Updated: Oct 19, 2019
THE ANTARCTIC-MARINE ENVIRONMENT; COSTS AND BENEFITS OF ANIMAL ADAPTATIONS
Adaptation is a process that allows organisms to better survive in their habitats, through changes of behaviour, anatomy and physiology (Dobzhansky, 1968) . Darwin (1859) states that from an evolutionary perspective, animals that are well adapted to their environment are more likely to survive and reproduce. Animals are adapted to almost all environments, including the most extreme. An extreme environment is one where conditions make it hard for life other than specially adapted organisms to survive. Extreme environments include the Atacama Desert (Chile), with less than a tenth of an inch of rainfall each year, Northern Ethiopia, where the average temperature is 41oC, and the oceans surrounding the Antarctic, where life must survive low temperatures and a marine environment (Garside et al, 2012).
The continent of Antarctica is the coldest on Earth , with the lowest natural temperature ever recorded being -89.2oC (Gavin, 2008). With very little rainfall and summers of constant daylight, it is one of the most extreme environments on the planet (USCIA, 2011). Antarctica is surrounded by the Southern Ocean, and sea temperatures vary from -2oC to 10oC. These marine waters however, still provide a habitat (both permanent and seasonal) to a range of fauna such as krill, birds, fish and mammals (El-Sayed, 1994), including the Antarctic Ice fish and the Orca (Rogers et al, 2012).
To discuss the challenges that organisms living in the Antarctic oceans are exposed to.
To explore how the orca and the Antarctic ice fish have adapted to the challenges.
To state the costs and benefits of each adaptation.
To make comparisons of the adaptations, and determine how well the species given have adapted to their environments.
As mentioned, sea temperatures around Antarctica will vary between -2oC and 10oC (El-Sayed, 1994). Heat will transfer 25 times faster in water than in air, meaning aquatic animals will potentially lose heat a lot faster than any land animal (Perrin et al, 2009). As physiological body processes work best in particular temperatures for individual species, animals need to be able to maintain their body temperature, or adjust their behaviour to reflect their physiological needs (Wolfe et al, 2008). Each species, regardless of being endo- or ectothermic will have an ideal range for optimal enzyme activity and overall biochemical or physiological processes (Campbell et al, 2008).
On a cellular level, the hydrophobic tails in a phospholipid bilayer which normally provide the cell with fluidity, will become rigid in cold temperatures. Cell movement and permeability will therefore be affected, and ultimately cellular reactions will slow down or stop (Chandler, 2009). Colder temperatures also inhibit enzyme activity. As molecules move less rapidly as the temperature cools, substrates collide with active sites less often. Metabolism, as a result, would be hindered (Campbell et al, 2008). Cinar, et al (2000), stated that when blood temperature was decreased from 36.5oC to 22oC, blood viscosity was increased by 26.13%, and a 20.72% decrease in blood flow rate, thus providing a problem with heat and oxygen transfer in the body.
The issue with living in the marine Southern Ocean environment is the salt concentrations. All animals need to balance the amount of water in their bodies, and this becomes a challenge when living in a high-sodium concentration aquatic environment. All marine life have to be specially adapted to live in salt water, and this can be by either osmoregulation or osmoconforming (Campbell et al, 2008).
The amount of oxygen dissolved in water will increase as the temperature rises, as the less molecules in water move around, the more oxygen remains trapped (Hince, 2000). The ways in which gas exchange takes place in an animal will depend on whether it gets oxygen from the air or from the water. Air breathers have the challenge of the length of time they can be away from the water’s surface if they rely on their lungs, while fish that take in oxygen from the water will require specialised organs (Campbell et al, 2008).
Orca Low Temperature Adaptations
The orca (Orcinus orca) is a marine mammal, commonly found in Antarctic water (Hince, 2000). As an endothermic mammal (Feldhamer et al, 2007), the cold waters of the Southern Ocean pose a problem to internal thermoregulation, and maintenance of their internal ~37oC body temperature (Perrin et al, 2009). They have behavioural, anatomical and physiological ways of maintaining thermoregulation in cold water.
