The bull shark, Carcharhinus lellcas, 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

Urist, Marshall R

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Investigations of the Ichthyofauna of Nicaraguan Lakes Papers in the Biological Sciences 1976 Calcium and Other Ions in Blood and Skeleton of Nicaraguan Fresh-Water Shark Marshall R. Urist University of California Follow this and additional works at: https://digitalcommons.unl.edu/ichthynicar  Part of the Aquaculture and Fisheries Commons Urist, Marshall R., "Calcium and Other Ions in Blood and Skeleton of Nicaraguan Fresh-Water Shark" (1976). Investigations of the Ichthyofauna of Nicaraguan Lakes. 42. https://digitalcommons.unl.edu/ichthynicar/42 This Article is brought to you for free and open access by the Papers in the Biological Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Investigations of the Ichthyofauna of Nicaraguan Lakes by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Reprinted from Science, September 21, 1962, Vol. 137, No. 3534, pages 984-986 Copyright © 1962 by the American Association for the Advancement of Science Calcium and Other Ions in Blood and Skeleton of Nicaraguan Fresh-Water Shark A bstract. The bull shark, Carcharhinus lellcas, 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 con-centrations 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 iden-tical in marine and fresh-water subspecies. Migrations into fresh water of the bull sharks of Lake Nicaragua in Cen-tral America, Lake Sentani of western Dutch New Guinea, and the Ganges and other Asiatic rivers have been re-ported by Herre (I) and Boeseman (2) . The physiology of these fishes presents extraordinary mechanisms of ion and osmotic regulation. Smith (3) reported a decrease in the serum con-centration of urea with relatively little change in phosphate or chloride in small sharks and sawfish found in rivers of Malaya and Siam; no data were recorded about the other solutes of the blood. My previous research (4) has included work on the chem-istry of calcium and the major com-ponents of the blood and skeleton of the bull shark of the Atlantic, Carcha-rhinlls leucas, and 13 other species of marine elasmobranchs, but none were fresh-water migrants. On 27 March 1962, one tarpon (Megalops atlanticus) and four bull sharks, three females and one male, ranging in length from 5 feet 11 inches to 7 feet 1 inch, were taken in south-eastern Nicaragua from Rio San Juan (5) near EI Castillo. Small fishes, alli-gators, and viscera of swine were used as bait, but only freshly caught tarpon brought results. The fish were photo-graphed after exsanguination by cardiac puncture and section of the caudal artery. Blood was collected in thermos containers. Serum was separated imme-diately after the blood had clotted, with a hand-operated centrifuge and then was frozen in liquid nitrogen for transporta-tion to Los Angeles. Vertebrae were excised and fixed in solutions of 10-percent formalin. Specimens of water were collected from Lake Nicaragua and the region of Rio San Juan in which the fish were caught. Chemical methods were the same as in previous reports (4); values given are the mean of three determinations on each speci-men. Photographs of two specimens were examined by Eugenie Clark and Stewart Springer but could not be positively identified as C. leucas. Possibly a sub-species can be recognized in which (i) the first dorsal fin is behind the pectoral fins, (ii) the third gill slit is short, and (iii) the dental formula is identical. There is now considerable uncertainty (6) about whether the data collected to date can justify two sub-species, C. leucas leucas and C. leucas nicaraguensis; only a statistically signifi-cant number of specimens can resolve this situation. The limits of the range of C. leucas nicaraguensis is not known. The general assumption is that it origi-nally inhabited oceanic lakes extending from the sea, adjusted to fresh water when the salt water turned fresh by a slow process of dilution, and became landlocked by historic earthquakes that raised the bedrock and produced the rapids of Rio San Juan. At EI Castillo, EI Toro rapids have channels through them that I found were easily passable for large tarpon, sharks, and even fisher-men traveling in dugout canoes during the dry season. The vertebrae of the shark of Nica-ragua were sectioned for radiographic examination of the calcium deposits and for comparison with typical specimens of C. leucas which had been