FISHERY BULLETIN; VOL. 72. NO. 2 



posteriormost pterygiophore is directlj' opposite 

 the last or 30th interneural space that is occupied 

 by the pterygiophore of the last dorsal finlet. 

 Thunnus spp. and T. atlanticus deviate from this 

 pattern. In TS'^ oimy AOThunnus spp. specimens, 

 only the first four interhaemal spaces were 

 occupied by more than one pterygiophore, the 5th 

 through 7th spaces had only one. Thunnus 

 atlanticus differed by having only six interhaemal 

 spaces available for the anal fin. Thus, 61% of 

 a total of 89 fish had their first five spaces 

 occupied with more than one pterygiophore and 

 the 6th space with one. 



BONES AND RAKERS OF THE FIRST 

 GILL ARCH 



(Figures 11, 12) 



In Thunnus the gillrakers develop within the 

 epithelium that overlies the three bones of the 

 first gill arch and their connective cartilage. In 

 its first stage of development, the raker can be 

 observed on cleared and stained preparations, 

 under 100 x power, as a tiny speck of weakly 

 stained material within the translucent epithe- 

 lium. As the speck grows it gradually assumes 

 the triangular shape of a raker. The tip of the 

 raker will finally break through the epithelium, 

 and its broad base remain anchored in the tissue 

 close to the bone. Ankylosed rakers were not 

 observed on the bones of the first gill arch, even 

 in the larger specimens. As the epithelium is 

 opaque in preserved specimens, gillraker counts 



for smaller juveniles should be made only on 

 cleared and stained material for accuracy. Mead 

 (1951) noted that no gillrakers could be seen in 

 fish smaller than 15 mm. I believe that he 

 referred to untreated specimens, because in my 

 collection all species of Thunnus had six or 

 seven rakers at their smallest size (8 mm SL). 



The ceratobranchial bone of all species has six 

 to seven rakers at 8 mm SL. Development and 

 acquisition of rakers over the ceratobranchial 

 proceeds distally from the angle towards the 

 hypobranchial bone. The raker in the angle is 

 always included in the ceratobranchial count. It 

 develops at about 8 mm SL. 



At 14 mm SL some specimens of all the species 

 develop a raker on the epibranchial bone next 

 to the angle. One exception was noted: an 11-mm- 

 SL T. atlanticus with one raker on the epi- 

 branchial. Additional rakers over the epibran- 

 chial bone develop distally from the angle. 



The last bone of the gill arch to acquire rakers 

 is the hypobranchial. The size of the juveniles 

 when this occurs depends on the size at which the 

 ceratobranchial becomes entirely occupied with 

 rakers. In the hypobranchial count I have in- 

 cluded the rakers (usually one, sometimes two) 

 found over the cerato-hypobranchial cartilage. 



The first rakers to appear are usually located 

 over the cartilage but are considered hypo- 

 branchial rakers. Occasionally rakers appear over 

 the hypobranchial bone leaving the cartilage 

 empty. Thunnus atlanticus has the lowest adult 

 gillraker counts and completes its entire cerato- 

 branchial complement at the smallest size (9-15 



CARTILAGE 



EPIBRANCHIAL 



CARTILAGE 

 HYPOBRANCHIAL 



r alalunga 

 (26.0 mm SL) 



T. atlanticus 

 (27.1mm SL) 



T. thynnus 

 (25.6 mm SL) 



Figure 11. — First right gill arches of juveniles of three Thunnus species. 



578 



