7 – Muscle Fiber Diversity and Plasticity

The main muscle fiber types of fish axial muscle, namely red, white, intermediate, or pink, so-called tonic, red muscle rim, transitional zone, and scattered dorsal and ventral fibers, are described with respect to their location, origin, fine structure, and biochemical and functional characteristics. Ontogenic changes in structural and functional features are discussed, as well as interspecific variations. Red muscle fibers, the most aerobic fiber type, are characterized by high amounts of oxidative enzymes, mitochondria, lipid droplets, and capillaries. In white muscle fibers, metabolic economy is less important than maximum power output, therefore their characteristics emphasize their recruitment for burst-swimming activities. Intermediate muscle fiber, as their name suggets, are between red and white muscle fibers in a variety of their characteristics. The variety of these and other fiber types is described. A further focus of attention is the plasticity of muscle phenotype. Particularly, the factors that lead to adaptational changes in fish axial muscle (seasonal, temperature, oxygen tension, and exercise training) are discussed. Cold acclimation/acclimatization, hypoxic conditions, and endurance training demonstrate a general trend for transition to more aerobic fiber types and a more aerobic type of muscle.