Chapter 1 General introduction Section 1 contains a brief review of the structure and function of different muscle fibre types in teleost fish. Section 2 concerns the structure and function of fish liver including the classification of different liver cell types, the ultrastructure of fish hepatocytes and liver metabolism Section 3 The concepts of evolutionary adaptation and physiological acclimation are explained and an outline is given of some of the underlying cellular and molecular mechanisms. Chapter 2 Seasonal influences on the biochemical composition of tissues in dab (Limanda limanda L.) Seasonal changes in the biochemical composition of liver and skeletal muscle have been investigated in juvenile dab (Limanda limanda L.) from the wild. Chapter 3 Seasonal influences on the ultrastructure of myotomal muscles of dab CLimanda limanda L.) Seasonal influences on the ultrastructure of myotomal muscles of the immature dab (Limanda limanda L.) have been studied. Fish specimens were taken from the same population of animals as in chapter 2. The fractional volume (%) of each fibre type occupied by mitochondria, myofibrils and lipid droplets was determined using a point counting morphormetric method. Chapter 4 Seasonal influenes on the ultrastructure of hepatocytes of dab (Limanda limanda L.) The dab (Limanda limanda L.) exhibited marked seasonal alterations in the ultrastructure of hepatocytes during the year. Endoplasmic reticulum (ER) was more dilated and Golgi complex were highly developed in the hepatocytes of winter- adapted dabs. Mitochondrial profiles were also found to be well organized in the hepatocytes of winter caught animals. The hepatocytes in the summer-adapted dabs were characterised by containing enormous lipid droplets whereas those in the autumn-adapted fish were featured by numerous lipopigment-like granules. Different forms of hepatic glycogen and triglyceride were observed in the hepatocytes of dab (Limanda limanda L.) throughout the year. In the autumn, the hepatic glycogen and triglyceride existed in a form of loosely packed homogeneous masses throughout the cytoplasm whereas a more densely packed form for both glycogen and triglyceride was observed during the winter. Chapter 5 General discussion The major findings in the present study are discussed in relation to seasonal influences on tissue composition and ultrastructure of myotomal muscle fibres and hepatocytes in the juvenile dab (Limanda limanda L.). Some suggestions for further works are given.