Paper : 06 Physiology Module : 28 Osmoregulation in terrestrial

Development Team

Principal Investigator: Prof. Neeta Sehgal Department of Zoology, University of Delhi

Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi

Paper Coordinator: Prof. Rakesh Kumar Seth Department of Zoology, University of Delhi

Content Writer: Dr. Kapinder and Dr Haren Ram Chiary Kirori Mal College, University of Delhi

Content Reviewer: Prof. Neeta Sehgal Department of Zoology, University of Delhi

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Description of Module

Subject Name ZOOLOGY

Paper Name Zool 006 Animal Physiology

Module Name/Title Osmoregulation

Module Id M28: Osmoregulation in terrestrial animals

Keywords Osmoregulation, water balance, kangaroo rat, hyperosmotic urine, terrestrial environment.

Contents: 1. Learning outcomes 2. Introduction 3. Terrestrial environment 4. Moist skinned animals 4.1 Earthworm 4.2 Snails and 4.3 and other 5. 5.1 5.2 and 6. Water balance in terrestrial animals 6.1 Water loss: membrane permeability 6.2 Water loss in feces and urine 6.3 Water in food 7. Reptiles 8. Birds and mammals 9. Osmoregulatory organs in terrestrial animals 10. Summary

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

1. Learning Outcomes

After studying this module you shall be able to:  Understand the concept of osmoregulation.  Understand various factors which causes water imbalance in the terrestrial animals.  Learn mechanism of osmotic regulation in amphibians and other moist animals.  Learn osmotic regulatory mechanisms in arthropods, reptiles, birds and mammals.

2. Introduction

Maintaining a constant water balance is the key of terrestrial animals for their successful survival. Animals living in the marine have their body fluid in osmotic equilibrium with the surroundings. When living organisms started colonizing the terrestrial environment they faced more osmotically challenging and eventually water balance became paramount. For non-marine life of multicellular organisms, their successful survival depends upon maintaining the cells at more or less constant water balance. Regulation of osmotic balance of cellular and extracellular region depends upon the principles of permeability of membranes and surface-associated all over the organism, various active processes involved in regulating ion levels and control of intracellular osmotic effector levels. These processes for whole animal exhibit interactive and additive effects between various tissues and organs, with particular epithelia execute regulatory functions to maintain composition of blood and therefore providing osmotic stasis for rest of the body. A variety of regulatory mechanisms are used to maintain proper internal osmotic concentrations and to prevent the development of destructive osmotic pressure. A term „Osmoregulation‟ was coined by Hober (1902) which refers to the collective activities of the varieties of mechanism used by organisms to regulate water movement and water volumes. Osmoregulation implies the maintenance of an internal osmotic concentration different from that of the external medium.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

3. Terrestrial environment

The physiological advantage of animals on terrestrial habitat is the easy access to oxygen and their greatest physiological threat to life is dehydration. There is no opportunity for skin of terrestrial animals to uptake water or ions (exceptions do occur such as amphibians, snails etc), therefore skin and allows only the possibility of loss of ions and water. The loss of ions and water is also caused by fecal and urinary deposition. Moreover, loss of small amount of water and ions may also associated with reproduction and unpredictable losses from injury, whereas, some of the animals such as insects may also lose water during the process of molting or from secretions such as slime (slugs and snails), mucus (frogs), saliva, toxins and defensive fluids. The most successful evolution of terrestrial life occurs in only two animal phyla, arthropods and vertebrates that can survive even in the driest and hottest habitat found on the earth. As dehydration is a serious problem on terrestrial life and therefore those depends upon the selection of suitable moist habitat except arthropods and vertebrates and are terrestrial only in the technical terms of the word. For example, an earthworm requires moist environment in the soil for its survival and is susceptible to desiccation when exposed to open environment because it its body can resist very little to water loss (figure 1).

