Morphometric analysis of axial muscle in cyprinid fish provides support that intracellular lipid plays a role in slow fibre oxygen supply *

In the present study, slow axial muscle of three species of cyprinid fish, Danube bleak, nase and chub, was examined for intracellular lipid. Data from fish held under constant conditions in the laboratory (with and without locomotory exercise training) were compared with data from fish living in the wild. The laboratory‐reared fish (trained and untrained) as well as the fish from the wild showed species‐dependent differences of intracellular lipid content in their slow muscle, with values being higher in Danube bleak than in nase and chub. This suggests that accumulation of intracellular lipid within slow muscle fibres is partly under the influence of factors other than environmental ones, for example, capillary density. To test whether intracellular lipid facilitates oxygen transport from capillaries to mitochondria within the muscle fibres in those fish with a relatively low capillary density but high lipid content, this work also quantified slow muscle capillary supply. Under each life regime, the capillary densities for chub and nase were higher than those for Danube bleak. The negative correlation of intracellular lipid and capillary density is in favour of this supposition.