Blubber is the name given to the thick layer of at under the skin of the cetaceans, including the orca (Kvadsheim, 1996). It is 7.6 to 10cm thick in the orca (SeaWorld, 2013), compared to just 5cm thick in dolphins, which live in warmer waters (Kvadsheim, 1996). Blubber is specialised for insulation, as it is dense with adipocytes which are poor heat conductors. It is also less metabolically active than other tissues meaning there is less blood perfusion near the skin, which would normally result in heat loss (Perrin et al, 2009).
Benefits of Blubber
It effectively allows the orca to maintain a warm internal temperature. It does not compress with depth so is therefore not affected by deep diving, the fat is used as an energy store, thus providing a back-up of energy in nutritional scarcity. It also provides buoyancy (Perrin et al, 2009).
Costs of Blubber
It is less effective than fur as an insulator and will degenerate if food supplies are low. It also makes up 50% of the orcas body weight, and a large mass increases the chances of stranding when carrying out hunting behaviour on beaches (Perrin et al, 2009).
Low Relative Surface Area
The Orca is considered a large mammal, reaching between 4700-6600kg (sex dependant) in weight(Ford, 2009). Due to its size and small, few extremities, it also has a large relative surface area (RSA), which means that there is less surface per unit volume to lose heat from (Perrin et al, 2009).
Benefits of a low RSA
Aside from the decreased loss of heat, large body size allows for blubber to be thicker, which further decreases heat loss from the skin (Perrin et al, 2009)
Costs of a low RSA
A low RSA will prevent heat loss, even when the orca needs to, such as in the case where there was a sudden increase of activity during hunting (Perrin et al, 2009).
Countercurrent Heat Exchange
The arteries carrying warm blood into the flippers and fins of the orca run alongside the veins, in which the blood is cooler. The heat is transferred to the cooler blood, thus allowing it to be warmer upon arriving back to core of the body (Campbell et al, 2008).
Benefits of Countercurrent heat exchange
While assisting in thermoregulation, this system works alongside oxygen transfer (and other blood functions) at no extra cost to energy (Perrin et al, 2009).
Costs of Countercurrent heat exchange
As the veins and arteries are laid out specifically to preserve heat, it will do so even in situations where heat needs to be lost (Campbell et al, 2008).
Ice Fish Low Temperature Adaptations
Lack of Haemoglobin and RBCs
The ecto-thermic Antarctic ice fish (suborder Notothenioidei), are the most common fish found in Antarctic waters; making up 35% of 90% of the fish found there. The family of icefish called the Channichthyidae, are unique in that they lack haemoglobin and red blood cells (RBCs) completely, providing a number of adaptive advantages for living in their environment. As mentioned, blood viscosity increases with temperature. In the ice fish, their lack of haemoglobin and RBCs makes their blood 25-40% less viscous compared to other bony fish, thus overcoming the issue (Sidell and O’Brien, 2006).
Benefits of no Haemoglobin and RBCs
Compensation for blood being generally more viscous in colder temperatures.
Costs of no Haemoglobin and RBCs
Huge changes in circulatory anatomy are required and twice as much energy is needed to pump their blood, compared to other teleosts (Sidell and O’Brien, 2006).
‘Antifreeze’ in their blood
The Notothenioidei are able to survive in such cold waters due to a glycoprotein in the blood that has antifreeze properties. They kinetically bind to small ice crystals, which inhibits the crystal growth and prevents any fluid from freezing as they cover the water-accessible areas (Jorov and Yang, 2004).
Benefits of Antifreeze
They allow the fish to survive in below freezing temperatures and is much more effective than sodium (Jorov and Yang, 2004). The glycoproteins have very little effect on osmotic pressure (Fletcher et al, 2001).
Costs of Antifreeze
The glycoprotein in fish are not are less effective than ones seen in other species (Ohio University, 2007) which could suggest that it is not as well-evolved, and therefore sometimes unreliable system.
While Campbell et al (2008) states that lower temperatures inhibit enzyme activity, the ice fish has adapted specialised enzymes that are most active at freezing temperatures (Margesin and Schinner, 1999).
Benefits of cold loving Enzymes
Allows adequate metabolic activity for the ice fish’s needs (Margesin and Schinner, 1999).
Costs of cold loving Enzymes
The ice fish enzymes are less efficient at optimal temperature compared to the enzymes in humans at optimal temperature (Margesin and Schinner, 1999).
Cold Water Adaptations
All of the orcas adaptations rely on it staying warm to maintain homeostasis, whereas the ice fish appears to have developed physiological processes that work in the environment temperature, and adapted uniquely. To survive in the freezing cold, it would appear that an animal must maintain its core temperature through metabolic processes, or conform to the surrounding temperature, and use less energy.
Orca Saltwater Adaptations
No Permeable Surfaces are Exposed
Being hypo-osmotic, the orca is likely to lose water from its low concentrated body to its high concentrated environment. Marine mammals including the orca, however, have thick skin that is impermeable to water, meaning water retention is maintained. The respiratory tract and lungs, which are highly permeable, are not exposed to the water, protecting the membranes from osmosis (Bradley, 2009).
Benefits of Impermeable Skin
The orcas thick skin is serving other functions, such as protection and insulation. The lungs require no additional energy to keep away from the water (Bradley, 2009).
Costs of Impermeable Skin
The skin cannot be used for other benefits, such as gas exchange, exploited by some fish species. The lungs (through respiration) cause water evaporation, greatening the requirement to retain water (Bradley, 2009).
Like all mammals, orcas have kidneys control osmoregulation. Due to the need for the increased sodium secretion, the orcas is much more powerful than that of a human. If one litre of saltwater is ingested by a whale, approximately one third will be pure water after salt secretion. The human however, will be worse off with a water loss of a third (Schmidt-Nielsen, 1997).
Benefits of a powerful kidney
Hormone secretion, waste excretion and blood pressure will also be well monitored (Bradley, 2009).
Costs of a powerful kidney
The kidney requires a lot of energy as it is almost constantly working.
Ice Fish Saltwater Adaptation
Ice fish, like most marine fish, will excrete as much salt as possible from the gills. The fish will first drink a lot of the seawater. In the gills, chloride cells will move chloride ions out, and salt ions will follow, leaving the fish hypo-osmotic to its surroundings (Campbell et al, 2008).
Benefits of Gill Excretion
Excessive amounts of sodium and chloride that are ingested are expelled (Bradley, 2009).
Costs of Gill Excretion
It does not allow any useful ions to be reabsorbed (Bradley, 2009).
The proximal tubes of the ice fish kidney reabsorb sodium, chloride and important amino acids and glucose. This increases the activity of water in the urine, and the water goes back into the blood via the proximal tubule cells, while ions are transported into the urine. The urine is high concentration and expelled with minimal water loss (Bradley, 2009).
Benefits of Urinal Excretion
It allows both water and useful organic compounds to be reabsorbed (Bradley, 2009).
Costs of Urinal Excretion
It doesn’t rid of all the excessive sodium and chloride ingested (Bradley, 2009).
Adapting to survive in Seawater
Both the orca and he ice fish regulate their internal concentration with the kidney. The organ itself is highly adapted to function, which suggests that regardless of species, it will provide the animal with means to expel unwanted sodium, and maintain the hypo-osmotic equilibrium within.
Orca Gas Exchange Adaptations
Mammalian lungs are designed to take in air and utilise primarily the oxygen. Once inhaled, air is passed into the bronchi and bronchioles, and then to the alveoli with a thin lining of a moist epithelium. Here is where the oxygen diffuses into capillaries, and carbon dioxide diffuses from the capillaries to the alveoli (Campbell et al, 2008).
Benefits of lungs
Gaseous exchange from the air, which is much less viscous and dense, so easier to move in and out of small passageways. Air breathing is relatively easy (Campbell et al, 2008).
Costs of lungs
Orcas need to always be able to reach the water surface, which limits diving time. Lungs also take up a lot of room, as they require a cavity to inflate and deflate.
The orca, like all cetaceans, has a blowhole. It is the opening of the trachea, the main tract of the respiratory organs, connecting the air with the lungs for gaseous exchange. The openings, situated high on top of the head, are controlled by contractions of the skeletal muscles, with closure being the passive process, thus preventing too much unwanted water from entering (Berta et al, 2006).
