Capillarisation, oxygen diffusion distances and mitochondrial content of carp muscles following acclimation to summer and winter temperatures.

Many species of fish show a partial or complete thermal compensation of metabolic rate on acclimation from summer to winter temperatures. In the present study Crucian carp (Carassius carassius L.) were acclimated for two months to either 2 degrees C or 28 degrees C and the effects of temperature acclimation on mitochondrial content and capillary supply to myotomal muscles determined. Mitochondria occupy 31.4% and 14.7% of slow fibre volume in 2 degrees C- and 28 degrees C-acclimated fish, respectively. Fast muscles of cold- but not warm-acclimated fish show a marked heterogeneity in mitochondrial volume. For example, only 5% of fast fibres in 28 degrees C-acclimated fish contain 5% mitochondria compared to 34% in 2 degrees C-acclimated fish. The mean mitochondrial volume in fast fibres is 6.1% and 1.6% for cold- and warm-acclimated fish, respectively. Increases in the mitochondrial compartment with cold acclimation were accompanied by an increase in the capillary supply to both fast (1.4 to 2.9 capillaries/fibre) and slow (2.2 to 4.8 capillaries/fibre) muscles. The percentage of slow fibre surface vascularised is 13.6 in 28 degrees C-acclimated fish and 32.1 in 2 degrees C-acclimated fish. Corresponding values for fast muscle are 2.3 and 6.6% for warm- and cold-acclimated fish, respectively. Maximum hypothetical diffusion distances are reduced by approximately 23-30% in the muscles of 2 degrees C-compared to 28 degrees C-acclimated fish. However, the capillary surface supplying 1 micron 3 of mitochondria is similar at both temperatures. Factors regulating thermal compensation of aerobic metabolism and the plasticity of fish muscle to environmental change are briefly discussed.