The energy needed to sustain life depends on the reduction of oxygen during glycolysis, thereby producing ATP, water, and carbon dioxide. As multicellular organisms began to evolve and grow in size, the ability of the inner-most cells to receive enough oxygen to carry out cellular respiration was compromised. The absorption of oxygen through the outer cellular layers, called cutaneous respiration, evolved to become an important method for obtaining enough oxygen to sustain the evolution of larger organisms (Farmer, 1997).
Ancient fishes depended on cutaneous respiration to survive in oxygen-poor aquatic habitats, such as rivers, swamps, and tidal pools (reviewed by Farmer, 1997; Taylor, Leite, Mckenzie, and Wang, 2010). Cutaneous respiration was sufficient as long as these fish remained small in size, but the need to avoid predation would have increased the evolutionary pressure to grow larger. The combination of size growth and hypoxic conditions are believed to have contributed to the development of first gills and then lungs, with the latter permitting terrestrial habitation. To better understand this evolutionary process, this essay will examine the anatomical evolution of respiration in fishes and frogs.
Respiratory Anatomy and Function in Fish
The gills of fish, located anterior, are specialized organs that allow efficient gas exchange between dissolved oxygen in the water and the oxygen-depleted blood (Farmer, 1997). The circulatory system carries the oxygenated blood throughout the fish's body and is returned through a circulatory system not unlike that of mammals. The heart is located just posterior and upstream of the gills and pumps oxygen-depleted blood into the gills. The heart is therefore exposed primarily to the equivalent of human venous or oxygen-depleted blood.
There are several species of fish that have lungs, for example the genus Lepisosteus (Florida gar) (Farmer, 1997). The existence of lungs allow these fish to breath the air in addition to the oxygen that can be obtained from gas exchange at the gills. Anatomically, the lungs complement the oxygen content of the circulatory system and provide direct support of the myocardial tissue by supplying freshly oxygenated blood.
Farmer (1997) argues that lungs may have developed specifically to improve the flow of oxygenated blood to the cardiac tissue, thereby improving cardiac function. This would have given air-breathing fish an evolutionary advantage by being able to survive extreme exertion during escape from predators. In support of this argument, Farmer points out that gill-dependent fish like trout will often die after intense physical activity, whereas the gar will not.
Respiratory Anatomy and Function in Frogs
As frogs grow from tadpoles to adults they live a double life, first as an aquatic creature dependent on gills and cutaneous gas transfer for respiration and then as semi-terrestrial tetrapods primarily dependent on their lungs for gas exchange (Gargaglioni and Milsom, 2007; Taylor, Leite, Mckenzie, and Wang, 2010). Throughout the lifespan of frogs though, their skin continues to function as an important gas exchanger, especially for eliminating carbon dioxide. During the tadpole stage of development, the skin accounts for up to 60% of the gases exchanged with the aquatic environment. As adults, cutaneous respiration continues to function but is believed to be most efficient when submersed in water (Janis and Keller, 2001). Frogs therefore have three functional respiratory systems at some point in their life cycle and they are cutaneous, gills, and lungs.
The larval respiratory system creates a constant flow of water across the gill membranes through the orchestrated contractions of the buccal (analogous to human cheeks) and pharyngeal chambers (Gargaglioni and Milsom, 2007). As the buccal chamber expands, this draws water in through the mouth and nares (nostrils). Near the end of the buccal expansion phase the pharyngeal muscles constrict to maintain pressure within the oral cavity. As the buccal chamber begins to contract, the mouth and nares close and the pharyngeal chamber opens. This forces the water to exit over the gills. The entire cyclical process is controlled by the brain stem.
Anatomically, the adult frog respiratory system resembles that of mammals, with a trachea connected to bilateral lungs, which are in turn directly upstream of the heart (Gargaglioni and Milsom, 2007). The control of ventilation is regulated by the central nervous system and depends on the muscular control of the nostrils (nares), trachea (glottis), buccal chamber, and lungs. During buccal driven ventilation, the buccal chamber expands and contracts without the nares participating. This mode of ventilation does nothing more than circulate the air within the buccal chamber and the adjacent oropharynx. The oxygen concentration in the lungs is minimally affected. The other three forms of ventilation, balanced, inflation, and deflation, depend on the expansion and contraction of the lung cavity. Balanced breathing is inhaling and exhaling the same volume, while inflation and deflation breathing involves a series of inhalations or deflations in the absence of the opposite flow direction.
