POST LAR\AL TUNA FROM CENTRAL AMERICA 



123 



Figure 1. — Ncolhunnus macropterus, 10.5 millimeters long. 



2b. First dorsal continuous or almost continuous with 



second dorsal. 



3a. Pigment spot on point of isthmus. First 



dorsal 14 to 16, heavily pigmented. Vertebral 



count usually 20+ 17 •= 37 -Euthynnu^ lineatus. 



3b. No pigment spot on isthmus. 



4a. First dorsal 13 or 14, entire fin heavily 

 pigmented. Vertebral count 18+21'=39 



Neoihiinnus macropterus. 



4b. First dorsal 16, bearing a few moderately large 

 spots distally. Vertebral count 20 + 21=41 

 Katsuwonus pelamis. 



NEOTHUNNUS MACROPTERUS (Temminck 

 and Schlegel) 



A total of 42 specimens of this species was taken 

 in the collection, ranging from 10.5 mm. to 26.5 

 mm. in length. Representative specimens were 

 cleared with potassium hydroxide and stained 

 with alizarin (Hollister 1934) so that the bone 

 structure could be examined and the fin rays 

 counted. Fin- ray counts in very small specimens 

 are virtually impossible if the specimens are not 

 stained. 



NeothunnxLs macropterus can be identified by its 

 characteristic shape, vertebral count (18 + 21), 

 and coloration, as described by Schaefer and Alarr 

 (1948b). No gill rakers can be seen in fish smaller 

 than 15 mm. The position and extent of the 

 visceral organs cannot be determined without 

 sectioning. Schaefer and Marr (1948b) note the 

 characteristics of the viscera and gill rakers in 

 specimens over 15 mm. With the exception of 

 the pectoral, the fins of a 10.5-mm. fisii have 

 within one or two rays of the complete complement 

 of spines or rays. The number of rays in the 

 pectoral fin increases from 13 in the 10.5 mm. 



specimen to 30 in fish of 30 mm. Each half of the 

 upper and lower jaws bears 11 small, pointed, 

 irregularly spaced teeth. It was found that these 

 young yellowfin can be separated readily from the 

 other specie»s taken, without a special preparation, 

 by the absence of any pigmentation on the point 

 of the isthmus and by the heavily pigmented first 

 dorsal fin. In aU Euthynnus lineatus and Auxis 

 thazard examined there is a pigment spot on the 

 point of the isthmus overlying the junction of the 

 pectoral and pelvic girdles. No post larval Kats% 

 wonus pelamis were available for study, but 

 Milner B. Schaefer of the Pacific Oceanic Fishery 

 Investigations informs me that this spot is not 

 present on a 21-mm. specimen taken off Costa 

 Rica. I have found no reference to this spot in 

 the literatiu-e. This character is most useful for 

 separating very small A^. macropterus and E. 

 lineatus since both have a black dorsal fin and 

 they resemble each other closely in botly shape 

 until they attain a length greater than 15 mm. 



Dermal pigmentation on a 10.5-mm. Neothunnus 

 macropterus is restricted to a thin strip along the 

 first dorsal fin insertion, a patch on the tip of the 

 snout and the heavily pigmented first dorsal fin. 

 Subcutaneous pigmentation occurs over the brain 

 and in the peritoneum overlying the dorsal third 

 of the viscera. In an 11-mm. specimen, the thin 

 strip along the first dorsal insertion extends 

 posteriorly to the base of the third ray of the 

 second dorsal fin; by the 12-mm. stage it lines the 

 upper margin of the body from the operculum to 

 the terminal rays of the second dorsal. Those two 

 specimens show a faint strip along the postcro- 

 ventral margin of the orbit. From this size up to 



