Myomeres are blocks of skeletal muscle tissue that serve as the primary engine for locomotion in fish and other aquatic chordates. These muscle segments are arranged in a repeating series along the body axis. The coordinated action of myomeres enables the characteristic side-to-side body movements that propel the animal forward. This muscular system constitutes a significant portion of a fish’s body mass, underscoring its importance for swimming.
Segmented Structure and Location
The most recognizable feature of myomeres is their characteristic segmented shape, often described as a series of overlapping cones. When viewed from the side of a fish, the outline of these blocks presents a distinct zigzag or W-pattern. The individual blocks of muscle tissue are separated by thin, strong sheets of connective tissue called myosepta. These collagenous partitions act as tendons, providing attachment points for the muscle fibers within the myomeres.
Myomeres are arranged sequentially along the length of the body, from the head to the tail, with each segment corresponding roughly to a vertebra. The entire muscle mass is divided into two main sections by a horizontal septum of connective tissue running along the flank. The tissue above this septum is called the epaxial musculature, and the tissue below it is the hypaxial musculature.
This division allows for separate neural control of the dorsal and ventral muscle groups, aiding in precise swimming movements. The complex, folded shape of the myomeres, particularly the nested cone-like arrangement found in modern fish, evolved from the simpler V-shape seen in ancestral chordates. This intricate folding maximizes the surface area for force transmission and allows a single myomere’s contraction to affect a greater length of the body, increasing movement efficiency.
How Myomeres Power Movement
Myomeres power aquatic movement through lateral undulation, where the body is flexed into a continuous wave traveling from the head toward the tail. This propulsive force is generated by the sequential and alternating contraction of myomeres on opposite sides of the body. When muscles on one side contract, the body bends into a convex curve, while muscles on the other side stretch and prepare for contraction.
This coordinated, wave-like contraction generates an S-shaped motion that pushes against the water, propelling the fish forward. The segmented nature of the myomeres allows for precise, localized control, activating muscle segments only where needed. The myosepta act as a system of internal tendons that transmit muscle force to the vertebral column and the caudal (tail) fin.
This unique muscle-tendon system ensures that the force generated by the contracting muscle fibers is efficiently translated into the necessary lateral bending moment. Myomeres contain different types of muscle fibers that contribute to varied swimming behaviors. Red muscle fibers, located superficially along the flank, are slow-twitch and suited for sustained cruising, being rich in myoglobin and mitochondria. In contrast, the deeper, white muscle fibers are fast-twitch and used for powerful, anaerobic bursts of speed, such as escaping a predator or ambushing prey. The ability to switch between these muscle types based on swimming speed is a key aspect of myomere function.
Developmental Origins and Significance
The existence of myomeres in fish reflects the fundamental chordate body plan, characterized by serial segmentation. Myomeres originate during embryonic development from blocks of mesoderm tissue called somites. The somites form sequentially along the embryonic axis, and a portion of each somite, known as the myotome, is the precursor to the segmented muscle blocks.
This developmental process highlights the ancient organization of the chordate lineage. The original function of this musculature was to power the side-to-side swimming motion of early, fish-like ancestors. Even in basal chordates like the lancelet (Amphioxus), myomeres are present as simple V-shaped blocks that contract to bend the body.
In humans and other terrestrial vertebrates, somites still form in the embryo, but the resulting musculature differentiates extensively. While back muscles remain somewhat segmented, the myomeres of the trunk fuse and reorganize to form complex, layered structures like the abdominal and intercostal muscles. This transformation is an evolutionary adaptation from aquatic undulation to the stability and limb-based locomotion required for life on land. The persistent, highly segmented myomere structure in fish represents a specialized continuation of the ancestral mechanism for propulsion.