Figure 1: An earthworm living in the moist soil. Other animals include snails and frogs which have a high rate of evaporation (figure 2). There are large numbers of physical factors that can affect the rate of evaporation.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Figure 2: Animals living in a moist environment having higher evaporation rate (snail and ). (Source: http://hawaiinaturejournal.weebly.com/uploads/2/0/8/3/20833844/5986643_orig.jpg?316, http://www.dw.com/image/38489366_401.jpg)

The process of evaporation is driven by vapor pressure difference between air at the body surface and surrounding air. The rate of evaporation increase with an increase in temperature on the free water surface and it is faster on dry environment as compared to humid conditions. The water vapour pressure at mammalian body temperature (380C) is about double (50 mm Hg) when compare to room temperature at 25oC (24 mm Hg) and more than ten times at freezing point (4.6 mm Hg) (figure 3).

Figure 3: The rate of evaporation driven by vapor pressure difference and temperature.

If some water is already present in the air, then the vapour pressure of water on the free surface will be less. To measure the rough approximation of this driving force for evaporation some scientist used the term saturation deficit. It is defined as the difference between the

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

vapour pressure over free water surface at the temperature in question and water vapour pressure in the air. If relative humidity of air is 50%, the saturation deficit increases with temperature. The extent of air movement also influences the rate of evaporation in the terrestrial organisms. In the presence of wind, air close to the body surface of the animal is frequently renewed and leads to increase in evaporation. That‟s why we feel cooling effect when moist skin is in contact with the wind. The cooling of body surface also changes the density of air adjacent to body, produces convention current. The extent of such convention current differs between a horizontal and vertical surface. As a consequence, under identical physical conditions, the evaporation from a moist surface varies with the orientation of the surface. Finally evaporation also depends upon the curvature of a surface.

4. Moist skinned animals

In the moist skinned animals, evaporation is high and rate of evaporation is determined by the transfer of water vapour into the surrounding air. It is described as the vapour limited system in which the resistance to evaporation lies in water transport in the air. 4.1 Earthworm: An earthworm when kept in open in dry air looses weight very rapidly due to loss of water and dies. When a partially dehydrated worm is kept in the U-shaped tube and covered with water but mouth and anus remains outside the water surface, the worm reabsorb water from the surroundings. The membrane of the earthworm is permeable to water in both the direction. Table 1: Evaporation of water from body surface in different animals at room temperature (Schmidt-Nielsen, 1969) Name of Organism Water evaporation (microgram/hr/cm2 at vapour pressure difference of 1mm Hg) Earthworm 400 Frog 300 Salamander 600 Garden snail (active) 870 Rat 46 Iguana lizard 10 Mealworm 6 Man (not sweating) 48

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

In tunnel in the soil where the soil is fully saturated and is in contact with soil particles covered with thin film of water. An earthworm is unable to live in completely dry soil, where it rapidly becomes dehydrated and dies. Table 1 represents water evaporation from body surface of different terrestrial animals 4.2 Snails and slugs Snails and slugs are also having moist skin and evaporation through skin is also high. A naked depends upon the humidity of their habitat and they are active after rain and at night when relative humidity is high. Otherwise, they withdraw to and remain in microhabitat where the humidity is comparatively high. Snail, in contrast, carries water impermeable shell with them into which they can withdraw. A terrestrial snail that is inactive or aestivates has an excellent barrier to water loss. It withdraws into its shell and covers the opening with a membrane, an epiphragm (operculum) which consists of dry mucus, in some cases it also consists of large amount of crystalline calcium carbonate. The lower rate of water loss from an inactive snail in the shell and covered with an epiphragm permit some species to survive in hot, dry desert area. One such snail , is present in desert surface of near east.

Figure 4: Snail Sphincterochila is present in desert surface of near east. (Source: http://www.biodiversidadvirtual.org/insectarium/data/media/8196/Sphincterochila-baetica-360426.jpg)

Withdrawn into the shell and dorman, this snail can be found on desert surface in midsummer. This snail becomes active after rain, which is concentrated in the winter month from November to march when they feed and reproduce. During the summer they remain dormant. The water loss from dormant Sphincterochila is less than 0.5 g water; it can survive

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

for several years without becoming severely dehydrated. In fact snail itself contains over 80% water. This high water content remains same even during hottest and dry part of year. 4.3 Frogs and other amphibians A frog has a moist and cool skin and the rate of evaporation from skin of the frog is similar to that of an earthworm. The magnitude of the evaporation is of the same magnitude as evaporation from a free water surface and skin does not exhibit significant barrier to evaporation. Therefore, amphibians that are terrestrial live near water and humid place where evaporation process is very low. When these animals enter into the water they behave osmotically like fresh water animals. Amphibians are generally lived near water but some species of frogs are found in the dry arid interior of Australia. These animals retreat to burrows several feet deep in the ground where they aestivate during the long periods of drought. They required water for reproduction therefore they breed only after rainfall which provides a sufficient but temporary supply of water. When it rains, these frogs reappear from their burrows, restore their water content and deposit their eggs which develop into tadpoles at exceptionally fast speed and metamorphose into adult before the water dries up. When the frogs enter into aestivation again, their urinary bladder is filled with dilute urine which is of considerable importance in their water balance. Some can store as much as 30% of their gross body weight as urine in their urinary bladder. The urine is dilute and has an osmotic concentration corresponding to less than 0.1% NaCl. This urine is main water reserve and it gradually depleted during aestivation. Australian aborigines use the desert frog (Chiroleptes) as a source of drinking water because frog store diluted urine in the urinary bladder that resembles like knobbly tennis ball. Once the animal has used up all water reserve in the urinary bladder, further dehydration causes increased body concentration and the animals then undergo progressive dehydration of blood and tissue. In the amphibians, cutaneous evaporation does not exhibit major factor to prevent the water loss. However, there are several exceptions in which skin is found to be effectively control water loss. The South African frog Chiromantis does not depend on a humid environment to prevent rapid dehydration. When these frogs were kept in dry air it loses water by

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

evaporation in smaller fraction as compare with the other frogs and water loss is similar to that of the reptiles. In South American frogs of genus Phyllomedusa also exhibit very low rate of water loss through skin that is about one twentieth times than other frog species. These frogs has specialized glands in the skin that secretes a waxy substance that makes the skin surface hydrophobic and thus water proof it. These frogs also bear some specialized physiological characteristic that is totally different from other frogs. Unlike other frogs which secrete urea, these frogs excrete uric acid that is characteristics of reptiles which is important for their water balance. 5. Arthropods The most successful terrestrial animals on the earth are found among the arthropods which comprise insects, crustaceans and other animals characterized by jointed legs, rigid exoskeleton. Arthropods living on land are usually covered by a dense and stiff cuticle of very complex structure with complicated waterproofing properties, whereas land bears thick outer layer of dead skin cells called as keratinized layer. Insects are more numerous with respect to number of species. Insects and arachnids are primarily terrestrial and are adapted for terrestrial life and respiration through air. Only a small number has secondarily invaded the fresh water and no marine forms are found in these two classes. Two additional classes millipedes (Diplopoda) and the centipedes (Chilopoda) are also terrestrial. In contrast most of the crustaceans are aquatic. 5.1 Crustaceans The semiterrestrial and terrestrial crabs are generally excellent regulators. Some land crabs (Cardiosoma and Gecarcinus) in their ability to take up water from damp sand or a moist substratum, even in the absence of visible amount of free water. The common terrestrial isopods known as pill bugs, sowbugs, or woodlice are mostly found in humid habitats, well away from exposure. They remain hidden during the day and move around night when relative humidity is higher. These crustaceans are completely independent of free water from reproduction and are thus truly terrestrial. Compared with that of an , the cuticle of insect is covered by a thin layer of wax, which greatly reduces water loss.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

5.2 Insects and arachnids Insects are most successful animals on the earth and are more abundant than all of other animals combined. They lived in almost all the habitat on land and in fresh water. Clothes moths that thrive on a woolen garment or flour beetles that complete their life cycle from egg to adult in dry flour.

6. Water balance in terrestrial animals

For any organism to maintain water balance, all loss of water must be balanced by an equal gain of water over a period of time. The components involved in water balance are: Water loss Water gain Through evaporation Drinking of water From body surface Uptake via body surface From respiratory system From air Through urine and feces Water in food Other specialized secretion Oxidation water (metabolic water)

A terrestrial insect living in a dry habitat has very limited water intake and all losses must be reduced to such a level that their sum in the long run does not exceed the total gain. Any organism can tolerate a certain level of variation in its water content and some of them are more tolerant than others (table 2). Hygrophiles such as earthworms, snails and slugs can often survive with loss of 40% to 80% water; however this loss of water causes extensive shrinkage of the body (table 2). Many insects with terrestrial habitat can survive with the loss of 30% to 50% loss of body water, whereas anurans such as toads and frogs living in the same environments can survive with 30% loss of body water. However, most of the terrestrial birds and mammals of xerophilic condition cannot survive with the loss of more than 5% to 10% of water except camel that can exceptionally tolerate 30% water loss.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Table 2: Tolerance of water loss (as maximum % weight loss) in different terrestrial or semiterrestrial animals. Allolobophora (earthworm) 75 Patella (limpet) 35–60 Helix (snail) 45–50 Limax (slug) 80 Sphincterochila (desert snail) 50–55 Gecarcinus (Crabs) 15–18 Uca 18 Temperate beetles 25–45 Temperate roaches 25–35 Desert , 40–70 Desert tenebrionid beetles 60–75 Rana 28–35 Scaphiopus 45–48 Bufo 42–45 Small birds 4–8 Rat 12–15 Camel 30 Human 10–12

6.1 Water loss: membrane permeability Water loss takes place both from respiratory organs and from the general body surface. The respiratory organ of insects and vertebrates differ greatly in structure and function (figure 5). The lining of the vertebrate lung is always moist, the respiratory air is saturated with water vapour and the water loss from respiratory tract is substantial. The respiratory organ of insect however, consists of tubes lined with chitin and only finest branch of these are relatively permeable to water. In the most xeric insects and arachnids, the presence of a hard, dry, waxy epicuticle ensures much slower water loss when compared to similar sized crustaceans with similar cuticle design but very small or no waxy component. This complex combination of epidermis, cuticle and waxy epicuticle provides a complete covering over the body that is adequately waterproof to allow survival of these animals in desert conditions where no water is available at all.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Figure 5: Comparison of respiratory system of insect and vertebrates. (source:https://media1.britannica.com/eb-media/16/55316-004-F93586DF.jpg, http://slideplayer.com/slide/4651144/15/images/9/Respiratory+Systems+of+Vertebrates.jpg)

However, the amniotic vertebrates such as reptiles, birds and mammals exhibits more impermeable skin layer lacking respiratory function to compromise their osmoregulatory function. Their skin is formed from several layers of dead keratinized cells lying over the living epidermis. Some time they thickened into plates and scales in case of reptiles or it modified into hair or feathers in the case of mammals and birds respectively. In mammalian skin, lipid is present in a dispersed fashion, whereas, in reptile skin, the stratum corneum specifically has high content of phospholipid. Due to all of the above adaptation of the skin layer, these animals are less susceptible to water loss as compare to and amphibians. In spite of all these adaptations, these vertebrates still do not reaches the extent of waterproofing present in the arthropods. The water loss through cutaneous layer in some of these groups of animals can be altered more significantly through planned and controlled losses related to cooling, the phenomenon of sweating. Mammals have lower water loss from their sweat glands superimposed on the loss through rest of the skin, with a more rapid loss that occurred during exercise and in hot climates. Similarly, several waterproof species of frogs can effectively sweat through their epidermis when there is stress of heat and few insects also sweat when gets overheated.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

