Physiological determinants of distribution in fish*

Fish are totally excluded, by virtue of their physiology, only from a permanently terrestrial existence, from bodies of water with a pH below 4 or above 10 or with a calcium content below 10 mmol l −1 , from permanently anoxic environments and from hypersaline water bodies with salinities above about 90‰. In less extreme, but still demanding, environments physiological limitations constrain the distribution of certain categories of fish. Extreme conditions are associated with low‐energy life styles.