collected in the Atlantic Ocean and supplied by Clark. The configurations of the cal-cium deposits are exactly the same, and somewhat similar to those of other Carcharhinidae. The vertebrae of tarpon consist of true bone rather than calci-fied cartilage (Fig. 1). Analyses of waters of Nicaragua re-vealed typical medium soft fresh water. In Lago de Nicaragua, and Rio San Juan, respectively, such analyses showed the following ions (in millimoles per liter) : total ions, 8.5 and 5.1; Na, 1.3 and 0.7; K, 0.1 and 0.1; Ca, 0.9 and 0.8; Mg, 0.6 and 0.2; CI, 1.8 and 0.8; Fig. 1. Radiographs in the frontal view, showing the anatomy of the lumbar vertebrae of C. lellcas /eucas (left), C. /ellcas nicaraguensis (middle), and the tarpon, M. at/an-ticus (right). These deposits consist of calcified cartilage in the elasmobranchs, but true bone exists in the teleosts. The identical pattern in the two sharks suggests that they are of very closely related. if not the same, species. 575 Published in INVESTIGATIONS OF THE ICHTHYOFAUNA OF NICARAGUAN LAKES, ed. Thomas B. Thorson (University of Nebraska-Lincoln, 1976). Copyright © 1976 School of Life Sciences, University of Nebraska-Lincoln.HCOa, 3.0 and 1.7; Sal, 0.7 and 0.7; and PO" 0.001 and 0.001. In C. leucas nicaraguensis from Rio San Juan, the total concentration of the serum is approximately 83 percent, the calcium 66 percent, and the uremia 30 percent of the levels which were found in C. leucas from a marine habi-tat (Table 1). To appreciate the re-markable adjustments that these elasmo-branchs can make in fresh water, the chemical composition of the serum in marine habitat (4) must be noted. In the sea, the level of serum urea fluc-tuates between 300 and 350 mmolel liter, and the total ion concentration rises and .falls only slightly in order to maintain osmotic equilibrium and home-ostasis. The ionic strength, p., is as high as 0.3 (7). Elasmobranchs do not drink sea water but absorb calcium and other ions from the diet through the gut. The active concentration of cal-cium ions is sustained by the kidneys and gill membranes at levels required for normal neuromuscular tone and for calcification of cartilage. For these functions, the optimum concentration of total serum calcium (3.0 mmolel liter is ionized) for the shark in marine Table I. Chemical composition (millimoles per liter) of serums of sharks and tarpon. Item C. iellcas fellcas Ions: Na 223.4 K 9.0 Ca 4.5 Mg 2.9 CI 236.0 HCO) 5.1 SO. 0.6 PO. 2.0 Total 483.5 Urea nitrogen 333.0 Ultrafilterable Ca 3.7 Protein (g (~) 2.4 Alkaline phosphatase 0.9 Acid phosphatase 1.0 "Hemolyzed blood. -----C. fellcas nicara-gllellsis 200.12 8.2* 3.0 2.0 180.5 6.0 0.5 4.0* 404.3 132.0 1.8 3.4 3.0 1.1 M. atiallticlIs 101.0 6.2 2.5 1.4 140.0 10.1 0.4 5.0* 266.6 6.0 J.I 6.0 2.0 12.0 habitat is 4.5 mmole/liter. Further efforts to investigate ion regulation in fishes requires experiments designed to study thermodynamic mechanisms. In sea water, the gills expend energy to: (i) hold back ions from moving too fast toward concentration and electro-potential gradients; (ii) maintain uni-576 directional flow; and (iii) regulate each ion separately. The concentration of calcium in sea water is 10.10 mmolel liter, but only a fraction is chemically active when this value is corrected to infinite dilution and ion association is taken into consideration. In the fresh water, the serum con-centration of urea is sustained at ap-proximately 132 mmole/liter. Elasmo-branchs require at least this concentra-tion of urea for normal cardiac function (7, 8). The concentration of total ions is 404.3 mmole/liter, 83 percent of the ion concentration of the serum of ma-rine elasmobranchs; the ionic strength, p., is estimated to be .2790. Under these conditions, the active concentrations of calcium ions increase to the extent that precipitation of calcium phosphate and depression of neuromuscular irritability can be avoided only if the gill mem-branes and the kidneys lower the cal-cium concentration in the body fluids. This level appears in the fresh-water shark to be approximately 3.0 mmolel liter of total and 1.8 mmole/liter of ultrafilterable calcium. The most abun-dant ions in Rio San Juan and other bodies of fresh water are Ca++ and HCO.