IX 



THE ADAPTIVE RADIATION OF BONY FISHES 



The variety of Actinopterygii is so great that it would be impossible 

 to try to give a complete idea of it and the best that we can do is to 

 consider various functions in more detail and specify some of the ways 

 in which the animals have become specially modified. 



1 . Swimming and locomotion 



The teleosts have perfected in various ways the process of swim- 

 ming by the propagation of waves of contraction along the body. 

 The situation is different from that of elasmobranchs on account of 

 the presence of the air-bladder, serving to maintain the fish steadily 

 at any given level in the water. The stabilization of the animal during 

 locomotion has therefore become a wholly different problem, and the 

 fins are correspondingly changed. In the sharks the pectoral fins serve 

 to correct a continual tendency to forward pitching and by adjustment 

 of their position they are used to steer the animal upwards or down- 

 wards in the water. 



With an air-bladder the fishes have become freed from the tendency 

 to remain at the bottom, which was prevalent in the more primitive 

 forms and is still so common in sharks that it has several times pro- 

 duced wholly bottom-living ray-like types. A fish with an air-bladder 

 needs only very little fin movement to maintain it at a constant depth 

 or to change its depth. As Harris puts it, 'the elaborate mechanism 

 of pectoral "aerofoils" and a lifting heterocercal tail is no longer needed 

 for the maintenance of a constant horizontal cruising plane. Con- 

 comitant with the loss of the heterocercal tail in evolution occurs a 

 rapid and tremendous adaptive radiation of the pectoral fin in form 

 and function.' A stage in this process seems to have been the use of 

 the paired fins to produce oscillating movements during hovering, 

 and this is still found in Amia and Lepisosteus, fishes that remain 

 relatively slow and clumsy. 



Many of the lower teleosteans are relatively poor swimmers and 

 some of them, like so many elasmobranchs, have become bottom- 

 living. Thus in the catfishes there is a large anal fin, acting, like a 

 heterocercal tail, to give lift and negative pitch. The pectoral fins are 

 used to balance this tendency, very much as in sharks. 