6.2 Water loss in feces and urine Insects eliminate their feces and urine through the same opening, the anus. The urine is formed by malpighian tubules, which opens into the posterior part of the gut. Liquid urine as well as fecal material from the intestine enters the rectum, where water reabsorption takes place. If excretory product instead of being eliminated in the urine, are withheld in the body, no water is expanded for their excretion. Because uric acid is highly insoluble compound, its retention in the body is in fact a feasible approach to the problem of excretion and the deposition of uric acid in various part of the insect body seems to be a regular feature, for example, in several species of cockroaches, as much as 10% of the total dry weight of the body may be uric acid. An insect that lives in very dry surrounding (eg meal worm) withdraws water from the rectal contents until the fecal pellets are extremely dry. This withdrawal of water has active transport of water as water moves from a high osmotic concentration in the rectum to a lower concentration in the blood. In general, most uphill transport of water in animal system can be explained by a primary transport of a solute, with water following passively due to osmotic forces, acting as explained by three compartment theory proposed by Curran in 1960.

0.1 M sucrose 0.5 M sucrose Water

Graduated pipette

A B C

Wire mesh Cellophane Porous glass membrane disc Figure 6: The Curran model to explain net movement of water against an osmotic gradient between chamber A, B and C.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

This hypothesis can be explained by figure in which cylinder is divided into three compartments. Compartment A and B are separated by a cellophane membrane, which is readily permeable to water but not to sucrose. Compartment B and C are separated by a porous glass disc. Compartment A is filled with 0.1 molar sucrose solution, compartment B with 0.5 molar sucrose solution and compartment C with distilled water. The concentrated solution in B will immediately draw water osmotically from A and C but if the liquid in B is kept from expanding, it will flow through the porous disc into compartment C. the overall result is therefore a movement of water from A to C, although the osmotic concentration in A is higher than in C. the movement of water against the overall the osmotic gradient obviously cannot continue forever, for the concentration differences will gradually disappear and the system run down. However, if somehow we can maintain a high solute concentration in compartment B, the water movement will continue. Movement of water through various biological membranes can be explained by Curran model.

Figure 7: Three compartment model for water transport across an epithelial cell. Arrow indicates active sodium ion transport and dotted area corresponds to compartment B in figure 6.

Figure 7 is represents an epithelial cell, which has deep infoldings of the membrane on the side facing the tissue fluid. Active transport of sodium establishes a high osmotic concentration within lumen of infoldings. Then, because of osmotic forces, water diffuses into the lumen and the increased hydrostatic pressure causes a bulk flow of liquid through the opening into the tissue fluid. The result is an overall movement of water from the outside

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

lumen to the tissue fluid. This water transport, which can be against an overall osmotic gradient from the outside of the cell to the tissue fluid, depends on the active transport of sodium as the primary driving force. 6.3 Water in food Insects that eat plant may obtain large amount of water in the food, for fresh vegetable material has a high content of water. When plentiful of water is present in the food, the problem is to eliminate the excess. This is the normal function of the kidney, which then produce dilute urine in large quantities. Under dry condition the function is to remove waste materials with minimum of water and produce concentrated urine. The animals living in dry condition, gain water from the oxidation of food. During food oxidation, the amount of water formed is depends upon the amount of hydrogen present in the food. One gram of glucose produces 0.60 gm water and other carbohydrate such as starch produces slightly less water because of lower hydrogen contents. The oxidation of starch produces 0.56 gm water per gram of starch. Oxidation of fat gives more oxidation water than carbohydrate (1.07 g water/gm fat). It also varies slightly with composition of fat and the degree of saturation. Protein metabolism is more complex because nitrogen present in the protein form excretory products which also associated with hydrogen. The amount of oxidation water depends on the nature of the end product of protein metabolism. If it is urea, the amount of oxidation water formed is 0.39 gm/gm of protein and if excretory product is uric acid, water formed is higher. Table 3: Arthropods that can absorb water directly from atmosphere but their absorption process is limited by relative humidity below which there is no net gain of water.