--, but the total ion concentration is very low (5.1 mmole/liter), com-pared with 404.3 mmole/liter in serum. Here, the gills expend energy obtained from intracellular glycolysis to pump ions against concentration and electro-potential gradients. The teleost fish that is found in the same geographic area as the Nicaraguan shark has an entirely different compo-sition of the serum; p. equals 0.15, total ions constitute 266.6 mmole/liter, and the total calcium is only 2.5 mmolel liter. Uremia is absent (Table 1). The blood is less concentrated, more vis-cous, better aerated, and homeostasis is' achieved presumably by dynamic mecha-nisms using the functions of bone cells and bone tissue as a buffer (7). Tele-osts are ionically independent and highly advanced, compared with the elasmobranchs; they evolved bone for finer regulation of ionic composition of the blood. The apatite mineral in bone is metabolized by means of a two-way process of resorption and formation, and constitutes the storage depot of a closed cycle or servosystem for utilizing hydronium, sodium, calcium, phosphate, and other ions. In addition to gills, kidneys, and bone, integument func-tions as an organ of ionic homeostasis. It serves as a semipermeable membrane in lamprey (which have gills but no calcium deposits) and in certain adult amphibians (which possess true bone but no gills and yet retain urea). In elasmobranchs and teleosts, the body covering is waterproof, and facilitates osmoregulation. Thus, the interactions and combinations of organs used by vertebrates for water balance and min-eral homeostasis are numerous and highly complex. Physiologic experi-ments are necessary to measure the capacity of the bull sharks for adapta-tion. A tagging project would be of great interest to determine the limits of their range, reproductive cycle, spe-cies specificity, and ascertain whether they readapt to marine habitat. MARSHALL R. URIST Division of Orthopedics, Department of Surgery, University of California Medical Center, Los Angeles 24 References and Notes 1. A. W. C. T. Herre, Science 121, 417 (1955). 2. M. Boeseman, Ibid. 123, 222 (1956). 3. H. Smith, Am. I. Physiol. 98, 279 (1931). 4. M. R. Urist, Endocrinology 69, 778 (1961). 5. I am indebted to: Dr. Thomas R. Howell, Marc Buchanan, and John Zoeger of the de-partment of zoology at UCLA, who assisted and made the arrangements in Nicaragua; Dr. J. Antonio Mora, Ing., Escuela Nacional de Agricultura y Ganaderia, who selected the area of exploration; the Nicaraguan Air Force, which transported personnel and equipment; Major Alvaro Pallais S., Commandant at San Carlos, who allocated a jeep and boats; Cap-tain Humberto Suarez A., of the Guardia Na-cional de Nicaragua, who gave invaluable time and energy and assisted in the surgery; Mar-shall McLean Urist of Cate School who served as laboratory technician and processsed the specimens; Dr. Eugenie Clark, Cape Haze Marine Laboratory, Placida, Florida, who pro-vided 'serum and vertebrae of C. leucas, col-lected in the Atlantic Ocean; Robert Dorion, Guatemala City, Guatemala, who generously loaned his private library on the Elasmo-branchii; Drs. Stewart Springer (Duke Univer-sity Marine Laboratory), T. B. Thorson (Uni-versity of Nebraska), and Malcolm Gordon, who contributed valuable suggestions. 6. A. B. Bigelow, W. C. Schroder, S. Springer, Bull. Mus. Compo Zool. 109, 213 (1953). 7. M. R. Urist, Perspectives Bioi. Med. (in press). 8. W. W. Simpson and E. Ogden, I. Exptl. Bioi. 9, 1 (1932). 9. These investigations were supported by con-tract NR 104-575 between the Office of Naval Research and the University of California Medical Center, Los Angeles, and in part by grants from the Squibb Institute for Medical Research. 10 August 1962 

Calcium and Other Ions in Blood and Skeleton of Nicaraguan Fresh-Water Shark

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Investigations of the Ichthyofauna of 
Nicaraguan Lakes Papers in the Biological Sciences 
1976 
Calcium and Other Ions in Blood and Skeleton of Nicaraguan 
Fresh-Water Shark 
Marshall R. Urist 
University of California 
Follow this and additional works at: https://digitalcommons.unl.edu/ichthynicar 
 Part of the Aquaculture and Fisheries Commons 
Urist, Marshall R., "Calcium and Other Ions in Blood and Skeleton of Nicaraguan Fresh-Water Shark" 
(1976). Investigations of the Ichthyofauna of Nicaraguan Lakes. 42. 
https://digitalcommons.unl.edu/ichthynicar/42 
This Article is brought to you for free and open access by the Papers in the Biological Sciences at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Investigations of the 
Ichthyofauna of Nicaraguan Lakes by an authorized administrator of DigitalCommons@University of Nebraska - 
Lincoln. 
Reprinted from Science, September 21, 1962, Vol. 137, No. 3534, pages 984-986 
Copyright © 1962 by the American Association for the Advancement of Science 
Calcium and Other Ions in 
Blood and Skeleton of 
Nicaraguan Fresh-Water Shark 
A bstract. The bull shark, Carcharhinus 
lellcas, 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 con-
centrations 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 iden-
tical in marine and fresh-water subspecies. 
Migrations into fresh water of the 
bull sharks of Lake Nicaragua in Cen-
tral America, Lake Sentani of western 
Dutch New Guinea, and the Ganges 
and other Asiatic rivers have been re-
ported by Herre (I) and Boeseman 
(2) . The physiology of these fishes 
presents extraordinary mechanisms of 
ion and osmotic regulation. Smith (3) 
reported a decrease in the serum con-
centration of urea with relatively little 
change in phosphate or chloride in 
small sharks and sawfish found in 
rivers of Malaya and Siam; no data 
were recorded about the other solutes 
of the blood. My previous research 
(4) has included work on the chem-
istry of calcium and the major com-
ponents of the blood and skeleton of 
the bull shark of the Atlantic, Carcha-
rhinlls leucas, and 13 other species of 
marine elasmobranchs, but none were 
fresh-water migrants. 
On 27 March 1962, one tarpon 
(Megalops atlanticus) and four bull 
sharks, three females and one male, 
ranging in length from 5 feet 11 inches 
to 7 feet 1 inch, were taken in south-
eastern Nicaragua from Rio San Juan 
(5) near EI Castillo. Small fishes, alli-
gators, and viscera of swine were used 
as bait, but only freshly caught tarpon 
brought results. The fish were photo-
graphed after exsanguination by cardiac 
puncture and section of the caudal 
artery. Blood was collected in thermos 
containers. Serum was separated imme-
diately after the blood had clotted, with 
a hand-operated centrifuge and then was 
frozen in liquid nitrogen for transporta-
tion to Los Angeles. Vertebrae were 
excised and fixed in solutions of 10-
percent formalin. Specimens of water 
were collected from Lake Nicaragua 
and the region of Rio San Juan in 
which the fish were caught. Chemical 
methods were the same as in previous 
reports (4); values given are the mean 
of three determinations on each speci-
men. 
Photographs of two specimens were 
examined by Eugenie Clark and Stewart 
Springer but could not be positively 
identified as C. leucas. Possibly a sub-
species can be recognized in which 
(i) the first dorsal fin is behind the 
pectoral fins, (ii) the third gill slit is 
short, and (iii) the dental formula is 
identical. There is now considerable 
uncertainty (6) about whether the data 
collected to date can justify two sub-
species, C. leucas leucas and C. leucas 
nicaraguensis; only a statistically signifi-
cant number of specimens can resolve 
this situation. The limits of the range 
of C. leucas nicaraguensis is not known. 
The general assumption is that it origi-
nally inhabited oceanic lakes extending 
from the sea, adjusted to fresh water 
when the salt water turned fresh by a 
slow process of dilution, and became 
landlocked by historic earthquakes that 
raised the bedrock and produced the 
rapids of Rio San Juan. At EI Castillo, 
EI Toro rapids have channels through 
them that I found were easily passable 
for large tarpon, sharks, and even fisher-
men traveling in dugout canoes during 
the dry season. 
The vertebrae of the shark of Nica-
ragua were sectioned for radiographic 
examination of the calcium deposits and 
for comparison with typical specimens 
of C. leucas which had been collected 
in the Atlantic Ocean and supplied by 
Clark. The configurations of the cal-
cium deposits are exactly the same, and 
somewhat similar to those of other 
Carcharhinidae. The vertebrae of tarpon 
consist of true bone rather than calci-
fied cartilage (Fig. 1). 
Analyses of waters of Nicaragua re-
vealed typical medium soft fresh water. 
