Air Breathing Fishes Evolutionary Adaptations

Air breathing fishes have developed diverse air-breathing organs (ABOs) for drawing oxygen from the surface of the water but maintain gills, pumped by cranial muscles, for a variable fraction of their oxygen needs, depending on species and conditions, and for excretion of most of their carbon dioxide. Many facultative air breathers, such as the tarpon (Megalops cyprinoides), rely on gill ventilation and restrict blood flow to the ABO in normoxic water but increase rates of air breathing and perfusion of the ABO with hypoxic exposure and particularly during exercise.

During these periods, opercular beating becomes imperceptible, indicating cessation of effective gill breathing or a switch to ram ventilation while swimming. Access to air reduces the lactic acid load during burst swimming and prolongs aerobic exercise in tarpon; however, they are able to repay an accumulated oxygen debt during recovery by increased rates of gill ventilation.

In all air-breathing fish, gulping of air at the water surface is achieved through the action of the same muscles as used for feeding or for forced ventilation in water-breathing fishes. These are elements of the jaw musculature, innervated by cranial nerve V, together with the hypobranchial musculature, innervated by occipital and anterior spinal nerves. They work together in a coordinated action either for feeding or for gulping air, actions that are independent of the visceral arches and may derive from their separate evolutionary origins as feeding muscles.

In the primitive ray-finned (actinopterygian) fish, the bowfin (Amia calva), which uses a well-vascularized swim bladder as an ABO, there seem to be two types of air breath, one that involves exhalation followed by inhalation (designated type I air breaths) and one that simply involves inhalation (type II air breaths). Type I breaths are possibly respiratory in nature, whereas type II breaths have a buoyancy-regulating function.

Spectral analysis indicates an inherent rhythmicity to type I (i.e., respiratory-related) air breathing, both in normoxia and hypoxia that may be driven by the changes in blood O₂ status during the interbreath interval, rather than by an RRG for air breathing. Control of the switch between ventilation of the gills and the ABO is likely to relate to stimulation of chemoreceptors by reduced oxygen levels at the gills or in the ABO. However, the central sites responsible for control of air-breathing reflexes in fish are still unknown.

CNS associated with the evolution of air breathing fishes

Reorganization of the CNS associated with the evolution of air breathing has been poorly studied in fish. Air-breathing fish belonging to the class Actinopterygian have, likely, an RRG for gill ventilation which is located in the RF of the hindbrain as in water-breathing fish. In the bowfin, catecholamine infusion stimulates gill ventilation apparently via a central mechanism but has no effect on air breathing either at normoxia or hypoxia, suggesting that the central sites governing gill ventilation and air breathing are pharmacologically and possibly spatially distinct.

However, the events sequence associated with air breathing in the bowfin suggests that the action of air breathing would require little change in the pattern of neural control required for suction feeding and/or coughing. The only exception is the control over opening of the glottis at the entrance to the ABO. Neural tracers showed that the ABO in the bowfin is innervated by vagal motor neurons that may supply the glottis or smooth muscle in the walls of the ABO.

An isolated brainstem preparation from the long-nosed gar (Lepisosteus osseus) showed two distinct motor patterns in fictive respiratory activity recorded from the root of the Vth cranial nerve, a high-frequency, low-amplitude pattern associated with gill breathing and a low-frequency, high-amplitude pattern associated with ventilation of the lung during air breathing. This latter activity was associated with phasic activation of the Xth cranial nerve. The differing motor patterns and recruitment of vagal motoneurons suggest the presence of two oscillators.

There may be distinct derivations for air-pumping mechanisms in actinopterygian fishes and the lobe-finned (sarcopterygian) lungfish along with their evolutionary cousins, the amphibians. However, both employ the same sets of muscles to pump air into the lung, and hence may share the same central oscillators.

African lungfish (Protopterus aethiopicus) may have two distinct central rhythm generators, one for gill ventilation and the other for air breathing. The pattern of airflow in the breathing cycles of lungfish and amphibians, such as bullfrogs, is essentially similar. In amphibians, there is evidence that evolution of air-breathing rhythms may have required a new motor pattern in the CNS rather than one evolved from progressive modification of the branchial rhythm generator.

Episodic breathing rhythms were recorded from an isolated brainstem of a bullfrog, implying that they are generated centrally in the absence of patterned inflow from receptors. Microinjection of glutamate into rostral areas of the bullfrog-isolated brainstem in an area of the RF that corresponds to that identified as responsible for respiratory rhythmogenesis in fetal mammals caused brief breathing episodes. It seems that the neural networks associated with respiratory rhythmogenesis have been well conserved during vertebrate evolution.

Summary

Respiratory rhythmogenesis resides in the brainstem of all vertebrates, including fish, and the isolated medulla oblongata can generate a respiratory rhythm. However, the site of the primary generator is poorly defined, and its activity is modulated by centers in the midbrain and stabilized by peripheral inputs from mechanoreceptors on the gills. The respiratory rhythm in fish may become episodic or cease completely during ram ventilation.

When oxygen demand is high, feeding muscles can be recruited into the respiratory cycle to provide active inspiration of water during the mouth-opening phase. Air-breathing fish utilize the same muscles to gulp air at the water surface. There is some evidence of separate RRGs for water and air breathing, but their properties and locations are unclear.

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