Image of a Gulf toadfish, Opsanus beta, from Wikimedia Commons
Teleost fish living in saltwater environments are constantly compensating for water loss. This happens because their surroundings have higher concentrations of salts than their plasma and the rule of thumb in Biology is: water follows salt. Thus these fish must somehow compensate for water loss in order to prevent dehydration. One way they do this is by drinking a lot of water. Ions are then removed from the water in the intestines which creates a gradient for water absorption into the body. A new study published in the American Journal of Physiology - Regulatory, Integrative and Comparative Physiology presents a related protective mechanism used by the fish. In this study, researchers discovered that the rectum of Gulf toadfish (pictured above) is also important in water balance and, in fact, is able to absorb more water than the intestines. Although the intestines and rectum use different ion transporters, the outcome is the same, water is absorbed to prevent dehydration. The intestine and rectum of the fish also secrete a hormone called guanylin, which shuts off water absorption long enough for water to build up in the rectum to help the fish defecate.
Source: