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In the light of global warming, climate change and its associated shifts in the geographic distribution of aquatic and terrestrial organisms, the physiological mechanisms that limit and adjust cold and heat tolerance have regained fresh interests. Comparative studies of different species have demonstrated that there are fundamental differences in thermal tolerance. Different taxa possess different physiological mechanisms of thermal tolerance and consequently their dramatic response to short term or long term temperature changes occasioned by global warming. Additionally, differences in thermal tolerance is also a function of species complexity as well as the existence of the systemic and molecular hierarchy of thermal tolerance which are sequences of thermal limits(Verde et al 2006).
Climate change affects many biological systems but the effects of greenhouse induced global warming is more pronounced at high altitudes; the antarctic regions(Eastman 1993; Peck et al 2004). While its comparatively easy to observe the effects of global warming and climate changes in plant species, studies of the effects of climate change on animals are much more complicated. Global warming and its impact on species survival and distribution may become the reason behind widespread species extinctions unless species employ anatomical, physiological and behavioral adaptability characteristics with the creation of novel ecosystems with novel physical conditions.
Just like the plant kingdom, the animal kingdom also exists in different forms and sizes. From microscopic zooplanktons to the more complex animals like insects, fish, worms and birds to even the large animals like mammals adaptation to the environment has always been at the basis of their survival. The human animal has reached to its predominance not a as a function of its size but due to its adaptability. In terms of numbers and distribution, insects are the most successful species. Insects have a system of rapid reproduction that is segmented into specialized developmental stages. This segmentation allows the insect to survive in almost all the environmental conditions and consequently their survival in the community. Insects also contribute to the survival of other communities of living forms through pollination; a process fundamental to plant reproduction. At one end of the life cycle of insects lies their decomposition purpose and the return of valuable nutrients to circulation(Kemp 2004).
Since the animal kingdom is home to a variety of life forms, the similarities that exist among these species is always very difficult to ascertain on the surface unless a critical scientific analysis of genetic correlation is carried out. For instance, a butterfly, a whale and an elephant seem completely unrelated on the surface, however they possess similar characteristics that differentiate them from the plant species as well as characteristics that differentiate one species from the other. For example, all these animals are directly or indirectly heterotrophic; dependent on green plants, for their existence. Since the animal kingdom is completely dependent on the animal kingdom for their food supply. Animals cannot use the basic raw materials of nutrients, air, water and solar radiation, yet these materials are abundantly available in the physical environment.
Additionally, most members of the animal kingdom are motile. Motility as a characteristic aids in the scavenging of nutrition for survival, defense against predatory members as well as in seeking environmental conditions that are suited to their survival. The importance of motility during environmental changes cannot be overstated. During seasonal changes, millions and millions of animals periodically migrate. As early as the Ice Age, animals have maintained their survival through moving into environmental conditions that can promote their production and species diversity.
As global warming continues, habitats that had hitherto been promoting of plant and species survival are changing causing a constant migration or adaptation mechanisms. In North America, researchers have demonstrated that species such as raccoons, armadillos and opossums are extending their normal ranges northwards due to the warming in their original habitat. However, migration is not only limited to the animal species, small members of the animal kingdom such as insects carry plant species to habitats that are alien to the plant species. Just as they disperse plant species, so does they disperse animal diseases. The spread of diseases by insects due to global warming is but one of the less studied negative environmental and economic consequences(Kemp 2004).
Response to changing physical conditions is as complex and complicated as the effects of these changes themselves. Changes in temperature and precipitation alter the geographic distribution of animals and these changes are also closely related to the distribution of plant species. Distribution boundaries of plants are more often correlated to specific isotherm or isohyet. This specific distribution has minimal application in animal distribution in the context where animals use their characteristic motility to change their distributions in response to short term and long term environmental changes.
Through anatomical, behavioral and physiological adaptations, the animal species survival remarkably in severe environmental conditions. From those surviving in high altitudes or in deserts adaptations to diverse physical conditions are as varied as the animal species resident in these animal species.
Environments with low temperatures are dealt with by anatomical modifications such as the development of thick coats and excessive deposition in the adipose tissue. Physiological and behavioral adaptations are associated with hibernation and sheltering in colonies so as to preserve body heat. Animals that are resident in high temperature areas have anatomical characteristics like large ear lobes for dissipation of body heat. Physiological conditions allow for increased evaporative cooling. Behaviorally, sheltering from the hot sun in dens are burrows aid in the preservation of body moisture.
The human animal is the most successful animal in terms of environmental adaptations. Anatomically, the human skin is devoid of hair, advanced perspiration system, advanced homeostatic systems and the upright posture makes human beings embrace existence in the warm climates(Kemp 2004). In addition to these; cultural, behavioral and technological advances have established the human race as an occupier of almost all the habitats.
Physiological Adaptation of Fish to Global Warming
Global climate change affects the biological, physical and biogeochemical characteristics of water bodies. It modifies the ecological structure, the functions of the ecological structure as well as the goods and services that these ecological systems provide to the aquatic life. On a large scale basis, global warming results to increased seal levels and sea surface temperatures, decreased sea ice cover, alterations in salinity, wave climate, alkalinity and ocean circulation(IPCC 2001).These changes impact on the status, productivity, sustainability, and biodiversity of both the ecosystems and the species that they support. Adaptations involve species adjustments to the changes in the ecological system so as to minimize their vulnerability and increase their resilience to the changing system.
Changes in species composition are correlated to discontinuities and gradients in temperature at the species habitat. It is natural for a single species or genus to completely or partially replace its congener as a result of changes in altitude, latitude, depth or any other environmental variable associated with shifts in temperature(Somero & Hofman 1997). In order to survive in changing environmental conditions, species employ a set of physiological, behavioral and in the long run anatomical adaptations.
Global warming poses as a direct threat to fish populations as rivers, lakes and ocean waters elevate their temperatures. Global warming continues to strain fish populations that are already plummeting due to population, overfishing and habitat loss. Although fish species have adapted to the increasing temperatures by increasing their metabolic rate, insufficient nutrient supplies due to insufficient zooplanktons and phytoplanktons means that the resultant effect of increasing metabolism is but slow growth and reproduction rates. In freshwater ecosystems, the effect of global warming is more disastrous as the increasing temperatures reduce oxygen solubility in water(FOXNEWS).
Fishes are very sensitive to acute temperature changes at the cellular level. To safeguard against the effects of these temperature changes on their physiological mechanisms, they have evolved a number of adaptive strategies which not only minimize the effects of temperature changes on but also maintain their swimming performance. Most of these studies have been carried out on cold adaptation, but the predicted effects with respect to the ensuing global warming have been extremely difficult to extrapolate on other aquatic ecosystems.
Metabolic costs vary directly with changes in temperature. Oxygen consumption is depressed in cold environments, in the extreme, the species enters torpor. As temperature rises, the unfavorable balance between the metabolic costs of ventilation and oxygen uptake is increased. The result is a limiting effect on the duration of sustainable aerobic exercise. This limiting effect may extend to the post exercise recovery period. Naturally, temperature sensitivity over the range of thermal tolerance is complicated by the presence of a zone of thermal insensitivity. This zone exists around the specific temperature range midpoint(Somero & Hofman 1997).
Adaptation mechanisms ensure that this zone of viability is extended upwards. In addition to this extension, fishes also employ several mechanisms aimed at the regulation of sustained locomotor activity. For instance; the regulation of oxygen uptake is achieved by altering the functional respiratory surface area via resorting to the surface secondary lamellae. Through instraspecific changes in the physical barrier thickness to oxygen exchange, fishes are able to regulate the effect of temperature changes on oxygen uptake. The rate and regulation of the peripheral oxygen uptake is a determinant of the cardiovascular system’s capacity to carry out convective transport and the capacity of the micro circulatory system to maintain a diffusional supply of oxygen for respiration(Wood & McDonald 1997).
These adaptive mechanisms, enable fish to manifest considerable resilience to acute and chronic temperature changes. Presently, structural limits existent in the gaseous exchange system increase the ability of the fishes to employ effective adaptive changes against the effects of global warming.
Global warming is pushing pressure on fish species evolve, adapt or modify functional features of Hemoglobin(Hb). A natural hemoglobin molecule is composed of two identical globin pairs of ∝ and ᵝ. these two identical globin pairs define the hemoglobin structure ∝2 ᵝ 2. Each globin is composed of the heme group that binds to the Fe++ ions. Hemoglobin is concerned with the transport of oxygen and release to respiring cells in accordance with metabolic needs. Antarctic fishes are adapting to the sub zero temperatures by reducing the number of hemoglobin content and or multiplicity and the number of red blood cells(Cocca et al 2007).
This is necessary due to the increased solubility of oxygen in water at low temperatures contrary to the reduced metabolic demand of oxygen in fish. Since the fish cannot control the rate of oxygen solubility in water, they employ this adaptive mechanism so that despite the amount of oxygen taken in, the cells can only partake of the oxygen captured by the hemoglobin deficient erythrocytes. The family Channichthyidae, has almost abolished hemoglobin as an oxygen carrier hence their colorless blood. In turn they have developed compensatory adaptations such as the use of gills, large capillary diameters, scaleless highly vascularized skin, a large blood volume and increased cardiac output(Riffenburgh 2007). Through these mechanisms of oxygen output, the operation of normal metabolic processes is maintained by directly using the physically dissolved oxygen in blood.
Since the colonization of habitats is almost solely dependent on molecular and functional differences in hemoglobin, this evolutionary ability to alter hemoglobin is going to be beneficial to these fishes while responding to increases in sea temperature. Multiplicity of hemoglobin is directly linked to the lifestyle changes and environment variability. In environment with higher variabilities, fishes increase their physiological adaptability by increasing the hemoglobin numbers. Trematomus newnesi together with two other species Pagothenia borchgrevinki and Pleuragramma antarcticum have succeded in using the evolutionary adaptive feature of adaptability in adjusting to lifestyles outside the antarctic region(Somero & DeVries 1967). As the impacts of global warming continues to exhibit profound environmental changes in the antarctic region, these pelagic species successfully adjusting to survival in non-Antarctic waters(Riffenburgh 2007). This survival mechanism is not limited to the three species only but to several other notothenioids(Verde et al 2006). Adaptability mechanisms are not only maintaining there population density but also species dispersal into alien habitats.
Given the uniqueness of the thermal history of the antarctic region, the order Nototheniodei; which is the most dominant order of fishes in the antarctic region, have been used for decades to determine the effect of environmental changes in the habitat. Through using features resident in their molecular structures, these species have been able to undergo an evolutionary adaptation facilitating their excursions into waters in temperate latitudes. These evolutionary adaptations constitute the convergent processes involved in species response to thermal variations(Verde et al 2006).
Understanding the nature and roles of physiological adaptations to changes in temperatures gives an insight on the species distribution patterns. With the average temperature increase of approximately 0.6 degrees centigrade over the last decade, meta analyses have demonstrated that species distribution patterns, phenology and the structures of ecosystems have also been profoundly altered. Recent consensus reveals that over global temperatures will increase with approximately three degrees centigrade(Somero, 2005). With this increase extensive biospheric changes can not be wished away.
The metabolic rate of an antarctic marine ectotherms such as Pagothenia borchgrevinki is strongly related to the environmental temperature. Increase in temperature within the physiological limits lead to an increase in the metabolic rate. Likewise the decrease of temperature within the physiological range results in decreased metabolic rates. These changes are directly related to oxygen uptake. While many species in the antarctic region have limited abilities to respond to temperature changes due to global warming; Pagothenia borchgrevinki is an exception to this general understanding. These fishes are able to acclimate their resting metabolic rate as well as the resting ventilation frequency when temperatures rise with as much as five degrees centigrade in a short duration(one month) after which no appreciable change occurs in acclimation. This signifies successful adaptability. (Robinson & Davison 2008).
Moreover, oxygen consumption in fish is not only determined by internal processes but also with a host of other environmental factors. Because of this both the rate of oxygen consumption and gill frequency can be used to measure species response to environmental stressors and thermal alterations manifested in the respiratory system. In a study carried out to determine the thermal sensitivity of activity scope of Pagothenia borchgrevinki, it was demonstrated that the scope was similar to the scope in temperate species. When these results are extrapolated to other marine fishes then the physiological adaptability mechanisms to global warming are almost similar(Lowe & Davison 2006). Both the ventilation frequencies and the oxygen consumption rates increase with elevation in temperatures. However, for Pagothenia borchgrevinki the maximum ventilation frequency is thermally insensitive. It is yet to be established whether this thermal insensitivity of the ventilation directly impacts on the thermal insensitivity at the maximum oxygen consumption rate.
In summary, during the process of cold adaptation, notothenioids; through an evolutionary trend, produced unique specializations such as the modifications of hematological features. Such characteristics include decreasing the amounts and multiplicity of hemoglobins and erythrocytes in response to low temperatures. The extremity of these modifications can be observed in the Antarctic family of Channichthyidae which is devoid of hemoglobin molecules. The process of this lose is related to the deletion of all globin genes except the inactive 3′ end of adult α-globin(Guido di Prisco et al 2007). These adaptation in the oxygen transport systems in notothenioids have its basis on the evolutionary changes that are currently being stimulated by temperature changes induced by greenhouse gas associated global warming.
Anatomical adaptations of these antarctic fishes include the development of thin, hardy and unblemished skin. Physiologically mediated slow development of gills is also ans adaptive feature of Pagothenia borchgrevinki. Adults thrive in ice cold waters with the assistance of the antifreeze protein(AFPs) in the blood. While spawning occurs at different depths, all fish larvae swim upwards into the ice platelets located below the surface ice. These larvae hatch on the platelets of ice which also acts as a hiding ground from predators(Science News 2006). With global warming and the gradual melting of all the surface ice, these species will be forced to seek newer breeding grounds or alter their modes of breeding. In essence, this implies that the physiological adaptations that support their excursions into temperate waters will through novel mechanism,s of evolutionary processes enable the larvae to adapt to the new ecological niches.
The impacts of global warming will continue to stimulate the development of physiological, anatomical and behavioral adaptive mechanisms to ensure species survival in the wake of increasing sea surface temperatures and other global warming associated impacts on ecological systems. These mechanisms will not only affect the survival rates of vulnerable species on changing ecosystems but also promote population increase and distribution of species to habitats that had hitherto been alien.