Name of organism Limiting relative humidity (%) Tick 94 Mealworm 90 Mite 90 Desert roach 83 82 Flea 50

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Some terrestrial insects and arachnids are able to absorb water vapour directly from atmospheric air (table 3). The desert roach Arenivaga takes up the water from the atmospheric air when it is partly dehydrated. If it has been placed in a very dry atmosphere until, say, 10% of the body weight has been lost, uptake of water begins when animal is moved to any relative humidity above 83%. The uptake of water will be continuing till the animal become fully hydrated then it ceases. One hypothesis for the uptake of water vapour is that temperature micro gradients within the animal lead to condensation of water vapour.

7. Reptiles

The skin of the reptile is dry and scaly and has been assumed to be impermeable to water. The cutaneous evaporation in a dry habitat reptile is only a small fraction of that in an aquatic reptile and that even in the aquatic reptile evaporation is one magnitude lower than in moist skinned animals such as frogs. It is suppose that evaporation from moist respiratory tract would be more than the dry skin of the reptiles. However it is not true as the evaporation through skin is always more as compare to respiration by two or more folds. For example, chuckawalla, a desert lizard loses two third of water through skin and only one third of water loss through respiration (figure 8). In addition of water lost through evaporation, water is also needed for urine formation. Reptiles excrete uric acid as an end product of protein metabolism. Uric acid is highly insoluble therefore it requires very small amount of water for its excretion.

Figure 8: Chuckawalla, a desert lizard. (Source: https://i.ytimg.com/vi/gJ6u8kE4HpM/maxresdefault.jpg )

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

8. Birds and mammals

Birds and mammals use water to keep their body cool during hot surroundings. Human and some other mammals sweat whereas dog and some mammals pant which keep them cool in warm conditions. Rodents provide the classical example of maintaining the water balance as they do not pant and sufficient number of skin glands for the regulation of heat. In most of the deserts, free water is rarely available during rainy seasons. Several birds and small rodents live in the area where there is rain only once a year or even less often. These animals obtain most of the required water through food and very small fraction is from dew. Many animals obtain water from green leaves, stem, roots, fruits, tuber and other part of the plants whereas; carnivores obtain water from the fluid of the prey that contains 50% to 80% of water.

Figure 9: Kangaroo rat. (Source: https://upload.wikimedia.org/wikipedia/commons/e/ea/Tipton_kangaroo_rat_8046-5_1991.jpg)

The kangaroo rat and pocket mice are found in deserts of North America (figure 9). They survive primarily on dry seeds and other dry plant material and intake of free water is minimal. A kangaroo rat contains about 66% of body water. Even when it survived on dry food such as barley or oats for weeks or months, its water content remains the same. When 35 g of kangaroo rat provided with 100 gram of barley, it consume the food in one month. The total water derived from the oxidation of food is 54 gram. The grains also have some free water which depends upon the relative humidity (RH). At 20% Rh, about 6 grams of free

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

water is available in 100 gm of barley, which give total 60 gm of water in the food. On the other hand, nearly one quarter of available water is used for the urine formation for excreting the nitrogenous waste. It has developed a remarkable ability to concentrate the urine to minimize the water loss. Table: Total metabolism of water in kangaroo rat during a period in which it consumes and metabolizes 100 gm barley balancing overall water gain and losses at air temperature 250C and RH 20% (Schmidt-Nielsen, 1964)

Water gain ml Water losses ml Oxidation water 54 Urine 13.5 Absorbed water 6 Feces 2.6 -- -- Evaporation 43.9 Total water gain 60 Total water loss 60

The most important factor of water loss is evaporation which generally occurs through respiratory tract. During lower atmospheric humidity, there is slightly reduction in water gain as small amount of free water is absorbed by the grain. At the same time, in drier air more water is lost through evaporated from respiratory tract. Kangaroo rat mostly spends its time in the burrows in which the humidity of air is higher than the outside which aid in their water balance. Loss of water through respiratory evaporation is reduced exhalation of air at a lower temperature than the body core. Although air in the lungs is at body core temperature and saturated with water, it is cooled as it passes out through the nose. Camels in the desert are adapted to further reduce the water content during exhalation of air through lungs. These animals not only decrease the temperature of exhaled air but they also remove excess water vapour from the air during exhalation. The combination of both cooling and desaturation of the exhaled air makes the camel to save 60% of water content relative to the loss of water content through exhalation of saturated air at body temperature.

