BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE ANIMAL SKELETAL AND MUSCULAR SYSTEMS

18.6. Locomotion in Vertebrates

18.6.3. Locomotion in Bony Fish (with the Herring as an Example)

The External Structure of a herring is shown in Fig. 2.66. Bony fish possess an organ known as the swim bladder, which is a sac located between THE Vertebral Column and the gut; it provides the fish with what is termed "neutral buoyancy." This ensures that the average density of the body equals that of the surrounding Water, eliminating The Need for the fish to expend energy merely to maintain its position in the water column.

The evolutionary Development of the swim bladder freed the paired fins from any supportive function. Consequently, they are much smaller than those of sharks and serve primarily for stability or braking—in the latter case, they are spread out vertically at a 90° angle to the body. The two pectoral fins can operate independently of one another, enabling the fish to execute sharp turns pivoting around either fin as an axis of rotation. When swimming in a straight line, the paired fins are held flat against the sides of the body, enhancing streamlining. Because the caudal fin is symmetrical, its propulsive force drives the fish directly forward. The unpaired dorsal and anal fins prevent lateral rolling of the body and help stabilize its anterior portion during tail movements.

Swim bladders are of two distinct types.

1. Physostomous swim bladder (e.g., in the goldfish and herring). It is connected to the Pharynx by a pneumatic duct, allowing air to enter or leave the bladder via the Mouth. This mechanism decreases or increases the average density of the body accordingly.

2. Physoclistous swim bladder, found, for instance, in the cod. Such a bladder has entirely lost its connection to the pharynx. The fish is able to match its body density to that of the surrounding water and maintain neutral buoyancy by automatically regulating the volume of gas within the bladder. Gas content is controlled by means of the red body (rete mirabile)—a dense network of Blood Vessels situated on the inner wall of the bladder.

18.5. List the adaptations that enable fish to achieve efficient aquatic locomotion.