In Lago de Nicaragua, and Rio San Juan, 
respectively, such analyses showed 
the following ions (in millimoles per 
liter) : total ions, 8.5 and 5.1; Na, 1.3 
and 0.7; K, 0.1 and 0.1; Ca, 0.9 and 
0.8; Mg, 0.6 and 0.2; CI, 1.8 and 0.8; 
Fig. 1. Radiographs in the frontal view, showing the anatomy of the lumbar vertebrae 
of C. lellcas /eucas (left), C. /ellcas nicaraguensis (middle), and the tarpon, M. at/an-
ticus (right). These deposits consist of calcified cartilage in the elasmobranchs, but 
true bone exists in the teleosts. The identical pattern in the two sharks suggests that 
they are of very closely related. if not the same, species. 
575 
Published in INVESTIGATIONS OF THE ICHTHYOFAUNA OF NICARAGUAN LAKES, ed. Thomas B. Thorson 
(University of Nebraska-Lincoln, 1976). Copyright © 1976 School of Life Sciences, University of Nebraska-Lincoln.
HCOa, 3.0 and 1.7; Sal, 0.7 and 0.7; 
and PO" 0.001 and 0.001. 
In C. leucas nicaraguensis from Rio 
San Juan, the total concentration of the 
serum is approximately 83 percent, the 
calcium 66 percent, and the uremia 
30 percent of the levels which were 
found in C. leucas from a marine habi-
tat (Table 1). To appreciate the re-
markable adjustments that these elasmo-
branchs can make in fresh water, the 
chemical composition of the serum in 
marine habitat (4) must be noted. In 
the sea, the level of serum urea fluc-
tuates between 300 and 350 mmolel 
liter, and the total ion concentration 
rises and .falls only slightly in order to 
maintain osmotic equilibrium and home-
ostasis. The ionic strength, p., is as 
high as 0.3 (7). Elasmobranchs do not 
drink sea water but absorb calcium and 
other ions from the diet through the 
gut. The active concentration of cal-
cium ions is sustained by the kidneys 
and gill membranes at levels required 
for normal neuromuscular tone and for 
calcification of cartilage. For these 
functions, the optimum concentration 
of total serum calcium (3.0 mmolel 
liter is ionized) for the shark in marine 
Table I. Chemical composition (millimoles per 
liter) of serums of sharks and tarpon. 
Item C. iellcas fellcas 
Ions: 
Na 223.4 
K 9.0 
Ca 4.5 
Mg 2.9 
CI 236.0 
HCO) 5.1 
SO. 0.6 
PO. 2.0 
Total 483.5 
Urea nitrogen 333.0 
Ultrafilterable Ca 3.7 
Protein (g (~) 2.4 
Alkaline 
phosphatase 0.9 
Acid phosphatase 1.0 
"Hemolyzed blood. 
-----
C. fellcas 
nicara-
gllellsis 
200.12 
8.2* 
3.0 
2.0 
180.5 
6.0 
0.5 
4.0* 
404.3 
132.0 
1.8 
3.4 
3.0 
1.1 
M. 
atiallticlIs 
101.0 
6.2 
2.5 
1.4 
140.0 
10.1 
0.4 
5.0* 
266.6 
6.0 
J.I 
6.0 
2.0 
12.0 
habitat is 4.5 mmole/liter. Further 
efforts to investigate ion regulation in 
fishes requires experiments designed to 
study thermodynamic mechanisms. In 
sea water, the gills expend energy to: 
(i) hold back ions from moving too 
fast toward concentration and electro-
potential gradients; (ii) maintain uni-
576 
directional flow; and (iii) regulate each 
ion separately. The concentration of 
calcium in sea water is 10.10 mmolel 
liter, but only a fraction is chemically 
active when this value is corrected to 
infinite dilution and ion association is 
taken into consideration. 
In the fresh water, the serum con-
centration of urea is sustained at ap-
proximately 132 mmole/liter. Elasmo-
branchs require at least this concentra-
tion of urea for normal cardiac function 
(7, 8). The concentration of total ions 
is 404.3 mmole/liter, 83 percent of the 
ion concentration of the serum of ma-
rine elasmobranchs; the ionic strength, 
p., is estimated to be .2790. Under these 
conditions, the active concentrations of 
calcium ions increase to the extent that 
precipitation of calcium phosphate and 
depression of neuromuscular irritability 
can be avoided only if the gill mem-
branes and the kidneys lower the cal-
cium concentration in the body fluids. 
