There are four gills and the pseudobranchiae are well 

 developedo The gill filaments are set closely together and 

 their length is roughly equal to the upper limb of the first 

 gill arch. The gill-rakers are well developed^ The gill 

 cleft is extremely largej the gill membranes of the left and 

 right sides are separate from each other and also from the 

 isthmuso 



The heart is generally large in comparison with other 

 fishj, and a large amount of blood and a large number of blood 

 vessels are characteristic of these fishesc 



The kidneys are concentrated in the pectoral regiono 

 This is especially marked in the case of the black tuna and 

 the albacoreo In the other species these organs are slender 

 and extend along the dorsal surface of the body cavityo 



The urinary bladder is generally smallj, but in some 

 species it may be conspicuously developedo 



The gonads are paired and lie on the dorsal surface of 

 the body cavityo The ovaries when ripe are very long and 

 also quite thicko The testes are generally slender and 

 irregular in shape™ 



Co Ecology 



io Outline of habits 



The fishes of this family are generally large and 

 migrate through the open sea forming great schoolso The 

 majority of them are fishes of tropical seas^ but^, depending 

 on the season, when the environment becomes suitable they 

 migrate into the waters of fairly high latitudes,. They 

 generally occur in the deep layers during the daytime, but 

 they characteristically come to the surface at nighto This 

 habit is common to fishes in general and is not limited to 

 the fishes of this family. It is thought that the reason for 

 it is related to the vertical movement of plankton and the 

 corresponding vertical movement of small animal s„ 



These fishes often swim in the surface layer during and 

 directly after the spawning season^, but it is said that at 

 such times they are extraordinarily active and difficult to 

 oatcho It is knowi that they have the characteristic of 

 frequently leaping on the surface of the water, it being 

 said that the big-eyed tuna in particular has the strange 

 habit of leaping at dawno 



Generally they migrate ceaselessly in the open sea„ but 

 at times they become *'shoal fish" and remain for a period in 

 the vicinity of hidden reefs or elevations of the ocean flooro 



11 



