Freshwater elasmobranchs: a review of their physiology and biochemistry
Only 5% of elasmobranch species live in freshwater (FW) compared to more than 40% of known teleost species. The factors affecting the poor penetration of elasmobranchs into FW environments are currently unknown, however, an important consideration may be the high urea requirement of many proteins in marine elasmobranchs. Urea is an important osmolyte in marine elasmobranchs and must be reduced in dilute environments. There are three identifiable stages in the successful colonization of FW. The euryhaline marine species freely entering and leaving FW represent the initial stage of FW colonization. In this group, there is an apparent inability to eliminate all urea due to protein integrity issues and this results in energy and nitrogen losses that may constrain growth and reproduction. The second stage is represented by those species that live entirely in FW but must also retain some urea. This group also suffers from the same constraints as the first group. These two groups have kidneys and sensory organs that more closely resemble strictly marine forms. The third and final stage is represented by the Potamotrygonid stingrays where the need for urea in FW has been eliminated. Consequently nitrogen and energy losses are reduced and those sections of the kidney needed for urea conservation have been eliminated. The driving force for such modifications is a reduction in urea levels and the concomitant saving of energy needed for urea synthesis. Other physiological adaptations associated with survival in FW include giving birth to live young, the capacity of sperm to be activated in freshwater and modifications of the electrosensory system to function in a low conductivity environment. The need for many anatomical, metabolic and physiological modifications for FW existence may constrain the rapidity and hence the frequency of FW colonization, compared to the situation in the more advanced osmoregulating teleosts. Once optimally adapted to FW, recolonization of sea water by elasmobranchs is problematic due to the loss of urea synthetic capacity and renal structures for urea retention.