For X24 heres one for you
LUNGS
Paleontological evidence shows that air-breathing appeared some 438-408 million years ago during the late Silurian period (Sundstrom 2002).
Two main theories are trying to explain its evolution.
1) Theory 1 more widely accepted one argues that warm tropical waters were suffering from either periodical or constant low concentrations of dissolved oxygen. Based on observations of modern species, an existing behavioral responce is fish moving to shallower, higher oxygen availabilty water and utilyzing aquatic surface respiration. Such pressure led to the evolution of specialized organs for breathing athmospheric air and retreaving its oxygen contents (Sundstrom 2002).
2) The second theory suggests that fish started gulping air to increase their buoyancy; the result was again an air-breathing system (Sundstrom 2002).
In fact, evidence supports both hypothesis. Studies have shown that the air-breathing fish exhibit various structural, behavioral, physiological characteristics, and differ in ventilatory control and central/periferal receptor interactions. This suggests that air-breathing has evolved independently. Graham (1997) stipulates between 38 and 67 independent occurences of air-breathing.
A study by Perry et al. (2001) on the origin of potentially homologous air-filled organs (lungs and swimbladders) also demonstrates the complexity of figuring out the evolution of air-breathing. The Polypteriformes, lungfish, and tetrapods are characterised by the existence of lungs; in all other bony fish the swimbladder is ancestral. Perry et al.(2001) defines lungs as "paired ventral derivatives of the pharynx posterior to the gills." The unpaired swimbladder in gar fish (Ginglymodi), bowfin (Halecomporphi), and basal teleosts is dorsal and evolved from the posterior pharynx. The authors of "Which came first, the lung or the breath?" (Perry et al., 2001) demonstrate that extreme variation exists even though air ventilatory mechanisms in the ray-finned fish (Actinopterygii) and Polypteriformes are similar among different groups, and are markedly different from those of lungfish and tetrapods. Evidence exists that supports the theory that homologous pharyngeal air-filled organs first appeared in early bony fish, but resulted from behavioral mechanisms for surface (water) breathing. The posterior gill pouches or the gills served by the sixth branchial artery might have been the early air-filled organs. It could have later evolved into the respiratory swimbladder and the lungs.
Wilson et al. (2000) also looked into the difference between the lobe-finned (Sarcopterygii) and the ray-finned (Actinopterygii) fish, both of which have evolved primitive air breathers. The tetrapods, which stem from Sarcopterygii, utilise a chemoreceptor bound central pattern generator. It is subject to control from both central and peripheral CO2/H+ chemoreceptors. On the contrary, Actinopterygii are believed to lack such chemoreceptors and rely instead on "reflexive" behavior. However, using in vitro brainstem from longnose gar fish, Wilson et al. (2000) show that air-breathing patterns occur in brainstem and are subject to changing levels of carbon dioxide. The authors make a case that a central neuronal controller for air-breathing appeared in a common ancestor before the split of lobe-finned and ray-finned fish. This illustrates the difficulty of quantifying the number of independent evolutions of air-breathing suggested above by Graham (1997).Note how the lung on the right runs almost the lenght of the body.
pic from digimorph
The wall of the asymmetrical saclike lungs of the fishes Polypterus and Erpetoichthys consists of several functionally different tissue layers. Their lumen is lined by a surface epithelium composed of (1) highly attenuated cells, termed pneumocytes I; (2) pneumocytes II with lamellar bodies, presumably indicating surfactant production; (3) mucous cells; and (4) ciliated cells. Underlying the pneumocytes I is a dense capillary net. The thin continuous endothelium of this net, together with the pneumocytes I, constitute the very thin blood-air barrier. The basement membrane of epithelium and endothelium fuse in the area of the blood-air barrier (thickness 210 m). Secretory and ciliary cells form longitudinal rows in the epithelium. Below the zone with a gas-exchanging tissue, a layer of connective tissue containing collagen and special elastic fibers occurs. The blood vessels that give rise to or drain the superficial capillary plexus are located in this connective tissue. The outermost layer of the lung consists of muscle cells, a narrow inner zone with smooth muscle cells, and an outer, broader zone with cross-striated muscle cells. The lung is innervated by myelinated and nonmyelinated nerve fibers. The morphology of the gas-exchange tissue in the lungs of these primitive bony fish is fundamentally very similar to that of the lungs of tetrapod vertebrates. The morphologic observations are in close agreement with physiologic data, disclosing well-developed respiratory capacities. Structural simplicity can be regarded as a model from which the lungs of the higher vertebrates derived. In addition to respiratory function, the lungs seem also to have hydrostatic tasks.
A. Lechleuthner 1, U. Schumacher 1, R. D. Negele 2, Dr. U. Welsch 1 *
1Department of Anatomy, University of Munich, Federal Republic of Germany
2Bavarian Institution for Freshwater Research, Wielenbach, Federal Republic of Germany
*Correspondence to U. Welsch, Anatomische Anstalt, Universität München, Pettenkoferstr. 11,8000 München 2, FRG
The respiratory behaviour in Polypterus senegalus. P. senegalus rarely resorts to aerial respiration under normal conditions.The consumption of atmospheric O2 depends on the activity and age of fish and the availability of aquatic oxygen.immature fish cannot utilize aerial O2 but older fish exhibit age-dependent reliance on aerial respiration in *hypoxic and** hypercarbic waters.Atmospheric O2 accounts for approx. 50% of the total oxygen needed when aquatic O2 levels are approx.3.5mg per liter (or CO2is about 5%) an fish rely exclusively on atmospheric O2 in oncentrations of less than 2.5mgl.Branchial respiration is initially stimulated by **hypercarbia (CO2: 0.5–0.8%) but increased** hypercarbia (CO2 – 1%) greatly depresses (by over 90%) brancial respiration and initiates (CO2: 0.5%) and sustains pulmonary respiration.
*hypoxic = very low oxygen levels
**hypercarbic = the presence of an abnormally high level of carbon dioxide
Development of dependence on aerial respiration in Polypterus senegalus (Cuvier)
M. M. Babiker1
(1) Department of Zoology, Faculty of Science, University of Khartoum, Sudan
