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A REVIEW OF OSMOREGULATION IN FRESHWATER AND MARINE ELASMOBRANCHS
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Since 1966, an extensive tagging program has demonstrated beyond any further doubt (Thorson, 1971) that the sharks move from the Caribbean Sea to Lake Nicaragua and vice versa. At the time of this writing, of 1450 post-juvenile sharks tagged at the various river mouths on the Caribbean Coast, ten have been recovered in Lake Nicaragua; and of 146 tagged at San Carlos, where the river leaves the lake, 28 have been recovered along the Caribbean Coast, most of them at the various outlets of the Rio San Juan. Except for these basic facts, the results of the tagging program have not yet been published.
Since the discovery of the phenomenally high urea content of the body fluids of cartilaginous fishes by Staedeler and Frerichs (1858), the unique osmoregulatory system of this vertebrate class has been studied by many investigators. In broad outline, the mechanisms are essentially similar in all three of the major subtaxa: the selachians, the batoids, and the holocephalans. A single exception is the genus Potamotrygon (fresh-water stingrays of South America and Africa), which has apparently lost the ability to concentrate urea even when transferred to salt water.
1. The major body fluid compartments were measured in two species of fresh-water Chondrostei, two species of fresh-water Holostei, three species of fresh-water Teleostei and seven species of marine teleosts. These were compared with previous measurements of an agnathan species and four species of Chondrichthyes.2. A general correlation was shown between the relative rates of respiratory movements and pulse rates, but neither of these appeared to be related to the taxonomic series. A faster pulse was more characteristic of marine than of fresh-water species.3. Plasma volume was measured by the dye dilution method, using T-1824. Whole blood volume was calculated from plasma volume and hematocrit. A progressive reduction in plasma and whole blood volume was noted, proceeding from the primitive to the more advanced groups. This is true both among the three classes of aquatic vertebrates and also among the three groups within the Osteichthyes. These volumes were remarkably similar in fresh-water and marine tel...
Under laboratory conditions, the gills of the striped dogfish Poroderma africanum (Gmelin) were studied as a possible site of ion and water transfer between internal and external media. Haematocrits showed that a drop in the external osmalarity produced increase in blood volume, measured as a decrease in blood pcv's (packed cell volume). Similarly, when fish were exposed to increased external osmolarity, the converse occurred, with resultant rise in pcv's. The use of phenol red showed that normally-fed fish did not drink the medium, and hence dilution of the blood was most probably due to water influx at the gills. Hypoosmotic fish (due to underfeeding) drank the medium in appreciable quantities, and dilution and concentration of external medium had a more pronounced effect on blood pcv's in these individuals. The pcv's of normally fed fish returned to initial values within 7 days after transfer to new medium is most cases, but hypo-osmotic specimens took longer to adjust to the new state of water balance. Surgical closure of rectal gland and urinary systems produced initial rises in serum sodium and chloride levels, but these reached equilibrium after 5 to 7 days, indicating compensatory regulation by some other organ, such as the gills. After removal of the sutures to the urinary systems of 3 fish, there was a noticeable drop in sodium and chloride levels of the serum in these individuals. All fish were kept for 14 days in the laboratory, with little change in blood composition (as measured) and with only the gills as regulatory organs in two of them. By using the radioisotopes Chloride-36 and Sodium-22, it was shown that both ions are lost at the gills, against the concentration gradient. Histochemical examination of gill tissue from several fish indicated that many cells contain high concentrations of chloride and are probably the site of chloride excretion. The number of such cells increased with increase in external salinity, and they were also abundant in tissue from hypo-osmotic specimens and those with inoperative urinary and rectal gland systems. From these findings it was concluded that the gills have a definite role in the ion and water balance of P. africanum.
Pyjama sharks (Poroderma africanum) were exposed to a wide range of salinities, over which blood serum was analysed for osmolarity, chloride and urea concentrations. Fish were divided into two groups, those fed twice weekly (high intake), and those fed once a month (low intake). Both groups were exposed to the same salinity range. High intake fish showed the characteristic elasmobranch osmolarity picture, with serum values slightly hyper-osmotic at all times. Low intake fish, however, showed a degree of hypo-osmotic regulation. Serum values for both groups overlapped at very low salinities. Serum urea was also affected by diet, so that again two distinct sets of values were produced, again with overlap at the lower salinities. When previously well-fed fish were starved over a period of one month, serum urea and osmolarity decreased simultaneously. Consequently, it is felt that serum osmolarity is directly related to serum urea levels. Serum chloride was not found to be affected by diet, both groups showing the same change in blood values when exposed to the same change in salinity. It is shown, however, that a reduction in food intake, over a period of more than a fortnight, can reduce metabolic urea to the extent of depressing serum osmolarity and, hence, shift the ionic and osmotic equilibrium between the fish and the sea water. This may result in varying degrees of hypoosmotic regulation.
