Study on the jaw-closing muscle, Adductor mandibulae, of the salmon, Oncorhynchus masou. 1. Fiber types.

A histochemical study has been made on the jaw-closing muscle, Adductor mandibulae, of the Japanese salmon, Oncorhynchus masou. 3 main types of fibers can be distinguished on the basis of differences in SDH activity: 1. fibers with very high SDH activity; 2. fibers with intermediate SDH activity, and 3. fibers with low SDH activity, containing only a few formazan particles. And, fibers with large ranges in diameter were clearly observed in each of the three fiber types classified on the basis of SDH activity. Fibers with the highest SDH activity were recognized in the most medial part of the deep portion, whereas fibers with intermediate activity were mainly distributed in the lateral side of the deep portion. But, there was no discrete zone in the distribution of these fiber types, differing from the locomotor myotomes in the fish. These facts suggest that the A. mandibulae muscle is composed of, at least, more than 3 types of fibers and the function of this muscle is also complex in the fish as it is in the mammalian.