9. Osmoregulatory organs in terrestrial animals

Some of the terrestrial animals can control their osmotic regulation through all or part of their general body surface. However, majority of terrestrial animals have specialized internal organ known as excretory organ or kidney through which they regulate their body water content as

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

well as a site for regulating salt balance. In fact the primary role of excretory systems of land animals probably had osmoregulation. Animals living in the moist environment (soil) tend to have similar osmoregulatory systems as their aquatic relatives. This is due to the fact that they often have an excess of available water in their surrounding and have to quickly excrete it. Earthworms may require a system that can remove excess water but it also act as water conservers during dry season. Animals living in the drought condition (Xerophilic) always required conserving as much water as possible. Most of the animals, except mammals, have combined their excretory opening with their gut opening to regulate removal of excreta at single site (the rectum) with minimal loss of water. Although there are great variation of osmoregulatory organs in their origin, size and apparent complexity, even then there are few common physiological principles operating in all of them. Almost all of them consist of one to several tubular structures that includes an initial collecting region where primary urine is formed, then one or more regions in the tubule where primary urine is further modified by either addition or removal of particular solutes. Many of these tubules include an even more distal area in which the urine can be made more concentrated (hyperosmotic) or more dilute (hyposmotic) than the internal body fluids. Soil dwellers such as earthworms have a nephridial system having extensive coiled resorptive tubular portion that is organized segmentally and exiting outside the body through nephridiopore (figure 10). The excretory product is hyposmotic in most of oligochaete species. In some larger earthworms, such as Pheretima, the nephridia are “enteronephric”, that opens into the gut instead to the outside of body, here a little drying of feces and urine occurs within the rectum.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Figure 10: Nephridial system of earthworm. (Source: https://urinarysystems.weebly.com/uploads/1/9/5/6/19562951/2535259.png?406) Terrestrial prosobranch snails have kidney, similar to their littoral and marine relatives, in which fluid flows from the pericardium and drains directly it directly to the outside with no special absorptive area. In these animals, excretion involves the loss of substantial amount of water because there is no spatial separation of the regions for nitrogenous excretion and regulation of water balance. Semiterrestrial crabs that lives at the top of beaches and in damp mangrove forest of the tropics, exhibits quite strong ability of osmoregulation. However, the osmoregulation is not achieved at the antennal glands, a kidney, as these glands only produce isosmotic urine. In woodlice, urine is released from their maxillary glands through capillary channels to flow over the flat ventral respiratory flaps called as pleopods. This flow of urine over the pleopods keep them moist for exchange of gases and it also allows ammonia to diffuse away which help in disposing the nitrogenous waste outside the body of woodlouse. However, some of the excreted water is reabsorbed through rectum and returned to the body, provides an unusual involvement of the gut in that fluid arrives in it via an external route. Insects and several myriapods have an excretory and osmoregulatory system based on the “Malpighian tubules and rectum” composite, in which urine formation and resorption occurs in widely separated structures. Production of urine occurred by secretion in the malpighian tubule cells. The hindgut of insect has various specializations to effectively resorb water and also to regulate the combined composition of the urine and feces.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Figure 11: Osmoregulatory and excretory system of an insect. (Source: https://www.prettysmile.xyz/wp-content/uploads/2016/11/malpighian-tubules.bmp)

Amphibians indeed inherited an essentially freshwater-adapted kidney from their ancestors that can produce only hyposmotic urine (figure 12). These animals have the ability to decrease urine formation rate and also increases urine resorption process from the bladder with the action of amphibian antidiuretic hormone. It can also regulate their osmotic balance with active salt uptake through the skin.