Benefits of the blowhole
It allowed easy access to the air, considering the orca is a fully aquatic mammals and must remain in the water. The separation of the oesophagus and the trachea means the orca can breathe without risking getting waterlogged lungs (Berta et al, 2006).
Costs of the blowhole
As the orca cannot breathe through its mouth (the blow hole being the only opening to the external environment), there is no ‘back-up’.
Ice Fish Gaseous Exchange Adaptations
Fish, which take in oxygen from the water, require no lungs. They instead have gills, with a large surface area and take in oxygen as water flows past them. The fish will continuously pumps water through its mouth, and passed the gill arches, back outside the body. The gill filaments is where the exchange takes place (Campbell et al, 2008).
Benefits of gills
Little or no effort is required to push the water through the gill arches, simply the movement forward through the water is enough (Campbell et al, 2008).
Costs of gills
Water is far more dense and viscous than air, and requires more effort to get oxygen from then air (Campbell et al, 2006).
Low Metabolic Rate
Ice fish (all the Notothenioidei) have evolved in an isolated environment and dominate the fish populations in Antarctica. With hardly any competition for resources, the ice fish has taken a laid back approach to living, which includes bottom-feeder behaviour, and a low metabolic rate. Having a low metabolic rate, means that far less oxygen is required to maintain life (Sidell and O’Brien, 2006).
Benefits of a low MR
Low MR means less energy expenditure (Campbell et al, 2008).
Costs of a low MR
Significantly less oxygen intake, which in turn means less activity (Campbell et al, 2008).
Specialised Circulatory System
As previously mentioned, the ice fish of the family Channichthyidae has no haemoglobin. This means that dissolved oxygen is circulated in the plasma. To compensate, the ice fish have large blood vessels, much more blood volume (to body weight ratio), and bigger hearts. These all create a five times greater cardiac output than other fish (Sidell and O’Brien, 2006).
Benefits of a Specialised Circulatory System
Bigger vessels and heart make circulation easier.
Costs of a Specialised Circulatory System
The system has to work hard to transfer the oxygen around the body, more so than fish with haemoglobin (Sidell and O’Brien, 2006).
Lack of Scales
The ice fish have a lack of scales on their body, which suggests that their skin is involved with oxygen intake. While it is known that in comparison to the gills, cutaneous oxygen absorption is minimal, it may play a part in providing the heart with additional oxygen (Rankin and Tuurala, 1998).
Benefits of Cutaneous Oxygen Absorption
Any additional oxygen intake is going to be beneficial to a fish that lacks haemoglobin, and has to work hard to circulate oxygen in the body.
Costs of Cutaneous Oxygen Absorption
Considering that the intake from the skin is so small, scales may have been better used for defence or camouflage.
Gas Exchange Adaptations for Aquatic Species
With completely different respiratory systems, the orca and the ice fish are difficult to compare. While air is easy to breathe, the orca is limited to how long it can hold its breath. The ice fish, on the other hand, cannot supply enough oxygen to break away from its laid back and slow lifestyle. Even though the two species express very different adaptations, neither are without costs.
The low temperature adaptations of the orca all appear to interlink; the streamlined shape is supported by the presence of blubber, which in turn provides heat that will be used in the counter current heat exchange. This suggests that the orca is well adapted to its cold environment, more so then the ice fish who has insufficient enzyme activity. But if you consider the slow lifestyle of the ice fish, the lack of enzyme activity seems appropriate, making both the species not only suited to their environment, but what they do and how they behave. Where the species do differ however, is with the use of their respiratory system. While the ice fish has overcome one excretion problem by using its gills, the orca has to rely solely on the kidney, as it is an air breather.
While the orca being an air breather poses some problems, the ice fish appears to have adapted a trait which does not benefit it for gas exchange. While this suggests that neither is well adapted to oxygen intake in its environment, the bigger picture has to be taken into account. If the ice fish were to have haemoglobin, yes the amount of work to allow oxygen circulation would decrease, but the blood viscosity would dramatically increase. This could potentially cause the circulatory system to work harder than it currently does. As the unique traits expressed by the ice fish are so environment specific, it would suggest that it was the species better adapted. This does not, however, mean that the adaptations are the best possible, but simply the best available, as there are still costs to be taken in account.
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