The influence of environmental temperature on microvascular development infish

Post-natal fibre hypertrophy in aerobic muscle reduces capillary density, a lthough the number of vessels surrounding a fibre increases as the fish mat ures. This increase in intramuscular diffusion distance during ontogenetic growth will impair peripheral oxygen transport, and is likely to limit the scope for adaptation to environmental challenges. Acute cold exposure also impairs aerobic swimming capacity in most species, while chronic acclimation induces an apparently paradoxical increase in fi bre girth which would tend to exacerbate the reduced oxygen delivery at low temperatures. However, cold-induced fibre hypertrophy is accompanied by a more powerful angiogenic response than that seen during muscle development, such that capillary density is much less sensitive to changes in fibre siz e. This interaction between environmental temperature and muscle growth sug gests that early developmental stages, or species with a small maximal size , ought to be least affected by cold exposure. Slow muscle fibres of juveni le eels showed a mitochondrial proliferation at low temperatures in the abs ence of an accompanying change in capillary supply, while adult goldfish sh owed a strikingly similar muscle composition over a 20 degrees C range of a cclimation temperature, consistent with a significant reserve transport cap acity as a result of the small fibre size.