Posts Tagged ‘ turtle ’

Surviving Anoxia

Introduction

Oxygen comprises a large proportion of our atmosphere (21%) and has become a vital gas for many organisms on Earth. Without it our ability to produce energy (as ATP) via aerobic respiration fails. Oxidative phosphorylation, the primary method of aerobic ATP production, produces large amounts of ATP (from our glycogen stores) just as long as we are able to continually supply O2. Without oxygen we resort to anaerobic respiration, where the primary method of ATP production is glycolysis. In comparison to oxidative phosphorylation, the amount of ATP produced is minimal.

Whilst we take oxygen for granted in our environment, there is a large variety of situations where oxygen levels are limited, or where oxygen is completely absent. Typically, such conditions arise in aquatic environments where the ability of oxygen to diffuse is greatly reduced and oxygen levels depend heavily on the ability of organisms to photosynthesise. Yet many respiring organisms are able to live in these environments due to adaptations to low oxygen/ zero oxygen situations.

When referring to the content of oxygen in an environment, we usually state its partial pressure i.e. the proportion of the total gas which comprises oxygen. At sea level, the total pressure of the gases in the environment is 101kPa (or 1 atmosphere), as oxygen makes up 21% of these gases, the partial pressure of oxygen at sea level is 21kPa.

From this we can define four terms:

Hyperoxia (>21kPa)  – When oxygen levels are greater than normal

Normaxia (21kPa)      – When oxygen levels are normal

Hypoxia (<21kPa)      – When oxygen levels are below normal

Anoxia (0kPa)              – When oxygen is completely absent

Why We Require Oxygen

Aerobic Respiration

The major role of oxygen is to provide the body with metabolic energy in the form of ATP. This is done during aerobic respiration via a process known as oxidative phosphorylation which occurs within the inner mitochondrial membranes. The role of oxygen in this process is to act as a terminal electron acceptor. The transfer of electrons to oxygen provides enough energy to phosphorylate ADP to ATP.

However, for oxygen to form ATP in this manner, there are a number of prerequisites. Glucose (a 6-carbon molecule) must be readily available, if it is not it must be mobilised from the body’s glycogen stores. There are 3 major steps which occur before oxidative phosphorylation:

Glycolysis – Glucose is metabolised into 2 molecules of pyruvate (a 3-carbon molecule). This provides a small amount of ATP as well (2 per glucose molecule).

Pyruvate is metabolised into 2 molecules of Acetyl CoA (a 2-carbon molecule). One molecule of carbon is lost as CO2.

The Krebs cycle – Each molecule of acetyl CoA undergoes a number of enzyme-catalysed reactions producing a number of highly energised electrons. These electrons are used in the electron transport chain to produce vast amounts of ATP.

Anaerobic respiration

However, oxygen isn’t always available. In this situation, respiration may continue but it can go no further than glycolysis. As a result, the ATP yield per molecule of glucose alters dramatically:

Glycolysis Krebs Cycle Oxidative Phosphorylation
Aerobic 2 2 34
Anaerobic 2 0 0

Anaerobic Glycolysis

The efficiency of ATP production drops to around 5% during anaerobic reproduction. One way the body can cope with this is to increase the glycolytic rate (the rate of glycolysis). Increasing the glycolytic rate by about 20x returns the level of ATP production to that seen in aerobic respiration. This is known as the Pasteur Effect.

There are of course negative effects as a result of this:

Increasing the glycolytic rate causes a rapid drop in the body’s store of glycogen. Without glycogen, the body will be unable to perform any form or respiration – anaerobic or not, which will inevitably lead to death.

The Krebs cycle stops soon after oxygen becomes absent. This means there is no demand for acetyl CoA. The result of this is excess production of pyruvate. Lactate dehydrogenase converts the excess pyruvate to lactate (and also recycles NADH + H+ to NAD+ allowing glycolysis to continue). A build up of lactate is also undesirable however as it immediately dissociates into lactic acid. The low pH of lactic acid will begin to reduce cellular pH and denature sensitive enzymes.

Decreasing Cellular pH

As a result of decreased pH and ATP, maintaining the membrane potential becomes very difficult. The Na/K ATPase pump which actively removes N+ from the cell and uptakes K+ in order to maintain the correct (-70mV) membrane gradient, stops functioning when ATP production diminishes. The maintained -70mV gradient begins to equilibrate at 0mV, causing a breakdown of the transmembrane gradient within a matter of minutes.

This has dire consequences on the mammalian brain in particular; the change in the membrane potential allows calcium ions to enter the cell. Intracellular calcium causes cell damage and hastens enzyme breakdown. The breakdown of these important brain cells causes permanent brain damage and if not supplied with oxygen quickly enough, will lead to death.

Strategies to Survive Anoxia

To survive anoxic conditions, an organism must be able to deal with the problems associated with anaerobic respiration; keeping up ATP levels (particularly in the brain) and avoiding acidification and associated cellular damage. Two organisms which are highly adapted to surviving anoxic conditions are; the crucian carp and the painted turtle. Both have different strategies for maintaining the body without oxygen, but both do so by combating the associated problems with anaerobic respiration.

Glycogen Stores in the Crucian Carp

One of the problems with anaerobic respiration is that, after a short time in anoxia, glycogen levels decrease rapidly. The Pasteur effect (where glycolytic rate increases for a short period to accommodate the lack of ATP production from oxidative phosphorylation) is only temporary. All body stores of glycogen are quickly depleted and the body is no longer able to survive.

The crucian carp (and to a smaller extent the painted turtle) has an unusually large glycogen reserve in the liver (30% of the total liver mass). Such large stores of glycogen allow the crucian carp to survive in anoxia by glycolysis alone for a much longer period of time.

The crucian carp will typically only face anoxic conditions during the winter months, when a thick layer of ice forms over the water sources they inhabit. Because of this, the crucian carp has adapted to build up glycogen stores throughout the year so that come winter, the glycogen stores are as great as possible. To do this, during the warmer summer months, instead of using carbohydrates for energy (i.e. Glucose / glycogen) lipids are used, allowing the crucian carp to build glycogen stores.

Metabolic Depression in the Crucian Carp

Although increased glycogen stores length the period of time the crucian carp is able to survive in anoxic conditions, it is not the only factor which prolongs this survival period. Metabolic depression means slowing down the rate at which the body consumes ATP, so combining reduced ATP requirements with greater stores of glycogen to produce ATP ensures a near maximum survival time in anoxia.

To reduce metabolic rate, locomotion is reduced during anoxia this means muscles use as little energy as possible. Also, some sensory functions are suppressed, these include vision and hearing. During the winter months when anoxic conditions arise, the thick layer of ice above reduces light levels to a minimum so vision is not a necessity.

Crucian carp will also induce voluntary hypothermia during anoxia. This is actually beneficial to the carp because at lower temperatures, metabolic rate decreases. Therefore when anoxic conditions arise, the crucian carp will swim to cooler waters to reduce their metabolism.

Avoiding Acidification in the Crucian Carp

The problem with glycolysis is the production of lactate and hence lactic acid, causing a decrease in cellular pH and thus acidification. If the crucian carp is producing ATP from glycolysis alone then cellular concentrations of lactic acid would normally quickly rise and pose a major threat for the carp. However, instead of producing lactate as the end product of glycolysis, the crucian carp has adapted to produce ethanol.

By producing ethanol instead of lactate, the crucial carp benefits in a number of ways. Primarily ethanol is non-acidic and thus will not decrease intracellular pH (therefore avoiding the associated damage to the cell as a result of acidification). Ethanol is also lipid soluble meaning it will not build up in the body as it is easily excreted across the gills.

Normally, during glycolysis, pyruvate is metabolised to acetyl CoA by pyruvate dehydrogenase – producing lactate as the end product. Yet a mutation in the pyruvate dehydrogenase enzyme of the crucian carp means that instead of acetyl CoA, acetaldehyde is produced. Acetaldehyde is then metabolised by alcohol dehydrogenase to produce ethanol.

However, the brain is unable to produce ethanol in this manner, only skeletal muscle can. Any glycolysis occurring in the brain will therefore produce lactate as the end product. This lactate however, is transported via the blood, to the skeletal muscle. Here, the lactate is turned into pyruvate where glycolysis will metabolise it into ethanol, it is subsequently excreted across the gills.

