The morphology and lactate dehydrogenase patterns of the sound muscle of the haddock (Melanogrammus aeglefinnus)

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Abstract
The proportion of sarcoplasm to myofibrils in the muscle fibers of the sound muscle of haddock (M. aeglefinus) is lowest in immature females and highest in mature males. Some mature females have a higher ratio than immature females, and immature males usually have a higher ratio than females. The myofibrils form convoluted patterns in the muscle fibers of mature males, resulting in a large area of contact between the myofibrils and the numerous mitochondria in the sarcoplasm. Bundles of parallel axons give rise to groups of circumscribed nerve endings on the sarcolemma. This arrangement may be associated with the fast twitch function of the sound muscle. The lactate dehydrogenase activity is predominately of the B type in immature fish, but that of the mature fish shows more A and C type activity. This change is correlated with an increase in muscle size with maturity and alteration from an aerobic to a more anaerobic metabolism. Some sexual dimorphism is present.