1. 1. Bonnethead sharks, Sphyrna tiburo, were acclimated to three salinities representative of hyposaline, ambient, and hypersaline conditions. 2. 2. Serum urea, TMAO, sodium, and chloride comprised the major portion of the osmotic pressure. 3. 3. Urea and TMAO made up most of the serum osmotic pressure difference noted between 20 and 30‰ Urea, TMAO, and sodium chloride contributed equally to the increase in osmotic pressure from 30 to 40‰ 4. 4. A number of possibilities are explored to explain the failure of the sharks to become hyperosmotic during the 40‰ trial, with particulate matter causing stress deemed the most likely explanation.
The blood plasma of elasmobranchs is isosmotic or slightly hyperosmotic to sea water, but ions account for only part of its osmotic concentration, a large fraction being made up by urea and trimethylamine oxide (Holmes and Donaldson, 1969). These nitrogenous compounds are also present in high concentration in muscle (Smith, 1929; Dyer, 1952). In this paper an attempt is made to outline the sea water-plasma and plasmamuscle steady states by comprehensive analyses of muscle and plasma of specimens of the spiny dogfish Squalus acanthias. Measurements of osmotic concentration of plasma and muscle have been made and compared with the sum of analyzed constituents, values of the latter, first obtained as milligram-ion or millimolar concentrations per kilogram solvent water, being converted to milliosmoles by the appropriate osmotic coefficients. Imprecision arises here owing to lack of knowledge of some coefficients, and because of the possibility that some of the constituents may be bound to protein, exerting little osmotic effect. Some idea of the amount of ionbinding in muscle has been obtained by analyses of the juice expressed from muscle by a tissue press or obtained by ultracentrifugation. Estimates have also been made of the extracellular space in muscle, thus enabling intracellular concentrations to be calculated.
1.1. In eight females, serum urea concentrations of 134–336 mM/l. were interpreted as indications of previous environmental salinities.2.2. Fetal serum solute concentrations usually resembled those of the mother, but in two cases where fetal urea was higher, it was interpreted as a lag in urea reduction following movement from salt to fresh water.3.3. Uterine fluid was similar to maternal serum in all parameters except for its very low protein content.4.4. No flushing of the uteri with environmental water occurs.5.5. Independent osmoregulation apparently does not occur before birth, although required mechanisms may be present.6.6. The full range of urea tolerance is present before birth.
All previously reported species of Chondrichthyes, from both marine and fresh water, have contained urea at concentrations
ranging from about 300 to 1300 milligrams of urea nitrogen per 100 milliliters of fluid. Body fluids from two species of Potamotrygon, permanent residents of the Amazon basin, contained only 2 to 3 milligrams of urea nitrogen per 100 milliliters. Although they
have abandoned the retention of urea exhibited by other chondrichthyans, the extent to which they have lost the mechanisms
of retaining and tolerating urea in a hypertonic medium has not been determined.
The rectal gland of the spiny dogfish, Squalus acanthias, secretes a fluid which is essentially a sodium chloride solution with a concentration about twice that of the plasma and
greater than that of sea water. Observed volumes of flow are sufficiently large to make it clear that the rectal gland can
remove from the blood relatively large amounts of sodium chloride, and presumably this is its function.
The bull shark, Carcharhinus leucas, employing archaic but effective means of regulating the physical-chemical composition of its body fluids, thrives in tropical fresh-water rivers and lakes. The ionic strength of the serum and the concentrations of total solutes, calcium, urea, and other ions are below the levels found in marine elasmobranchs but higher than the levels in teleosts. The patterns of the calcium deposits of the vertebrae are identical in marine and fresh-water subspecies.
The relative volumes of major body fluids of freshwater and marine sharks are remarkably similar in spite of the differences
in external medium and in osmotic pressure of body fluids. The small differences detected are in agreement with differences
reported in comparisons of freshwater and marine teleosts: a slightly higher total water content and a smiller ratio of extracellular
to intracellular fluids in freshwater forms.
The rectal glands of elasmobranchs perform the function of salt-excreting organs. These glands are smaller and show regressive
changes in specimens of the bull shark, Carcharhinus leucas found in fresh-water environment, compared with specimens of this
and other species from a marine habitat.
Salt Secretion. 241-292
- F P Conte
Conte, F.P. 1969. Salt Secretion. 241-292. In W.S. Hoar and D. J. Randall
(eds.), Fish physiology. Academic Press, New York.
- P K T Pang
- R W Griffith
- J W Atz
Pang, P.K.T., R.W., Griffith and J.W. Atz. 1977. Osmoregulation in
Elasmobranchs. Amer. Zool., 17: 365-377.