Avoiding Acidification in the Painted Turtle

The painted turtle deals with anoxia in a different way to the crucian carp. Whereas the crucian carp has very large glycogen stores and avoids acidification by producing ethanol, the painted turtle favours metabolic depression. Although the crucian carp also depresses its metabolism, it is on a much smaller scale. The crucian carp is still aware and the brain functions as normal. The painted turtle on the other hand becomes almost ‘comatose’ as many more body systems are shut down.

The painted turtle must too deal with the excess of lactate produced during glycolysis to avoid acidification. The painted turtle produced lactate as the end product of glycolysis and does not have the ability to form ethanol from pyruvate. To avoid acidification, it must therefore buffer the low pH of the lactic acid.

To buffer the lactic acid, the painted turtle uses bicarbonate and carbonate. The turtle has large stores of these in its shell (which has a rich blood and nerve supply). The carbonates are able to ‘mop up’ excess H ions (which make the blood acidic) and through a number of reactions, the carbonate and H ions and turned into carbon dioxide and water.

The shell is also a rich store of calcium ions. These calcium ions are able to bind with lactate and the resulting molecule becomes stored in the shell until it is possible to excrete the lactate during aerobic respiration.

This method of avoiding acidification is arguably not as efficient as the crucian carp (producing non-acidic ethanol) but it does help the turtle to avoid acidification for a much longer period of time than would normally be possible.

Metabolic Depression in the Painted Turtle

The painted turtle is able to survive anoxia for such extended periods of time due to the amazing amount at which it is able to decrease its metabolic rate by (90-95%). All non-essential systems are shut down which causes the turtle to become ‘comatose’ as the brain becomes quiescent. Even essential systems such as protein synthesis cut their ATP demand drastically. However on system reduces its ATP demand as little as possible -the Na/ K ATPase pump.

As we know, if the Na/ K ATPase pump stops functioning, then the transmembrane gradient breaks down due to the leakage of Na & K ions across the membrane. This allows calcium ions to enter the cell and cause fatal destruction if normoxic conditions are not resumed. However, the painted turtle is not able to maintain 100% efficiency of this pump during anoxia due to the large demand of ATP it requires. Its efficiency is essential cut down to 25%.

To combat the reduction in the ATPase pump’s efficiency a phenomenon known as ‘Channel Arrest’ occurs. This allows the retention of the transmembrane gradient. Normally Na & K are able to leak into and out of a cell via transmembrane ion channels, down their respective concentration gradients. However during channel arrest these ion channels are forced to close greatly decreasing the amount of ion leakage into and out of the cell. This essentially counteracts the 4-fold reduction in the ATPase pump’s efficiency and allows for the preservation of the transmembrane gradient – thus preventing cell damage.

Another phenomenon of the painted turtle which allows for further depression of the metabolic rate is ‘Spike arrest’. This inactivates Na & K ion channels normally associated with producing action potentials. However as these channels are inactivated the action potential threshold becomes greatly elevated – thus forcing the brain into a quiescent state and preventing responses to stimuli.

The Role of Neurotransmitters and Metabolites

Certain molecules are involved with mediating the transition to an anoxic state. The main ones are the neurotransmitters; glutamate and GABA as well as the metabolite adenosine.

Adenosine release is associated with low levels of oxygen. When low blood oxygen is detected, adenosine is released shortly after. The immediate result is an increase of cerebral blood flow. This decreases soon after in the painted turtle as the metabolic rate is lowered – however as the crucian carp remains ‘active’ during anoxia, cerebral blood flow does not reduce.

Adenosine is basically responsible for ensuring ATP production is able to meet demand as it is responsible for a number of factors contributing to the metabolic depression associated with anoxia. It blocks the production of glutamate (an excitatory neurotransmitter) and has been found to suppress protein synthesis and down-regulate NDMA receptor activity.

The NDMA receptor is highly permeable to Ca2+ ions, which are responsible for cell damage in anoxic conditions of unspecialized organisms. Normally in anoxic conditions excessive glutamate over-stimulates this receptor and causes a massive influx of calcium ions into the cell. Therefore the ability of adenosine to down regulate this receptor is highly beneficial during anoxia.

GABA and glutamate are highly related neurotransmitters; GABA is an inhibitory neurotransmitter and glutamate an excitatory neurotransmitter. During anoxia, we see a steady rise in GABA and a steady decline in glutamate. An initial steep rise of GABA in the painted turtle is due to increased levels of adenosine (released at low blood oxygen levels). Because GABA is an inhibitory neurotransmitter it is able to effectively reduce the amount of ATP the body requires, as the painted turtle reduces its metabolism by 90-95% we see very large amounts of GABA released. Release of GABA is seen in the crucian carp too, but as the carp remains active, GABA levels observed are much more modest.

