Some Inorganic Constituents of the Muscles and Blood of the 
Oceanic Skipjack, Katsuwonus pelamis 1 
Bryant T. Sather 2 and Terence A. Rogers 
Cellular metabolism, anabolic and catabolic, 
regulates the production of macromolecules 
and the content of intracellular and extracellu- 
lar ions. It is mainly the latter which are con- 
cerned with providing osmotic homeostasis. 
Much literature exists pertaining to the ionic 
composition of various tissues of aquatic and 
marine fish (Vinogradov, 1953). However, as 
noted by Love (1957), many of the values 
should be re-examined because of heterogene- 
ity in sampling which makes comparisons al- 
most impossible. Also, very little is known 
about the inorganic composition of a true 
pelagic form. 
Only recently has it been possible to im- 
pound successfully for a fairly long period a 
fast swimming fish such as the oceanic skip- 
jack, thus greatly enhancing the sampling of 
fresh tissues. Many of the fast-swimming pe- 
lagic fish have two distinct striated muscle 
types, white and red ("chiai”). When the tuna 
is cross-sectioned just anterior to the secondary 
dorsal fin, the two muscle types are readily 
discernible. The less plentiful red muscle pairs 
are located as bundles adjacent to the vertebrae. 
Both the dorsal and ventral pairs have horns 
that course, mediolaterally, toward the lateral 
line. At the skin the terminations of these 
horns are quite narrow. The remainder of the 
musculature is composed of the white muscle 
which approximately surrounds the red muscle 
bundles except at the lateral line. 
Until recently, little was known about the 
physiological function of these two muscle 
types. It was thought that some indication 
might be obtained by determining the major 
1 A contribution from the Pacific Biomedical Re- 
search Center, University of Hawaii. Supported by 
Contract No. 14-17-0006-66 from the Bureau of 
Commercial Fisheries. Manuscript received April 8, 
1966 . 
2 Present address: Department of Zoology and 
Physiology, Rutgers University, Newark, New 
Jersey 07102. 
electrolyte composition of the muscles and of 
the blood. Also, the inorganic components of 
the oceanic skipjack, Katsuwonus pelamis, may 
be very informative to the comparative physi- 
ologist as well as to the nutritionist. 
MATERIALS AND METHODS 
The tuna ("aku”) were caught from a ship 
by the barbless hook-pole fishing method in 
waters adjacent to Oahu, Hawaii. The fish were 
transported to Oahu in large circular tanks with 
a constant supply of circulating sea water. The 
tuna were impounded in tanks similar to, but 
larger than, those aboard ship. The fish were 
fed frozen smelt and beef liver once a day, 
and were sacrificed for sampling 24 hours after 
feeding. 
Cardiac blood was drawn into heparinized 
syringes. After the hematocrit was determined, 
the plasma was prepared. The pH of the plasma 
was determined by using the Beckman Model 
G pH-meter fitted with microelectrodes. The 
plasma’s osmolality was ascertained employing 
the Fiske Osmometer. 
The carcasses were sectioned, after which 
pieces (1. 5-2.0 gm) of the two muscle types 
were removed. After weighing, one-half of the 
samples were dried overnight at 110°C and 
the percentage water content was determined. 
These samples were digested in concentrated 
nitric acid, appropriately diluted, and the so- 
dium and potassium contents determined. The 
remaining samples were homogenized in 10% 
trichloroacetic acid (TCA), centrifuged, and 
the supernatants were drawn off for the cal- 
cium, magnesium, and chloride analyses. 
The sodium and potassium content of the 
plasma and the muscles were determined by 
flame spectrophotometry employing a Coleman 
Jr. spectrophotometer equipped with a propane- 
oxygen burner. The plasma calcium was ascer- 
tained by the Ferro-Ham method (Ferro and 
Ham, 1957^ and 1957&), using a Beckman 
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