CHANGES IN MUSCLE STRUCTURE ASSOCIATED WITH SOMATIC GROWTH IN IDIOSEPIUS-PYGMAEUS, A SMALL TROPICAL CEPHALOPOD

Mantle muscle tissue of Idiosepius pygmaeus was examined to describe c hanges in structure and organization associated with growth. Growth in I. pygmaeus was a function of both an increase in muscle fibre number and fibre size within muscle blocks. Continuous fibre production over the observed life span of I. pygmaeus was indicated by the presence o f very small muscle fibres (<1.0 mu m in diameter) in substantial prop ortions in all sizes of individuals. Muscle blocks became larger as an imals increased in size, although new muscle blocks were generated in all sizes of individuals. Mantle muscle fibres had a maximum size of 1 1 mu m. Therefore, for an individual to continue increasing muscle blo ck sizes, new fibres must be produced. This is further evidence of con tinuous fibre production throughout the size range of I. pygmaeus exam ined. The relative rates of muscle fibre generation and fibre growth d epended on the size of the animal and position along the mantle (anter ior, mid or posterior mantle). The predominance of small fibres and bl ocks at the anterior end of the mantle suggested that this was the pri mary growth region. Mitochondria-poor and mitochondria-rich muscle fib res from small individuals had much larger mitochondrial cores than mu scle fibres from larger animals. Changes in the muscle structure are d iscussed with respect to the metabolic and energetic requirements of I . pygmaeus, and how these may change with growth.