Gas Secretion and Absorption
Fish with swimbladders are divided into two categories defined by which type of swimbladder is present. A fish with an open swimbladder is called a physostome while others with closed swimbladders are called physoclists.
The open swimbladder of physostome fish is the ancestral version of the swimbladder and it still retains a somewhat similar function to that of the lungs (Pough 1999). Gases for the filling of this bladder are retrieved from just above the surface of the water. In order for the bladder to hold and release these gases, there is a channel that connects the bladder to the esophagus called the pneumatic duct. This connecting duct is present in most larval fishes while their swimbladder completes its development, and only some-- the physostomes-- retain this duct through maturity (Jones 1957). Figure 2 illustrates the layout of a typical open swimbladder.
Through the pneumatic duct, fish can gulp and burp atmospheric gases from just above the surface of the water into their bladder in order to maintain their buoyancy. For this reason, open swimbladders are usually found in fish that live near the surface of the water. This is so that they may easily uptake atmospheric gases for the filling of the bladder. It would be illogical for deeper dwelling fish to have open swimbladders because of the nearly impossible task of intaking the tremendous volumes of gas that would have to be gulped at the surface in order to achieve neutral buoyancy at such great depths (Schmidt-Nielson 1997).
Therefore, open-bladdered fish (i.e. carp and trout) are mainly found in the shallower depths of freshwater, where few live below ten meters. The only common exceptions are the eels and the herring-like fishes that bear open swimbladders, but can live at greater depths. It is hypothesized that the closed swimbladder was evolved to allow for the filling of niches at greater depths as a result of the movement of primitive fishes from freshwater to marine water (Marshall 1966).
Physoclist fish, on the other hand, have a swimbladder that is completely closed off from any external sources of air (Figure 3). The pneumatic duct is only present in early larval stages of these fish (Marshall 1966). The gases essential for maintaining buoyancy in physoclists are retrieved from the blood, instead of from the atmosphere as in physostomes (Schmidt-Nielsen 1997). To aid in this method of gas exchange, new parts of the swimbladder are introduced: gas gland and rete mirabile (explained further in Gas secretion and retention).
|With the evolution of the closed swimbladder came an independence from the water's surface, which has allowed fish to venture into deeper waters than those fish with open bladders. Some deep water fishes may even have a swimbladder that is reduced size or even absent because they have little or no need for its hydrostatic properties (Jones 1957).|
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