Figure 12: Amphibian kidney inherited from their aquatic ancestors. (Source: http://www.biology-pages.info/F/FreshwaterKidneys.gif, http://lh4.ggpht.com/-JgOeTT6E2Ds/Uxb6TaG8sOI/AAAAAAAAC- 0/L8og_vU-EVQ/frog-excretory-system%25255B11%25255D.jpg?imgmax=800)

Reptiles are not able to produce hyperosmotic urine, even when these are placed in extremely dry habitats (figure 13). They produce isosmotic urine and excretion of excessive salts occurred through their nasal salt glands. Moreover, several birds and mammals also do

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

not produce hyperosmotic urine. However, in the presence of plenty of drinking water or where the diet rich in water (fruit bats, hummingbirds), many of them produces hyposmotic urine. In water stressed condition, they generally produce isosmotic urine. Some of the birds and reptiles species releases their urine into the cloaca from where it moves back into the rectum where further resorption of water occur which produces nearly isosmotic urine.

Figure 13: Excretory system of reptile. (Source: http://www.biology-pages.info/R/ReptileKidney.gif)

Few birds and almost all mammals have a loop of Henle which is interposed between the ascending and descending parts of their nephrons that gives a site of countercurrent mechanism (figure 14). These mechanisms make the kidney of the animals to produce hyperosmotic urine when needed. In the above mechanism, the cells in the hairpin tip of the countercurrent loop are exposed to high levels of urea and are almost anoxic; they manage by incorporating high levels of betaine and sorbitol into their own cytoplasm as counteracting osmotic effectors.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

Figure 14: Comparison of avian and mammalian nephron. (Source:http://people.eku.edu/ritchisong/554images/avian_nephron_9.gif, https://www2.estrellamountain.edu/faculty/farabee/biobk/excretsys_3.gif)

The length of the loop of Henle is not the only key factor for concentrating the urine. For example, the desert mouse Notomys has up to 5.2 mm long loops that can produce urine of 9370 mOsm, whereas, the horse has up to 36 mm long loops and can produce urine of only 1900 mOsm. Indeed, urine-concentrating ability in the birds generally decreases with increasing length of the loop, perhaps partly because most of their loops placed crosscurrent rather than countercurrent to blood flow.

The concentration of the urine is mostly determined by the specific metabolic rate of the tubule tissues (higher in smaller animals) than by tubule length. Most tetrapods excrete either urea (mammals) or uric acid (birds and reptiles). However, in some tetrapods composition of diet also causes certain variation in urine. For example, humming birds are able to switch to secrete ammonia as nitrogenous waste when they feed on dilute nectars at low temperatures.

10. Summary

 Maintaining a constant water balance is the key to terrestrial animals for their successful survival.  Osmoregulation refers to the maintenance of an internal osmotic concentration of the organisms that are different from that of the external medium.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals

 The physiological advantage for terrestrial animals is the easy availability of oxygen and major threat is danger of dehydration.  In moist skinned animals, evaporation is high and rate of evaporation is determined by the transfer of water vapour into the surrounding air.  For maintaining water balance in an organism, overall loss of water through different processes must be balanced by an equal gain of water.  Water loss generally takes place both from respiratory organs and from the general body surface in terrestrial animals.  In the most insects and arachnids the presence of a hard, dry, waxy epicuticle insures minimum loss of water as compare to similar sized crustaceans.  The animals living in dry condition, gain water from the oxidation of food in which the amount of formed water is depends upon the number of hydrogen present in the food.  Birds and mammals use water to keep their body cool during hot surroundings.  Rodents provide the classical example of maintaining the water balance as they do not pant and sufficient number of skin glands for the regulation of heat.  The excretory system of terrestrial animal primary function as an osmoregulatory organ. These animals therefore show a great variety of mechanisms of excretory water loss and their regulation.

Animal Physiology ZOOLOGY Osmoregulation in terrestrial animals