Fish Musculature: Trunk and Head Muscles
Trunk Muscles in Fish
Trunk muscles in fish consist of a series of muscle blocks called myotomes or myomeres, separated by connective tissue sheaths called myosepta. These myosepta represent the segmentation of the body. Some fish have red and white muscle cells (myofibers). The red fibers are oriented more or less parallel to the axis of the body, while white fibers can deviate as much as 45 degrees from the axis of the body. The myotomes are folded, just under the skin, and their outer edges resemble the letter “W” from the side. In sharks and bony fish, the bending angles are strong and obvious.
Musculature of the Trunk
The muscles of the tail and body wall are called parietal or somatic muscles. They are W-shaped segments separated by connective tissue septa. Each segment is called a myotome, and the connective tissue septum of each is called a myoseptum. The myotomes consist of muscle fibers that run lengthwise and attach to the spine or the myosepta. The white line running along the body near the dorsal side is the outer edge of the horizontal skeletal septum, a layer of connective tissue that extends from the vertebral bodies to the skin.
The parietal muscles that lie above the horizontal skeletal septum are called epaxial muscles. The parietal muscles that lie below the horizontal septum are called hypaxial muscles. Epaxial muscles consist of two or more longitudinal ridges on each side and extend from the base of the skull to the tail. Hypaxial muscles consist of a lateral longitudinal bundle, which extends from the pectoral girdle to the tail, and two longitudinal ventral bundles, which extend from the pectoral girdle to the pelvic girdle. A septum called the linea alba lies in the ventral midline, separating the left and right halves of the hypaxial muscles.
Head Muscles in Fish
The gill muscles, which move the gill arches and jaws, can be divided into three groups: superficial constrictors, elevators, and interarcuals.
Superficial Constrictors
Superficial constrictors cover part of the head and branchial region between the eye and the sixth branchial cleft. They consist of dorsal constrictors, which lie dorsal to the gill slits, and ventral constrictors, which lie ventral to the gill slits. Superficial constrictors are numbered from 1 to 6, each constrictor corresponds to a gill arch of the same number. The pharyngeal constrictors squeeze the pharyngeal chamber, eject water, and close the gill slits. The first and second dorsal constrictors raise the upper jaw and the hyoid arch. The first and second ventral constrictors assist in opening the mouth. Constrictors 3 to 6 are similar. They are attached to the connective tissue, which separates and overlaps the top of each other so that one is partially covered by the one before it.
The second constrictor corresponds to the second (hyoid) arch and is wider than the others. It extends from the second gill opening to the angle of the jaw and continues anteriorly as the epihyoid and interhyoid. The epihyoid lies just posterior to the spiracle. It originates from the otic capsule and surrounding fascia and inserts on the hyomandibular. The interhyoid lies near the ventral midline just posterior to the lower jaw, covered by the intermandibular. It originates from the medioventral area and inserts on the ceratohyal. Like the other constrictors, the epihyoid and interhyoid compress the gill pouches.
The first constrictor corresponds to the first gill arch, from which the upper and lower jaws are derived. It is represented by four separate muscles: the levator palatoquadrati, preorbitalis, quadratomandibularis, and intermandibularis.
- The levator palatoquadrati is a small muscle that lies just anterior to the spiracle. It originates from the otic capsule and inserts on the palatoquadrate cartilage, which is part of the upper jaw.
- Anterior to the levator palatoquadrati is the preorbitalis, which belongs to the group of elevators. The preorbitalis is a small muscle that lies between the upper jaw and the eye. It originates from the medioventral surface of the chondrocranium, and its fibers are inserted into the quadratomandibularis muscle.
- The quadratomandibularis is the large muscle of the angle of the jaw. It originates from the back of the palatoquadrate cartilage and inserts on Meckel’s cartilage. Both the preorbitalis and the quadratomandibularis close the mouth.
- The ventral intermandibularis of the first constrictor is a large muscle that lies posterior to the mouth. It originates from the medioventral area and inserts on Meckel’s cartilage and the fascia of the quadratomandibularis. This raises the floor of the mouth and pushes water out of the gill slits.
Elevator Muscles
The elevator group consists of three muscles: the levator palatoquadrati, the levator hyomandibulae, and the cucullaris.
- The levator palatoquadrati, which represents the levator of the first gill arch, lies anterior to the craniomaxillary. It originates from the otic capsule and inserts on the palatoquadrate next to the quadratomandibularis, acting to lift the upper jaw.
- The second, or levator hyomandibulae, lies deep to the epihyoid, with which it fuses. The two muscles cannot be easily separated. The levator hyomandibulae raises the hyoid arch (second branchial arch).
- The cucullaris represents the levator of gill arches 3 to 7. It is a large triangular muscle that lies dorsal to the constrictors and extends from the pectoral girdle anteriorly to the epihyoid. It originates from the fascia of the dorsal longitudinal bundles and inserts on the epibranchial cartilage of the last gill arch and the scapular process of the pectoral girdle. This raises the gill arches and the pectoral girdle.
Interarcual Muscles
The interarcuals are small muscles that act on the cartilage of the arches.
- The coracoarcuales communes lie immediately anterior to the coracoid bar, from which they take their origin. Anteriorly, the fibers of the coracoarcuales communes are continuous with those of the coracohyoid.
- The coracomandibularis is a thin muscle that lies at the midline. It originates deep to the intermandibularis from the fascia of the coracoarcuales communes and inserts on the posterior border of Meckel’s cartilage.
- The coracohyoids are paired muscles that lie dorsal and lateral to the coracomandibularis. They originate from the coracoarcuales communes, with which their fibers are continuous, and insert on the basihyal cartilage ventrally through the hyoid arch.
- The coracobranchiales lie deep to the branchial pouches. The coracobranchiales consist of five parts, which are inserted on gill arches 2 to 5 and on the basibranchial cartilage. Afferent branchial arteries pass between them. The fifth coracobranchial and the coracoarcualis communis form the anterolateral wall of the pericardial cavity.