Histological Characteristics of Fast and Slow Muscle Fibers in Skeletal Muscle of Fishes with Three Different Swimming Habits

Abstract

Swimming is of great significance for the survival of fish and directly affects their ability to avoid predators and enemies, hunt and capture prey, carry out mating and reproduction, and migrate. The skeletal muscles of bony fish, which provide power for swimming and account for approximately 40%–60% of the body mass, can be divided into red and white muscle fibers. Red muscle fibers have a slow contraction, strong endurance, high mitochondria content, glycogen, and myoglobin; mainly employ aerobic metabolism; and effectively use oxygen to produce ATP. They are also known as slow-twitch muscle fibers, whose main function in fish is to provide stable and continuous power for the swimming process. White muscle fibers contract quickly but also tire rapidly and mainly use glycolic metabolism. They are also known as fast-twitch muscle fibers and, in fish, provide power for fast swimming behaviors (such as predation and escape).Many studies have indicated a strong correlation between fish swimming habits and the composition of slow and fast-twitch muscles. Most of these studies focused on the correlation between swimming motion and muscle fiber types, but differences in the histological characteristics of fast- and slow-twitch muscle fibers of fish with different swimming habits have rarely been reported. The histological characteristics of muscle fibers include shape, diameter, and density, which are important indicators describing the histological structure of the skeletal muscle in fishes. In this study, we selected three species (Scomber japonicus, Larimichthys crocea, and Paralichthys olivaceus) representing different swimming styles, to clarify the histological characteristics of fast- and slow-twitch muscle fibers, and compared them using hematoxylin-eosin staining of paraffin sections and morphometric methods.The staining showed that the transverse sections of the fast- and slow-twitch skeletal muscle fibers were irregular and the diameter of the fast-twitch muscle fibers was larger than that of the slow-twitch muscle fibers. In S. japonicus, a species engaged in sustained swimming, the fast-twitch muscles were multi-angular, whereas the slow-twitch muscle fibers were multi-columnar. In L. crocea, a species swimming in an extended style, the muscle fibers were long, oval, and had cells with round edges. In P. olivaceus, a species engaged in prolonged swimming, the fast-twitch muscle fibers were oblate and had more connective tissues than the slow-twitch fibers. The slow-twitch muscle fibers of S. japonicus and the fast-twitch muscle fibers of P. olivaceus were finer than the slow-twitch and fast-twitch muscle fibers of these two species, respectively. The longitudinal section of the muscle fibers in the three species were distributed in strips alternating with connective tissue. In addition, the muscle fibers in S. japonicus occupied a larger space and were more loosely arranged than those in the other two species. However, the muscular space between fibers of both types was smaller in P. olivaceus and the muscle cells were more closely arranged.Morphometric results showed that the diameters of fast-twitch muscle fibers were significantly larger than those of slow-twitch muscle fibers (P L. crocea [(205.43±12.63) unit/mm2] > S. japonicus [(118.92±10.74) unit/mm2]. The density of the fast-twitch muscle fiber of P. olivaceus was 2.31 and 1.34 times that of S. japonicus and L. crocea, respectively. The order of slow-twitch muscle fiber density was S. japonicus [(1 442.33±28.25) unit/mm2] > P. olivaceus [(1 073.92±39.40) unit/mm2] > L. crocea [(945.74±19.53) unit/mm2]. Furthermore, the slow-twitch muscle fiber density of S. japonicus was 1.53 and 1.34 times that of L. crocea and P. olivaceus, respectively. The above-described methodology and analysis of differences in the shape, diameter, and density in the skeletal muscle fibers of teleost fish with different swimming habits will provide basic data for further studies on the adaptive evolution and movement physiology of this taxonomic group.

Keywords

• skeletal muscle
• fast and slow muscle fibers
• histological features
• swimming habits