Lungfish are interesting animals, "dependent on their watery environment for food but dying if they can't breathe air with their lungs." It is not unusual for them to be mentioned in accounts of the evolution of fish to land-dwelling tetrapods because they demonstrate an ability to bridge the gap between water and air. (Outside this, they are not good candidates for being transitional).
Water-breathers have a distinctly different biology when compared to air breathers. "All air breathing land dwellers deal with acid - an excess of protons - by reacting protons with bicarbonate ions to create water and CO2, which is breathed out. To deal with too much base, breathing slows down, keeping CO2 and therefore protons in the body. Water breathing fish take a different approach, relying on metabolic processes at their gills and kidneys to restore a normal blood pH." So it is an interesting research question - how do lungfish deal with the problems?
"The lungfish have the best of both worlds" says Gilmour, one of the co-authors. Blackburn's summary is: "Like land dwellers, they rely more on air breathing to redress a more acidic blood pH, but their gills and kidneys deal with an excess of base to return blood pH back to normal, just like water breathing fish."
The research findings are interesting for students of origins. Lungfish do not demonstrate a transitional physiological system, but employ two developed systems side-by-side. They have an air-breathing system for controlling acidity (respiratory compensation) and they use their gills and kidneys to reduce excess base (metabolic compensation). In other words, the ability to operate in both watery and land environments requires two complex systems to be in place: one for living in water and the other for living in air. The case of lungfish shows that biological information precedes and permits biological function.
Mechanisms of acid-base regulation in the African lungfish Protopterus annectens
K. M. Gilmour, R. M. Euverman, A. J. Esbaugh, L. Kenney, S. F. Chew, Y. K. Ip and S. F. Perry
Journal of Experimental Biology 210, 1944-1959 (2007) | doi: 10.1242/jeb.02776
Abstract: African lungfish Protopterus annectens utilized both respiratory and metabolic compensation to restore arterial pH to control levels following the imposition of a metabolic acidosis or alkalosis. [. . .] These findings suggest that lungfish, like tetrapods, alter ventilation to compensate for metabolic acid-base disturbances, a mechanism that is not employed by water-breathing fish. Like fish and amphibians, however, extra-renal routes play a key role in metabolic compensation
Blackburn, L., Lungfishes' balancing act, Journal of Experimental Biology 210, 0ii (2007), doi: 10.1242/jeb.007419
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