Conclusion

Effectively we see two different strategies for combating anoxic conditions. The painted turtle, although it has larger glycogen stores than other animals, chooses to reduce its metabolism by such a large amount that it becomes almost comatose. This is achieved by Spike arrest, channel arrest, nearly a total suppression of nervous activity and a huge increase in extracellular inhibitory neurotransmitter, GABA. The limiting factor of how long the painted turtle can remain in anoxia is therefore the build-up of lactate. Although it is steadily removed and stored in the shell, it eventually builds up enough to reduce cellular pH to fatal levels if normoxic conditions are not returned.

The crucian carp on the other hand has glycogen stores so large it is able to survive on glycolysis alone at a metabolic rate not too far below normal. In terms of metabolic depression the crucian carp only moderately suppresses nervous activity, coupled with moderate release of GABA and suppression of locomotor activity (as well as voluntary hypothermia to decrease metabolic rate further). The key strategy for survival is the production of the non-acidic glycolytic end product – ethanol, thus preventing cellular pH from falling to fatal levels. Because of this, the limiting factor of anoxia survival in crucian carp is therefore the amount of glycogen stored in the liver.

Summary

Crucian Carp Painted Turtle
Activity level during anoxia Slightly reduced Comatose, quiescent brain
Production of ATP from glycolysis Great Above average
Glycogen storage Great Above average
End product of glycolysis Ethanol Lactate (Lactic acid)
Blood pH and cellular pH Stable (ethanol is non-acidic) Slowly rises when uptake of lactate decreases
Overall metabolic depression 70% 90-95%
ATP levels and membrane potential of the brain Maintained at normoxic levels – Brain function is preserved Maintained at normoxic levels – Brain function is preserved
Suppression of neuronal activity Above average Great
Channel & Spike Arrest No Yes

Comparative Digestion

Introduction

The method of digestion which an animal uses depends on its diet i.e. carnivore, herbivore or omnivore. For example not all mammals are carnivores (e.g. dogs and cats), herbivorous mammals include rabbits, chinchillas, guinea pigs etc.

Generally, carnivores consume animal tissue which is similar to their own; therefore all the body needs to do is break down the tissue and absorb the different components which can then be used in the carnivores own body. Omnivores have very similar digestive systems to carnivores with the addition of a caecum.

Herbivores consume plant matter which is more difficult to break down than tissue. Therefore herbivores have evolved fermentation systems which contain specific microflora, the microflora breakdown the plant material releasing useful nutrients which the herbivore utilises.

Digestion by Diet:

Carnivores

Because meat is easily digested, the gastric system of carnivores is typically short and simple. They are monogastric meaning they have only one stomach (unlike a ruminants’ stomach which has four chambers). Due to the ease at which components required for growth are obtained from food, some carnivores have lost the ability to synthesis them (e.g. cats are unable to synthesis taurine).

The teeth of carnivores are sharp and strong, this makes it easy to rip and tear meat from bones of prey. When possible, the meat is broken down further by the teeth to ensure maximum surface area for digestion by enzymes in the stomach and small intestines. True carnivores do not have digestive enzymes in their saliva.

Due to the lack of salivary enzymes, food spends little time in the mouth of a carnivore, it is shortly swallowed and travels down the oesophagus. The oesophagus is a tube which runs from the pharynx (back of the oral cavity) to the stomach. The walls of the oesophagus are protected from damage by food by stratified squamous epithelium arranged in longitudinal folds, this also allows for expansion as the food travels down to the stomach. Food is passed down the oesophagus by peristalsis which is the contraction and relaxation of longitudinal and circular muscles, pushing food down to the stomach in wave like motion.

