way of the rectal gland. 



In freshwater teleosts, the osmotic imbalance between body fluids 

 and environment is reversed, since the salt content is regulated at only 

 a very slightly lower level than in the marine teleosts. But when 

 euryhaline elasmobranchs such as the bull shark Carcharhinus leucas and 

 the sawfish Pristis perotteti move from the sea to fresh water the imbalance 

 is not reversed but simply increased. The osmotic influx of water therefore 

 increases and the quantity of urine increases accordingly and its concen- 

 tration is reduced, both by a factor of 15 to 20. The urea content of the 

 body fluids decreases to about 30 to 50 percent of the marine level, and 

 presumably the rectal gland stops excreting salt. When the elasmobranchs 

 return once more to the sea, they revert to the marine osmoregulatory 

 pattern. 



The completely freshwater stingrays of South American rivers (family 

 Potamotrygonidae) have abondoned the accumulation of urea as an osmoregu- 

 latory agent and are unable to retain urea even in response to transfer 

 into saline water. Essentially they deal with their osmotic problems as 

 freshwater teleosts. In contrast to these rays, freshwater stingrays of 

 the Benue River of Nigeria (family Dasyatidae) deal with their osmotic 

 problems essentially as the euryhaline shark and sawfish, retaining urea, 

 but at a reduced level. 



In recent years, more studies have been conducted in the general area 

 of osmoregulation than in any other area of elasmobranch physiology except 

 perhaps neurobiology. Many facets of the overall problem have been at 

 least touched on, for instance, the biosynthesis of urea and trimethylamine 

 oxide and the influence of various experimental regimens on that synthesis 

 and the enzymes involved; the influence of various substances, procedures 

 and environmental salinities on urea levels; membrane transport of urea 

 and various inorganic ions; ietention of urea by tubular reabsorption and 

 gill membrane impermeability; various aspects of rectal gland function; 

 and the influence of endocrine secretions on osmoregulation. 



Nevertheless, much remains to be learned before we have a reasonably 

 complete understanding of how elasmobranchs deal with their osmoregulatory 

 problems, especially as they move between areas of differing environmental 

 salinities. 



No aspect of osmoregulation is fully understood and any of the areas 

 mentioned above would profit by study in greater depth and particularly 

 by comparison of a greater variety of species. Perhaps most enlightening 

 would be the application of already used techniques and experimental pro- 

 cedures to the truly euryhaline species. In this way the rates and 

 directions of the processes that make up the total osmoregulatory function 

 can be watched under controlled manipulations in species that are able to 

 make the requisite changes to accommodate to both sea water and fresh 

 water. 



In the past, many attempts have been made to observe changes when a 

 shark is transferred from sea water to fresh water, but almost invariably 

 the wrong species have been selected — species that are not truly 

 euryhaline and therefore are not fully able to make the necessary physio- 

 logical adjustments. Therefore, when placed, for instance, in 50 percent 

 sea water for several hours, they retain excessive water, are unable to 



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