Found another paper on ray osmoregulation. It looks like the subject has been broached, but never extensively.
I found this to be an entertaining read. It's in PDF.
http://www.pewoceanscience.org/publications/files/papers/HammerschlagReview.pdf
This is a more detailed study on the differences of osmoregulation, especially between freshwater and marine rays, and gives insight into the evolution of the freshwater species. It's a 5 minute read and worthwhile.
Pertinent portions:
Fresh water elasmobranchs retain and synthesize less urea than that of their
marine counterparts. Their body fluid solute concentrations are relatively low
and urine is dilute and copious (Thorson et al, 1967; Thorson, 1970; Goldstein
and Forster, 1971; Poulsen, 1981). This greatly reduces their osmotic problem
of water retention. The freshwater stingrays of South America have abandoned
retention of urea, they lack a functional rectal gland and they osmoregulate as
much as do the freshwater teleosts (Thorson et al, 1967; Thorson, 1970;
Goldstein and Forster, 1971; Thorson, 1976; Poulsen, 1981).
Urea biosynthesis and retention
Urea levels, which in high concentrations have shown to be largely responsible
for the high osmolarity found in marine elasmobranchs, result from the
difference between the rate of biosynthesis and excretion of the compound. The
South American freshwater ray, Potamotrygon, has low rates of urea
biosynthesis and has also lost the ability to reabsorb urea in the kidneys
(Goldstein and Forster, 1971).
Water content
Although there is a significant difference in the concentrations of body fluid
solutes between salt and freshwater elasmobranchs, including in euryhaline
species that can move between the two mediums, there is no shift or difference
in the total water content and in the distribution of water among the fluid
containing body compartments (Thorson, 1962)
Osmoregulatory functions of the Kidneys
The high level of urea in marine elasmobranch blood is maintained by the
kidneys. Renal tubules are capable of reabsorbing urea, insuring that this
important osmoregulatory compound is not wasted (Pang et al, 1977).
Elasmobranchs that are adapted to dilute seawater increase the renal excretion of
urea, thus effectively lowering their plasma urea levels and osmolarity; however
as branchial loss of urea remains largely unaffected, the renal mechanism is
likely more significant (Pang et al, 1977). The obligate freshwater rays,
Potamotrygon, have abandoned renal reabsorption of urea (Goldstein and
Forster, 1971)
Osmoregulation by the Rectal Gland
Marine elasmobranchs face the problem of a natural and continuous diffusion of
salts into the body from the external sea water, where the concentrations are
higher (Haywood, 1973). The rectal gland of marine elasmobranchs functions
as a salt secreting mechanism (Conte, 1969; Oguri, 1964; Haywood, 1975);
however, disturbances produced by the cessation of its activity can eventually be
compensated for internally by other means that are as yet unknown (Burger and
Hess, 1960; Conte, 1969; Haywood, 1975).
Freshwater elasmobranchs do not face the problem of continuous diffusion of
salts into the body and in fact, the rectal gland of Bull sharks moving from salt
to freshwater becomes regressive (Oguri, 1964). No functional rectal gland in
the freshwater rays, Potamotrygon, has been found (Goldstein and Forster,
1971).
Osmoregulation of the Gills
Branchial elimination of salts in elasmobranchs has generally been considered of
little importance. However, to keep serum salt levels low, two-thirds of total
sodium and chloride excretion in elasmobranchs may take place through the
gills (Payan and Maetz, 1973; Pang et al, 1977).
I found this to be an entertaining read. It's in PDF.
http://www.pewoceanscience.org/publications/files/papers/HammerschlagReview.pdf
This is a more detailed study on the differences of osmoregulation, especially between freshwater and marine rays, and gives insight into the evolution of the freshwater species. It's a 5 minute read and worthwhile.
Pertinent portions:
Fresh water elasmobranchs retain and synthesize less urea than that of their
marine counterparts. Their body fluid solute concentrations are relatively low
and urine is dilute and copious (Thorson et al, 1967; Thorson, 1970; Goldstein
and Forster, 1971; Poulsen, 1981). This greatly reduces their osmotic problem
of water retention. The freshwater stingrays of South America have abandoned
retention of urea, they lack a functional rectal gland and they osmoregulate as
much as do the freshwater teleosts (Thorson et al, 1967; Thorson, 1970;
Goldstein and Forster, 1971; Thorson, 1976; Poulsen, 1981).
Urea biosynthesis and retention
Urea levels, which in high concentrations have shown to be largely responsible
for the high osmolarity found in marine elasmobranchs, result from the
difference between the rate of biosynthesis and excretion of the compound. The
South American freshwater ray, Potamotrygon, has low rates of urea
biosynthesis and has also lost the ability to reabsorb urea in the kidneys
(Goldstein and Forster, 1971).
Water content
Although there is a significant difference in the concentrations of body fluid
solutes between salt and freshwater elasmobranchs, including in euryhaline
species that can move between the two mediums, there is no shift or difference
in the total water content and in the distribution of water among the fluid
containing body compartments (Thorson, 1962)
Osmoregulatory functions of the Kidneys
The high level of urea in marine elasmobranch blood is maintained by the
kidneys. Renal tubules are capable of reabsorbing urea, insuring that this
important osmoregulatory compound is not wasted (Pang et al, 1977).
Elasmobranchs that are adapted to dilute seawater increase the renal excretion of
urea, thus effectively lowering their plasma urea levels and osmolarity; however
as branchial loss of urea remains largely unaffected, the renal mechanism is
likely more significant (Pang et al, 1977). The obligate freshwater rays,
Potamotrygon, have abandoned renal reabsorption of urea (Goldstein and
Forster, 1971)
Osmoregulation by the Rectal Gland
Marine elasmobranchs face the problem of a natural and continuous diffusion of
salts into the body from the external sea water, where the concentrations are
higher (Haywood, 1973). The rectal gland of marine elasmobranchs functions
as a salt secreting mechanism (Conte, 1969; Oguri, 1964; Haywood, 1975);
however, disturbances produced by the cessation of its activity can eventually be
compensated for internally by other means that are as yet unknown (Burger and
Hess, 1960; Conte, 1969; Haywood, 1975).
Freshwater elasmobranchs do not face the problem of continuous diffusion of
salts into the body and in fact, the rectal gland of Bull sharks moving from salt
to freshwater becomes regressive (Oguri, 1964). No functional rectal gland in
the freshwater rays, Potamotrygon, has been found (Goldstein and Forster,
1971).
Osmoregulation of the Gills
Branchial elimination of salts in elasmobranchs has generally been considered of
little importance. However, to keep serum salt levels low, two-thirds of total
sodium and chloride excretion in elasmobranchs may take place through the
gills (Payan and Maetz, 1973; Pang et al, 1977).