This level appears in the fresh-water 
shark to be approximately 3.0 mmolel 
liter of total and 1.8 mmole/liter of 
ultrafilterable calcium. The most abun-
dant ions in Rio San Juan and other 
bodies of fresh water are Ca++ and 
HCO.--, but the total ion concentration 
is very low (5.1 mmole/liter), com-
pared with 404.3 mmole/liter in serum. 
Here, the gills expend energy obtained 
from intracellular glycolysis to pump 
ions against concentration and electro-
potential gradients. 
The teleost fish that is found in the 
same geographic area as the Nicaraguan 
shark has an entirely different compo-
sition of the serum; p. equals 0.15, total 
ions constitute 266.6 mmole/liter, and 
the total calcium is only 2.5 mmolel 
liter. Uremia is absent (Table 1). The 
blood is less concentrated, more vis-
cous, better aerated, and homeostasis is' 
achieved presumably by dynamic mecha-
nisms using the functions of bone cells 
and bone tissue as a buffer (7). Tele-
osts are ionically independent and 
highly advanced, compared with the 
elasmobranchs; they evolved bone for 
finer regulation of ionic composition of 
the blood. The apatite mineral in bone 
is metabolized by means of a two-way 
process of resorption and formation, 
and constitutes the storage depot of a 
closed cycle or servosystem for utilizing 
hydronium, sodium, calcium, phosphate, 
and other ions. In addition to gills, 
kidneys, and bone, integument func-
tions as an organ of ionic homeostasis. 
It serves as a semipermeable membrane 
in lamprey (which have gills but no 
calcium deposits) and in certain adult 
amphibians (which possess true bone 
but no gills and yet retain urea). In 
elasmobranchs and teleosts, the body 
covering is waterproof, and facilitates 
osmoregulation. Thus, the interactions 
and combinations of organs used by 
vertebrates for water balance and min-
eral homeostasis are numerous and 
highly complex. Physiologic experi-
ments are necessary to measure the 
capacity of the bull sharks for adapta-
tion. A tagging project would be of 
great interest to determine the limits 
of their range, reproductive cycle, spe-
cies specificity, and ascertain whether 
they readapt to marine habitat. 
MARSHALL R. URIST 
Division of Orthopedics, Department 
of Surgery, University of California 
Medical Center, Los Angeles 24 
References and Notes 
1. A. W. C. T. Herre, Science 121, 417 (1955). 
2. M. Boeseman, Ibid. 123, 222 (1956). 
3. H. Smith, Am. I. Physiol. 98, 279 (1931). 
4. M. R. Urist, Endocrinology 69, 778 (1961). 
5. I am indebted to: Dr. Thomas R. Howell, 
Marc Buchanan, and John Zoeger of the de-
partment of zoology at UCLA, who assisted 
and made the arrangements in Nicaragua; Dr. 
J. Antonio Mora, Ing., Escuela Nacional de 
Agricultura y Ganaderia, who selected the area 
of exploration; the Nicaraguan Air Force, 
which transported personnel and equipment; 
Major Alvaro Pallais S., Commandant at San 
Carlos, who allocated a jeep and boats; Cap-
tain Humberto Suarez A., of the Guardia Na-
cional de Nicaragua, who gave invaluable time 
and energy and assisted in the surgery; Mar-
shall McLean Urist of Cate School who served 
as laboratory technician and processsed the 
specimens; Dr. Eugenie Clark, Cape Haze 
Marine Laboratory, Placida, Florida, who pro-
vided 'serum and vertebrae of C. leucas, col-
lected in the Atlantic Ocean; Robert Dorion, 
Guatemala City, Guatemala, who generously 
loaned his private library on the Elasmo-
branchii; Drs. Stewart Springer (Duke Univer-
sity Marine Laboratory), T. B. Thorson (Uni-
versity of Nebraska), and Malcolm Gordon, 
who contributed valuable suggestions. 
6. A. B. Bigelow, W. C. Schroder, S. Springer, 
Bull. Mus. Compo Zool. 109, 213 (1953). 
7. M. R. Urist, Perspectives Bioi. Med. (in press). 
8. W. W. Simpson and E. Ogden, I. Exptl. Bioi. 
9, 1 (1932). 
9. These investigations were supported by con-
tract NR 104-575 between the Office of Naval 
Research and the University of California 
Medical Center, Los Angeles, and in part by 
grants from the Squibb Institute for Medical 
Research. 
10 August 1962 


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Calcium and Other Ions in Blood and Skeleton of Nicaraguan Fresh-Water Shark

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