no. 3647 PERCIFORM FISHES — GOSLINE 13 



contact with the bottom (at least during the day). Though the defen- 

 sive armature of percoids is less extensive than that of most living 

 berycoids, the percoids seem to have provided the pelvic spines with a 

 firmer base in the development of a direct pelvic-cleithral attachment. 

 In bringing the pelvics forward under the pectorals, the percoids also 

 seem to have increased their ability to maneuver. Harris (1938) 

 showed that acanthopteran pectorals are so constructed as to give an 

 upward thrust to the front of the fish when erected for the purpose of 

 turning or stopping and that erection of the pelvics at the same time 

 offsets this. In this respect, the pelvics seem to counteract the pectorals 

 more efficiently if they are directly below the pectorals rather than 

 behind them, as they are in lower fishes and still, to some extent, in 

 most Beryciformes. 



For the paired fins to be effective in stopping or turning, a forward 

 speed ("headway") must have been generated previously. This is usu- 

 ally developed by the vertical fins and the body. Among the lower 

 percoids, the forked caudal fin, a basal teleostean feature, plays a 

 large role. Gero (1952) has shown that, for a swimming fish, a forked 

 tail shape is the most efficient. From this basal type, found in such a 

 lower percoid as Roccus (=Morone), two divergent lines of develop- 

 ment have occurred. One is carried to its extreme in the Scombridae. 

 Here, the widely forked fin has a short, high, relatively stiff blade 

 firmly attached to the caudal skeleton at the end of a slender caudal 

 peduncle. This type provides great power and speed, but it has its 

 limitations. Harris (1953, pp. 26, 27) stated: "Tails of this type are 

 found in fishes which are fast continuous swimmers (scombroids) ; if 

 a sudden burst of speed from a standing start is required, the angle of 

 attack of this type of tail would be too high and the tail would 'stall'." 

 At the opposite extreme is the rounded caudal that has been developed 

 again and again in percoids and their derivatives. Such a caudal shape 

 not only provides a better "getaway" mechanism but seems to be a 

 more efficient (or perhaps accurate) propulsive force at slow speeds 

 and in enclosed areas. 



Aside from caudal shape, there are other factors that affect the 

 forward locomotion of the percoids and their derivatives (fig. 3). 

 Thus, when a fish becomes either very deep-bodied or very elongate, 

 the potentiality for rapid locomotion seems to be lost. At both ex- 

 tremes, the importance of the caudal fin as a source of forward thrust 

 diminishes. Such a deep-bodied form as Chaetodon has a relatively 

 long posterior border to the body, covered by the soft dorsal and anal 

 fins, and a short, brushlike tail. In moving forward, it flaps the whole 

 rear portion of the body, of which the tail is only an insignificant part. 

 The end point in such a line of development is of course the tetraodon- 

 tiform Mola, which has no caudal fin at all. 



