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UNDERWATER GUIDE TO MARINE LIFE 



SPINX DORSAL FIN 

 LATERAL LINE . | SOFT DORSAL FIN 



KEEL SCUTES 



OPERCULUM \y 



y PECTORAL FIN 



t 

 PELVIC, (ventral) fin 



THORACIC POSITION 



r 



CAUDAL (tail) fin 



PELVIC (VfNTRAL) FIN 

 ABDOMINAL POSITION 



Fig. 83. External anatomy of hony fishes. 



through their medium, air, have been well studied, but fish movement is less 

 understood, though they, too, fly, in a sense, through water. Bony fishes are 

 more efficient swimmers than sharks and rays because of the combination of 

 three factors: (1) the shortened, compact body allowing short, efficient, lateral 

 strokes of the tail (a tendency seen in the mackerel sharks), (2) the possession 

 of an internal hydrostatic organ, the swim bladder, which adjusts the specific 

 gravity of the fish so that stabilization in any depth of water is possible (sharks, 

 as we pointed out earlier, cannot do this), and (3) the great mobility of the 

 fins, resulting in increased maneuverability, and the ability to start and stop 

 suddenly (sharks and rays cannot do this). Fish movement is so intimately 

 related to the habits of fishes and is so useful in identification of groups that 

 we will give it considerable attention in our review of the families. 



Breder (1926) has set up three main categories of fish movement: 



1. Carangiform (like the jack, family Carangidae). The typical fish movement 

 in which propulsion is accomplished by short lateral strokes of the tail, and 

 steering is accomplished largely by the paired fins. 



2. Anguilliform (like the eel, family Anguillidae). The long-bodied fishes 

 such as eels progress by a long series of sine waves which pass down the 

 body. (Most sharks, being long-bodied, have a modified and shortened 

 movement of this type.) 



3. Ostraciform (like the trunkfishes, family Ostraciidae). Propulsion becomes 

 largely a task of the pectoral, dorsal, and anal fins. The tail is used mainly 

 for steering. 



Within these very broad categories many variations occur, and these shall 

 be mentioned as they are met, particularly as departure from the typical 



