Marine sharks accumulate and tolerate a high level of urea in their blood.
These high levels of urea effectively cancel the tendency of water the leave by osmosis, and there is consequently no net movement of water across a shark's surfaces. It would therefore appear that the high urea concentration is an adaptation to living in salt water. There are, however, freshwater sharks. 3.5 shows the distribution of rainfall over the earth. The Allee effect is named after W. C. Allee who observed that, at very low densities performance may decline compared to a somewhat higher density so that performance peaks at some low-intermediate densiny.