The next stop is the stomach, the stomach has multiple roles in digestion, including:

  • A reservoir for food
  • A sterilising chamber, due to the low pH (high acid content – HCL)
  • A churning chamber to mix food with digestive gastric juices a
  • The initial site of protein digestion, primarily by pepsin – secreted by the epithelial lining of the stomach

Food is moved to the next site of digestion, the small intestine, by peristalsis. The small intestine is a long and narrow ‘tube’ with a structure and epithelium that maximises surface area. This is important because the small intestine is the primary site of digestion by enzymes. Food continues to travel along the small intestine by peristalsis. The small intestine can be divided into the duodenum, jejunum and the ileum. The pancreatic duct connects the pancreas to the duodenum – the majority of the digestive enzymes enter the small intestine by this duct. To aid in lipid digestion, bile is secreted by the liver (stored in the gallbladder). Bile emulsifies lipids which gives them a larger surface area, increasing enzyme efficiency.

The small intestine joins to the large intestine, which consists of the caecum, colon and rectum. In carnivores the caecum has no function (as it is used in herbivores/omnivores as a site of bacterial fermentation of plant matter). The colon absorbs minimal nutrients from the ingested food; instead its primary role is the reabsorption of water, vitamins and electrolytes from the mixture of food, saliva and gastric & pancreatic juices passing through. This prevents excessive water loss and therefore dehydration. The remnants are excreted via the rectum and anal sphincters.

Herbivores

Herbivores only consume plant material which is very difficult to digest. No vertebrates make an enzyme capable of breaking down cellulose, the tough sugar that makes up plant cell walls which is unfortunate as its digestion yields glucose. As the diet includes large amounts of fibre the digestive tract of herbivores is comparatively much longer than carnivores, due to fibre being much more difficult to digest.

To overcome this herbivores have developed a symbiotic relationship with a population of microflora that inhabit a specialised region of the gut for fermentation e.g. the caecum or rumen of ruminants. The microflora population of the gut is able to breakdown cellulose and use the glucose for its own metabolic needs. As a waste product of this process, the microflora population releases volatile fatty acids (e.g. acetate, butyrate & propionate) which the herbivore utilises for energy. The production of these fatty acids is known as fermentation (fermentation also produces heat which keeps the animal warm).

There are two types of fermenting herbivores, those which ferment in the foregut and those which ferment in the hindgut. The difference between them is the site of fermentation and the organ used for fermentation; the attributes of the fermentation chamber remain the same however – Anaerobic, plenty of fluid, regulated pH, steady nitrogen supply and the correct temperature.

  • Foregut Fermentation – The majority of foregut fermenters are ruminants (including cow, sheep, goat, ox and deer) who ferment their food before it reaches the ‘true’ stomach. The stomach of a ruminant exists as four chambers which are the rumen, reticulum, omasum and abomasum (true stomach). Non-ruminant foregut fermenters (e.g. camels, llamas and whales) do not have the four distinct chambers; instead they simply have modifications to the gut before the true stomach which allows them to ferment. Ruminants digest food more efficiently than hindgut fermenters as they are able to consume food into the rumen – the site of fermentation, allow microbial digestion and then regurgitate the ‘cud’ and chew it some more. This means by the time the ingested food reaches the abomasum, all the extractable nutrients have been metabolised (some microflora from the rumen may also be digested in the abomasum which increases nutrient intake).
  • Hindgut Fermentation – Hindgut fermenters (e.g. e.g. elephant, horse, guinea pig, rabbit, herbivorous reptiles, e.g. tortoise and herbivorous birds) have a digestive system very similar to carnivores, except due to the large amounts of fibre and other difficult-to-digest components of the diet, the complete digestive tract is much longer. Hindgut fermenters also have a working, enlarged caecum which is the site of bacterial fermentation. The process of fermentation is the same as that of foregut fermenters, however as the caecum is located after the stomach and small intestine, the majority of food reaches the caecum undigested. Bacterial fermentation occurs in the caecum and colon allowing some volatile fatty acids to be absorbed, but then the digested food is excreted (along with the microflora). This is why some hindgut fermenters are seen eating their faeces – the food making up the faeces has been digested by the microflora making it of nutritional value. The ingestion of the faeces allows the restoration of the microflora population.

The foregut fermenter herbivores are a lot more efficient as the food is digested on the first pass through the digestive system. Unfortunately for hindgut fermenters digestion is more difficult; however they do have the ability to expel their microflora population which is useful during times such as hibernation.

Omnivores

Omnivores consume both meat and plant matter; they have a digestive system very similar to carnivores but also have a working caecum (not as well adapted as in herbivores). Due to this flexibility they are able to consume a wide diet, which has also prevented them losing the ability to synthesise certain products in the body (as in carnivores).

