Muscle tissue growth and muscle fibre dynamics in the tropical loliginid squid Photololigo sp. (Cephalopoda: Loliginidae)

Growth of somatic tissue in Photololigo sp. was examined in terms of muscle fibre recruitment and growth. Muscle blocks and muscle fibres were measured and size frequency distributions compared between different size-classes of squid. Muscle blocks increased in size as individuals grew. The size frequency distribution of the muscle blocks suggested that this increase was due to both the generation of new muscle fibres and an increase in the size of existing muscle fibres. The size frequency distribution of muscle fibres was very similar in all size-classes of squid examined, and the presence of small muscle fibres in all individuals suggested that fibre recruitment may be continuous. Growth of muscle tissue, by muscle fibre growth and recruitment, provides a mechanism to explain constant growth throughout the life cycle described for tropical squid. Two structural types of muscle fibres, mitochondria-poor and mitochondria-rich, are present in juvenile and adult squid. A weak relationship between the ratio of the two muscle fibre types and dorsal mantle length suggests that generation of mitochondria-rich fibres may not be influenced by growth