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PACIFIC SCIENCE, Vol. XXI, July 1967 
sible that the muscle ATP content of fast 
swimming fish is greater than that in less 
active forms. Studies on the phosphorus com- 
pounds of fish muscle may produce a strong 
correlation between magnesium and ATP- 
creatine phosphate contents. 
SUMMARY 
1. The major electrolyte constituents of the 
plasma, red muscle, and white muscle of the 
oceanic skipjack, Katsuwonus pelamis, were 
determined. The potassium content and the 
greater mitochondrial density of the red muscle 
suggest that this muscle is utilized for normal 
swimming activity rather than being an energy 
source for the white muscle. 
2. The plasma electrolytes were compared 
with those of other marine fishes. In general, 
the sodium content of the skipjack plasma is 
less than that found in the cyclostomes, the 
skate and the shark, but is slighly greater than 
that found in the majority of other teleosts. 
The plasma potassium is less than that in the 
cyclostomes and elasmobranchs and greater 
than that in other teleosts. The plasma chloride 
content of the skipjack, as well as the calcium 
and magnesium, is less than that of the other 
investigated species. 
3. Comparison of the differences in the elec- 
trolyte composition of the red and white mus- 
cles reveals that the white tissue contains a 
larger amount of water, potassium, and mag- 
nesium. However, the red muscle contains a 
greater amount of sodium. 
4. Using C 14 -inulin, the extracellular space 
of the red and white muscles was determined 
to be approximately 0.24 1/kg muscle and 0.19 
1/kg muscle, respectively. 
5. The muscle electrolyte content of K. 
pelamis was contrasted with the muscle con- 
tents of other teleosts. The order of decreasing 
composition is as follows. For Na+: mack- 
erel > herring > grouper > skipjack red > 
eel > skipjack white; for K+ : skipjack white > 
herring > eel > mackerel > skipjack red > 
grouper; for Cl - : skipjack red > skipjack 
white > herring > mackerel > grouper > eel. 
Both muscle types of the skipjack contained less 
calcium and more magnesium than did the 
muscle of the eel. 
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