The process of digestion is extremely similar to carnivores, except a few minor adaptations which allow them to digested plant matter – although not as efficiently as herbivores.

Digestion by Species:

Many species have digestive systems very similar to those shown above; however there may be slight tweaks to the systems between the species, below are some examples.

Birds

Birds do not have teeth and so cannot chew; they are able to break up food however by using their beak. Only some species of bird (e.g. sparrow) are able to produce saliva with the amylase enzyme (for digestion of carbohydrates prior to the stomach).

When a bird swallows food, it passes down the oesophagus into a structure called the crop. The crop is primarily a storage area for food consumed by the bird, differing in size between species however certain adaptions in some species allow it to produce ‘crop milk’ which is rich in protein and fat and fed to the young during their first few days of life. Another adaptation found in one species so far, is that the crop acts as a foregut fermentation chamber.

The stomach of a bird exists as two parts, the proventriculus and the gizzard (ventriculus). As the bird is unable to chew, powerful muscles in the gizzard allow it to grind food up, the presence of grit in the gizzard aids this process.

From this point, the digestive system is similar to other species – small intestine, large intestine, etc.  until it reaches the end of the tract. At the end of the large intestine, the digestive tract opens into the cloaca which is simply a common exit, shared with the urinary and reproductive tracts. Shortly before the cloaca there is a pair of caecum or caeca, unlike the single caecum found in other species. (The caeca are rudimentary or absent in species such as hawks and parrots).

Rabbits

Rabbits are herbivorous hindgut fermenters able to rapidly pass food through their digestive system and quickly eliminate fibre. Due to this ability, he rabbit has remained small and agile, able to quickly escape predators.

Rabbits have a very typical herbivorous digestive system, the only differences are they are unable to vomit and have a large caecum.

One variation, which rabbits are well known for is that they consume their own faeces – coprophagia. This is due to them being hindgut fermenters and losing a fair amount of nutrients and vitamins in the faeces. The rabbit produces two forms of faeces:

  1. Hard, fibrous –No nutritional value
  2. Soft, caecotrophs – High protein content as well as vitamins B & K and volatile fatty acids. The caecotrophs are not digested/damaged in the stomach acid as a layer of mucus surrounds them and protects them, this allows all the nutrients and vitamins to be absorbed in the small intestine

Myomorphs (Rodents)

Myomorphs are omnivorous and have a very typical digestive system. Due to a structure between the oesophagus and cardiac region of the stomach however, it is almost impossible for them to regurgitate food.

Most of the rodents in this group lack a caecum or similar specific organ involved in the fermentation of cellulose, hamster however do have a foregut – similar to ruminants, which has a high pH and a large microflora population.

All myomorphs, like rabbits show some degree of coprophagia for the same reasons – to consume the vitamins and nutrients lost in the faecal pellets.

Sciuromorphs (Chipmunks)

Also an omnivore and has a very similar digestive system to myomorphs.

Hystricomorph (Guinea Pigs/Chinchilla)

The guinea pig is an herbivorous hindgut fermenter, which a large caecum for its body size – containing up to 65% of the total contents of the digestive tract at one time. As will myomorphs, guinea pigs exhibit coprophagia.

Chinchillas are very similar to guinea pigs, they have evolved however to survive on the nutritionally poor yet highly fibrous grasses of the Andes, this means over indulgence of highly nutritious treats (captive kept chinchillas) can cause fatal constipation or diarrhoea.

Chelonians (Turtles, Tortoises & Terrapins)

Chelonians lack teeth so have to use their horny, beak like structure to cut up food. The small intestine of chelonians is relatively short when compared to mammals. Like birds, reptiles have a common exit from the body – the cloaca (urinary, digestive and reproductive systems exit from here).

Snakes

Snakes are carnivorous and possess many teeth which are regularly replaced. Due to the shape of the snake (elongated) the digestive system remains the same, but all the organs are also elongated appropriately.

Lizards

The diets of lizards vary greatly, so the digestive system adapts accordingly, from herbivorous to insectivorous. Variations occur in the efficiency of the caecum (herbivorous/omnivorous lizards).

Fish

Predatory, carnivorous fish have ‘throat teeth’ located just before their oesophagus used for catching and holding prey. Structure of the digestive system are more tube shaped than in other species and can vary in length greatly, depending on the diet of the fish.