Complex sentence: Although many of the Antarctic species have antifreeze proteins in their body fluids, not all of the Antarctic species do, some sub-polar species either produce no or very little antifreeze, and antifreeze concentrations in some species are very low in young, larval fish.
Split into simpler sentences: Although many of the Antarctic species have antifreeze proteins in their body fluids, not all of the Antarctic species do. Some sub-polar species either produce no or very little antifreeze, and antifreeze concentrations in some species are very low in young, larval fish.