Since frogs typically have a comparatively low metabolic rate, buccal ventilation is generally continuous and lung ventilation sporadic (Gargaglioni and Milsom, 2007). The same muscles control both and for this reason, buccal ventilation does not occur at the same time the lungs ventilate (Taylor, Leite, Mckenzie, and Wang, 2010). By comparison, mammals depend on the expansion and contraction of the rib musculature (costal ventilation) to ventilate the lungs (Janis and Keller, 2001). The emergence of costal ventilation is believed to have been essential for sustaining higher levels of physical activity, which would have been otherwise limited by the buildup of acidosis. This in turn would have eliminated the need for cutaneous respiration and allowed the emergence of a dry skin suited for habitats without large supplies of water.
Lungfish and Other Descendants
Lungfish have often been viewed as representing an evolutionary intermediate between gill fish and amphibians (Farmer, 1999). Meyer and Wilson (1990) discovered that the mitochondrial DNA for lungfish is more closely related to frogs than a ray-finned fish. However, as Farmer (1999) discussed, the overwhelming anatomic and morphological similarity between fish and lungfish convinced scientist they were fish with lungs, rather than amphibians with gills. The findings by Meyer and Wilson suggest classifying lungfish as fish may have been hasty.
Lungfish are important to the debate about the evolution of respiratory systems because lungs have been viewed as an adaptation permitting terrestrial habitation (Farmer, 1999). The force behind this evolution has been argued by many to be the hypoxic conditions often occurring in freshwater environments. By comparison, tidal pools and other marine habitats are considered well oxygenated due to tidal actions.
A recent study examined the habitat habits of coral-dwelling fishes to better understand the role hypoxia plays (Nilsson, Hobbs, Ostlund-Nilsson, and Munday, 2007). Of the nine species they examined, all revealed a high tolerance for hypoxic conditions, down to 15% to 15% of ambient air concentrations. This would prove advantageous in a coral environment when oxygen levels drop precipitously at night. Being able to withstand hypoxic conditions would allow these fish to remain protected inside the coral during the night. The air-breathing ability of the nine species varied considerably, from being able to breathe normally out of water for over four hours to experiencing respiratory distress almost immediately. Further experiments revealed that the air-breathing fish relied heavily on cutaneous respiration for gas exchange and subsequent sectioning of the skin tissue revealed a rich bed of capillaries immediately below the surface. By contrast, the non-air-breathers tended to have a thick scaly skin.
Nilsson and colleagues (2007) interpreted their findings as being consistent with the evolution of respiratory systems that allow coral-dwelling fish to hide in the coral at deep hypoxic depths at night or remain inside coral in the shallows at low tide. The primary purpose of these abilities would be to evade predation. These results are consistent with Farmer's (1999) hypothesis that hypoxia may not be the only evolutionary force driving the emergence of lungs.
Perry and colleagues (2001) reviewed the fossil record in light of contemporary examples of gills and lungs and came to the conclusion that the first evolutionary step away from gills was gill-derived air sacs. These air sacs would have had a respiratory and buoyancy function. The dorsal and ventral pharyngeal pouches 7-8 are believed to be the tissues from which air sacs and lungs, respectively, were derived. The airs sacs would have allowed fish to linger at the surface of hypoxic water for ventilation purposes and provide a source of air for long stays below the surface to avoid predation from above.
The gradual evolution of the respiratory system in fish to that of frogs represents an adaptive process that allowed terrestrial habitation in wet regions. This in turn allowed the evolution of costal ventilation (Janis and Keller, 2001) and surfactants (Perry Steven F., Wilson, Richard J.A., Straus, Christian, Harris, Michael B., and Remmers, 2001) that permitted habitation of dryer environments. Most of the scientists cited here agree that the evolutionary process was not linear, but filled with dead ends and parallel emergences (Farmer, 1997). However, all the evidence clearly suggests…
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