High latitude ocean habitats are a crucible of fish body shape diversification

A strong decline in species richness from the equator to the poles is a
common feature of Earth’s biodiversity. However, little is known about how
phenotypic diversity varies across the same latitudinal gradient. Here, we
examine body shape diversity in marine fishes across latitudes and explore
the role of time and evolutionary rate in explaining the diversity
gradient. Marine fishes' occupation of upper latitude environments
has increased substantially over the last 55 million years. Latitude
strongly affects the rate of body shape evolution and its disparity.
Fishes in the highest latitudes exhibit nine times the rate of body shape
evolution and one and a half times the disparity compared to equatorial
latitudes. The more dynamic evolution of body shape may be due to
increased ecological opportunity in polar and subpolar oceans due to (1)
the evolution of anti-freeze proteins in certain temperate clades that
allowed them to invade regions of cold water, and (2) periodic
environmental disturbances driven by cyclical warming and cooling in upper
latitudes. Our results suggest that decreasing water temperature, through
its effects on the activity levels of fishes, may have elevated the
relative frequency of body shapes associated with less-active lifestyles.