3 z6 DISCOVERY REPORTS 



must presumably make continual compensating movements. Now the setting of the pectoral fins in 

 alepisauroids gives the strong impression that one of their functions is to produce an upward lift 

 during forward motion. Moreover, during the swinging of the tail, the combined caudal and anal 

 fins, which are set close together, may act rather like a heterocercal caudal fin also giving an upward 

 lift. If this is so, the alepisauroids have certain dynamic parallels with the sharks. 



Table 4. Pectoral fin 'angles' in some iniomous fishes. 

 Alepisauroidea 



Yet this is to regard the pectorals merely as relatively passive hydroplanes. A study of their structure 

 soon shows that the pectorals of any alepisauroid must have greater mobility than those of a shark. 

 No observations on alepisauroids are available, but Breder and Krumholz (1943) have analysed the 

 functions of the pectoral fins in Anchoa mitchilli, which like most clupeids and alepisauroids has low 

 set pectorals, each with an axis making less than 45 with the horizontal axis. (In A. mitchilli this 

 angle is about 30 ). In spite of a well-developed swim-bladder, an adult A. mitchilli is evidently a 

 little heavier than the surrounding water. During swimming there is apparently a slight tendency for 

 the fish to roll, this being counteracted by the constant, irregular beating-down movement of the 

 pectorals. Between these movements, the pectorals also move together in simultaneous down-beats 

 so as to raise the head for swimming. Perhaps the pectorals of the Alepisauroidea have rather 

 similar functions. At all events it is clear that pectorals, with the axis nearer to the horizontal than 

 to the vertical, may act not only as hydroplanes but also as roll-compensating and elevating 

 devices. 



But there is another aspect to these considerations. While the evolution of a swim-bladder made 

 possible the evolution of pectoral fins as brakes, the inference that all teleosts with well-developed 

 swim-bladders should have pectorals with this function is only partly true. The remarks above on 

 A. mitchelli show this very clearly. When discussing locomotion in the Isospondyli, Breder (1926) 

 remarked that their pectoral fins ' . . . while capable of being folded back do not have the flexibility to 

 be seen in many of the higher teleosts and are not often capable of being used effectively to back water. 

 Their use is more nearly like that of the shark's pectorals. . . '. 



