A swim bladder, or air bladder, is found in most fishes. It is a derivative of the anterior region of the digestive tube and, if not primarily respiratory in function, is at least found in suspicious intimacy with respiratory organs.
Located dorsally in the body cavity just outside of the peritoneum under the vertebral column (Fig. 330), it is ordinarily a single elongated structure, although it may be bifurcated or paired, as for example in Polypterus and Calamoichthys among ganoids and in the swellfish, Sphaeroides, the curious headfish, Mola, and the sea robin, Prionotus, among teleosts.
Great variation in the shape, form, and size of the swim bladder is connected with its hydrostatic function, as well as with variation in the center of gravity in different shaped fishes.
In amphioxus, cyclostomes, elasmobranchs and a few of the higher fishes, particularly bottom-feeders and deep-sea forms, the swim bladder is absent. The flatfishes (Pleuronectidae) possess a swim bladder only in early life during the period when they maintain a vertical position. As they tip over on one side in the course of development and assume their lazy adult habit of life, the swim bladder degenerates.
Fishes which possess swim bladders in the adult form may be divided into two groups according to the character of the swim bladder, namely, the physostomi, having a pneumatic duct leading from the swim bladder to the alimentary tract; and the physoclisti, having the outlet duct closed or atrophied. To the first group belong the bony ganoids, the dipnoans, and the soft-rayed teleosts. In the latter group are included all the spiny-rayed fishes.
The pickerel, Esox, represents a simple type of physostomous fish having a single sac for a swim bladder, with a pneumatic duct at the anterior end opening into the esophagus (Fig. 331A). In some fishes the single swim bladder is made up of two connecting sacs (Fig. 331B).
In physoclistous fishes, upon the degeneration of the pneumatic duct, the swim bladder becomes a closed sac having two chambers separated by a partition through which there is a sphincter-like opening, regulated in size by both circular and radiating muscles similar in arrangement to those in the iris of the eye. The posterior chamber is formed by an enlargement of the pneumatic duct that is no longer needed for its original function. This is quite apparent in the eel, Anguilla, a physostomous fish on the verge of becoming physoclistous, in which the duct is caught in the very act of enlargement into a separate chamber (Fig. 331c). The formation of the posterior chamber by the enlargement of the pneumatic duct has been clearly indicated by Tracy in a series of diagrams based upon wax reconstructions of serial sections, showing stages in the development ol the swim bladder in the pipefish, Siphostoma (Fig. 332). The manner in which the anterior chamber is produced by the forward growth of the budding swim bladder has also been demonstrated by Tracy in the early stages of the toadfish, Opsanus (Fig. 333), which later develops a typical closed swim bladder, with the same three histological layers of tissue that characterize the alimentary tract from which it was derived, shown in section in Figure 333F.
An important modification in the epithelial lining of the anterior chamber of the swim bladder results in a structure unique among animal tissues, the red gland, which produces free oxygen (O2) by the reduction of oxyhaemoglobin in the red blood corpuscles when brought into close contact with secreting epithelial cells. No other gland is capable of isolating pure molecular oxygen. As Tower has demonstrated, this oxygen constitutes a large part of the gas that distends the swim bladder.
Excess gas produced by the red gland escapes through the pneumatic duct in all physostomous fishes. Since the two-chambered swim bladders of physoclistous fishes have the red gland located in the anterior chamber, the mechanism for the removal of excess gas is of necessity different from that in fishes with a pneumatic duct. The entire posterior chamber is lined with a thin epithelium beneath which is a capillary network, the rete mirabile, through which excess gas generated in the red gland is absorbed directly into the blood. By enlarging the opening in the partition between the chambers, more gas is admitted to the posterior chamber for disposal through the blood, while by restricting it, the gas is retained. In this way the degree of distention of the swim bladder is automatically regulated by the interaction of the red gland and the rete mirabile.
A further modification of the closed swim bladder sometimes appears, for example in the squeteague, Cynoscion (Fig. 334), when the posterior chamber with the rete mirabile becomes flattened almost to obliteration and is designated as the oval.
There are various uses for the swim bladder in fishes. Although primarily respiratory, it has become in most instances a hydrostatic organ, or “float,” for the purpose of maintaining a certain level in water without muscular effort. When its gaseous content is increased, the fish rises to higher levels, or if diminished, sinks deeper in the water. By shifting the volume of gas from one end of the swim bladder to the other through muscular compression, changes in the center of balance in the body also occur, which enable the fish to make a variety of movements easily at the same level.
In some fishes, particularly Siluroids, Cyprinoids, and Gymnoti, anterior prolongations of the swim bladder are present that come into intimate relation with the inner ear either directly or through a chain of bones derived from parts of the first three vertebrae, forming the so-called Weber’s organ. Variations in the distention of the swim bladder are conveyed to the inner ear by means of this device, that probably acts as .a regulatory sense organ either after the fashion of a manometer or a barometer. Whether Weber’s organ aids in any way as an organ of hearing is doubtful.
Another use for the swim bladder is that of respiration, for which reason its description is included in the present chapter. This function applies particularly to lungfishes, whose swim bladder becomes alveolar inside like the lungs of amphibians and the lower reptiles, being usually paired as well as taking on all the essential features of simple lungs. It even derives, after the fashion of true lungs, a supply of venous blood from the last pair of aortic loops, whereas the typical hydrostatic swim bladder receives arterial blood only, and gives off venous blood. The swim bladder of lungfishes, therefore, apparently is a more efficient breathing organ than the primitive lungs of the perennibranchiate urodeles.
A third incidental use of the swim bladder is the production of sound. Drum fishes (Sciaenidae), “grunters” (Haemulonidae), and a few other forms, such as the sea-robin, Prionotus, and the toadfish, Opsanus, are exceptional noise-producers in a modest way among the otherwise mostly silent brotherhood of fishes. According to Tower, who has carefully investigated the matter, the chief source of the drumming noise in drum fishes is the contraction of a “drumming muscle,” musculus sonificus (Fig. 335), which, being superficially attached to the swim bladder, “produces a vibration of the abdominal walls and organs, and especially of the swim bladder.” D. S. Jordan says that the “grunting” of the Haemulonidae is caused “by forcing air from part to part of the complex swim bladder.”