differences in the physiological status of organisms of both endogenous and environmental origin. Regarding the intraspecific analysis of this relationship in bivalve molluscs, one important source of metabolic variation comes from the large inter-individual differences in growth performance characteristic of this group. In the present study, we aimed to address the association of growth rate differences recorded among individual mussels (Mytilus galloprovincialis) with variable levels of the standard metabolic rate (SMR) resulting in growth-dependent shift in size scaling relationships. SMR was measured in mussels of different sizes and allometric functions fitting SMR vs. body-mass relationships were compared both inter- and intra-individually. The results revealed a metabolic component (the overhead of growth) attributable to the differential costs of maintenance of feeding and digestion structures between fast and slow growers; these costs were estimated to amount to a 3% increase in SMR per unit of increment in the weight specific growth rate. Scaling exponents computed for intraindividual SMR vs body-mass relationships had a common value b = 0.79 (~ ¾); however, when metabolic effects caused by differential growth were discounted, this value declined to 0.67 (= ⅔), characteristic of surface dependent processes. This last value of the scaling exponent was also recorded for the interindividual relationships of both standard and routine metabolic rates (SMR and RMR) after long-lasting maintenance of mussels under optimal uniform conditions in the laboratory. The above results were interpreted based on the metabolic level boundaries (MLB) hypothesis.

Differential tissue development compromising the growth rate and physiological performances of mussel

Differences in the food acquisition rates and in the energetic costs of metabolism seem to affect the growth rate variability of mussels. The aim of this study was to analyze if the physiological performances responsible for such growth rate variability are accompanied by structural differences at tissue or cellular level in the main organs involved in energy acquisition (gill) and processing (digestive gland). Fast growers had higher cilia density and metabolic efficiency in their gill, and well-developed digestive tissue with barely no connective tissue or atrophy. Slow-growing mussels displayed stress signs that impede the proper acquisition, digestion and absorption of food: low cilia density, low mitochondrial capacity and high antioxidant activity levels in the gills, and high atrophy of the digestive gland. The data herein explains the growth rate variability of mussels, demonstrating that morphological and functional differences exist between fast and slow growers.