Myomeres are segmented blocks of skeletal muscle tissue found in animals like fish and are a defining characteristic of the phylum Chordata. They constitute the bulk of the muscle mass in these animals and their arrangement is key to movement. This muscle organization is fundamentally different from that of many terrestrial vertebrates.
The Structure of Myomeres
Myomeres are organized into blocks along an animal’s body, often folded into a “W” or zigzag shape, though simpler “V” shapes are also found. The geometry of this nested, cone-like structure varies between species. For example, the myomeres of eels and sharks are W-shaped, while those of jawless fish are arranged in more vertical strips.
Each myomere is separated from its neighbors by thin sheets of connective tissue known as myosepta. The myosepta serve as attachment points for the short muscle fibers that make up the myomeres. This arrangement of muscle blocks separated by connective tissue is a primary reason for the unique texture of fish muscle.
How Myomeres Enable Movement
The primary function of myomeres is generating force for locomotion. This is achieved through the coordinated contraction of muscle blocks in a sequential, wave-like pattern that starts near the head and moves toward the tail. As one myomere contracts, it pulls on the myosepta and the animal’s spine, causing a section of the body to bend.
This action creates a side-to-side undulation that propels the animal forward. The overlapping, folded shape of the myomeres allows force from a single contracting segment to be distributed over a large portion of the body. This distribution ensures a smooth, S-shaped swimming motion, and the independent contractibility of each myomere allows for fine control and agility.
Myomeres in the Animal Kingdom
Myomeres are a feature found across the phylum Chordata. They are present in their simplest V-shaped form in primitive chordates like the lancelet, where they transmit force to the notochord for movement. These structures are most famously associated with fish, from jawless species like lampreys to sharks and bony fish like salmon.
The complexity of the myomere shape, such as the W-shape in many jawed fishes, is an adaptation for more advanced swimming. Myomeres are also present during the larval stages of amphibians, such as in tadpoles, before they undergo metamorphosis into their adult forms.
Why Fish Meat is Flaky
The flaky texture of cooked fish is a direct result of its myomeric muscle structure. The “flakes” that easily separate are the individual myomeres. This is caused by the delicate connective tissue that holds these muscle blocks together.
When fish is cooked, heat breaks down the thin myosepta separating the myomeres. This connective tissue is more delicate and less abundant than in terrestrial mammals like cattle. In beef, muscle fibers are longer and organized into large bundles with tough connective tissue that does not easily separate with heat. The short muscle fibers and weak myoseptal walls in fish allow the muscle blocks to slide apart, creating the familiar soft and flaky texture.