HYPOXIA ADAPTATION IN FISH OF THE AMAZON - A NEVER-ENDING TASK

In addition to seasonal long-term changes in dissolved oxygen and carb on dioxide, water bodies of the Amazon present periodic short-term epi sodes of hypoxia and even anoxia. To preserve gas exchange and acid ba se balance, fish of the Amazon have developed multiple adaptive soluti ons which occur at all biological levels. These solutions are thought to represent adaptive convergence rather than phylogenetic relatedness . Fish of the Amazon exposed to different experimental conditions adju st, for example, several parameters to improve oxygen transfer from th e gas-exchange site to the tissues. These parameters include morpholog ical changes such as the development of the lower lip in Colossoma, ch anges in ventilation rates, changes in circulatory parameters, increas ed circulating red blood cells, decreased levels of intraerythrocytic phosphates, and adjustments of intraerythrocytic pH (pHi). These adjus tments that allow fish to survive both short- and long-term hypoxia oc cur in different degrees in different fish species and may or may not occur simultaneously. In addition, these adjustments in oxygen transfe r affect many other parameters, particularly acid-base status. We sugg est that these adjustments are initiated as soon as the animal detects the environmental change in oxygen availability and are mediated by a single factor, possibly one of the catecholamines. In this paper we a im to show that adaptation to hypoxia is a never-ending task for the f ish of the Amazon.