ABSTRACT
Lungfi sh provide a unique insight into the evolution of metabolism in the vertebrates. Th ey are “living fossils anatomically” and this is also refl ected in their metabolic organization. Th e complexity of enzyme isoforms and their tissue specifi c distribution are indicative of the early evolutionary position they occupy. Th e bile salts and forms of surfactant used in the lungs are also considered “primitive”. Th e ability of some species to estivate provides an understanding of the mechanisms for down-regulating metabolism and the metabolic reorganization that must accompany such changes. During estivation, nitrogen metabolism is also modifi ed to allow them to detoxify ammonia and convert it to urea reducing osmotic stress. Th e subcellular organization of the system for urea synthesis is diff erent from that of mammals or elasmobranchs. Continued study of the metabolism of lungfi sh is certain to provide a more detailed understanding of the early stages of vertebrate evolution and the transition from an aquatic existence to a terrestrial lifestyle. Keywords: metabolism, enzyme, lipid, estivation, urea
Lungfi sh have long been considered of evolutionary interest. Th e phylogenetic relationship between lungfi sh and the tetrapods has been examined in a series
of studies with increasing evidence that lungfi sh are the closest ancestor of the tetrapods (Brinkmann et al. 2004; Meyer and Dolven 1992; Takezaki et al. 2004; Yokobori et al. 1994; Zardoya et al. 1998; Zardoya and Meyer 1996). Th us the six extant species of lungfi sh living in Africa (Protopteridae, Protopterus – 4 species), South America (Lepidosirenidae, Lepidosiren – one species) and Australia (Ceratodontidae, Neoceratodus – one species) provide a unique window into the evolution of metabolism of the vertebrates at that critical period when the transition from an aquatic to a terrestrial existence was made. Anatomical structures such as lungs and sturdy appendages were prerequisites for terrestrial survival but metabolic prerequisites were also required. Consequently, an important question is, if extant lungfi sh are living fossils anatomically, can we assume they also retain many of the primitive features of metabolism? As this review will show, there is considerable evidence that this assumption is valid. Th is review outlines the current understanding of the metabolic organization of the extant lungfi sh. Th e 6 extant species of lungfi sh diff er metabolically in several ways. Th e Australian species diff ers from the other species in its low reliance on aerial respiration and its inability to estivate. It is considered more primitive than the other species based on anatomical criteria. Th us it represents an early stage in the transition to a terrestrial existence. All 4 species of African lungfi sh have the ability to form cocoons to avoid desiccation, however only P. annectens has been routinely observed to do so in its natural environment (Greenwood 1986). Th is is likely due to diff erences in habitat type and the likelihood of experiencing complete seasonal drying. Th e ability to breathe air with a primitive lung and to estivate for long periods, have interested physiologists for decades. Th e earliest recorded studies of the physiology of lungfi sh are those of Homer Smith (Smith 1930; Smith 1931; Smith 1935) who measured the rate of oxygen consumption in Protopterus in air and water, and demonstrated changes in the pattern of bimodal gas exchange occur during fasting and estivation and at diff erent developmental stages. In the 1960’s and 1970’s Kjell Johansen contributed substantially to understanding the respiratory physiology with studies of the Australian (Johansen et al. 1967), South American (Johansen and Lenfant 1967) and African species (Johansen and Lenfant 1968). Th ese studies demonstrated that in the primarily water breathing Australian lungfi sh, blood fl ow increased to the lung during hypoxia. In the late 1970’s the studies of Fishman et al. on African lungfi sh provided more details on the regulation of respiration in active and estivating lungfi sh (Delaney and Fishman 1977; Laurent et al. 1978). Th ey also examined cardiovascular physiology in active (Arbel et al. 1977) and estivating fi sh (Delaney et al. 1974). In Africa, G.M.O. Maloiy encouraged fi eld and lab studies of African lungfi sh in Kenya (Dunn et al. 1981; Johansen et al. 1976a, b; Lomholt et al. 1975; Maina and Maloiy 1985; Weber et al. 1977). From the mid 1980’s until recently, studies of the metabolism of lungfi sh were sporadic.
