Nitrogen Excretion and Osmotic Regulation
All aquatic organisms must deal with the problem of maintaining internal homeostasis, a constant internal chemical environment in which that organism's enzymes can operate efficiently. As we saw earlier, this is largely a matter of osmoregulation, but other factors are involved also. Many marine organisms can get by with minimal osmoregulation since the oceans, where the enzymes evolved, are already a good environment; other marine organisms maintain their body fluids at ionic concentrations different from the surrounding ocean and must actively regulate ions. Freshwater also calls for active measures to maintain proper osmotic balance.
One method to avoid having to deal with osmotic balance is to cover the body with an impermeable membrane. Many aquatic organisms do just that, but this protection is necessarily incomplete, because three other processes involve intimate contact between a water-permeable body membrane and the surrounding fluid. In addition, these three other processes demand large surface areas in order to occur at sufficient rates to satisfy bodily needs. These three processes are, of course, respiration, absorption of food, and nitrogen excretion. Well, actually, only respiration is required of all aquatic organisms; plants do not ingest food (although they do need to take up plant nutrients), and plants do not have to excrete nitrogenous wastes, since they posses the chemical machinery needed to incorporate N into amino acids.
We will consider the elimination of nitrogen first, a process technically known as excretion. Excretion is a necessary consequence of protein breakdown; when proteins are converted to carbohydrates to provide energy, the amino group is removed and must be dealt with. In the body, the amino group is quickly oxidized to form ammonia (or, at high body pH the ammonium ion). Ammonia is highly toxic and highly soluble in water. If the organism has a sufficient source of water, ammonia can simply excreted in the water. This is the course taken by many (if not most) aquatic organisms, particularly those in freshwater. In any event, ammonia must be dealt with quickly because of its toxicity. Ammonia will diffuse passively out of respiratory structures such as gills. It takes a lot of water to dissolve and flush ammonia, however, and each ammonia molecule carries only one nitrogen.
Organisms with less fresh water available, such as some marine organisms and all terrestrial organisms, are not as likely to waste water excreting nitrogen one atom at a time. They will often invest some energy to convert the ammonia into urea, which is less toxic, has two nitrogen atoms, and therefore takes less water to excrete. Because it is less toxic, it can be allowed to accumulate in the blood to some extent, and many organisms have specialized organs to remove urea and other wastes from the blood and excrete them. Urea is commonly used as an excretory product in vertebrates, and is rarely used in invertebrates. Some organisms, such as sharks and snails, allow urea to accumulate in their blood to help with overall osmotic balance. Sharks, for instance, use urea in the blood to make them hyperosmotic in relation to seawater, thus they tend to gain water from the ocean and do not have to worry about dehydration.
Some organisms go to greater lengths still to deal with nitrogen. Where water is at a real premium, even the low toxicity and reduced water loss possible with urea excretion is not enough. Uric acid is a purine even less toxic than urea, and it precipitates from solution, allowing the 4 nitrogen atoms per uric acid molecule to be excreted with just enough water so that the crystals don't scratch on the way out. It has evolved in two groups with major water loss problems - terrestrial invertebrates and egg-laying vertebrates (obviously an embryo can't just step out for a drink, and whatever it excretes is going to be very close by until hatching). Figure 1 shows the three common nitrogenous wastes.
Figure 1. Common nitrogenous wastes of animals. Note that for each molecule excreted, ammonia will carry off 1 atom of N, urea 2, and uric acid 4. Ammonia is the most soluble, followed by urea and uric acid; the latter actually precipitates out of solution.
With the problems of respiration and nitrogen excretion settled (we will cover feeding later), we can now deal comprehensively with the issue of osmotic regulation. First, let's review the basic situations that aquatic organisms face.
The most abundant of the salts found in the oceans is NaCl, sodium chloride or table salt. We measure salinity in terms of the number of grams of dissolved salts in 1000 g (one l) of seawater. Seawater ranges in salinity, but a useful approximation is 35 g/kg; or 35 parts per thousand or 3.5%. Places like the Great Salt Lake, certain tidal pools, etc., can have higher salinities; most freshwater systems have dramatically lower salinities.
In seawater, the ions are dominated by Cl- (19.353 gr/kg), Na+ (10.76 gr/kg), SO4-2 (2.712 gr/kg), Mg+2 (1.294 gr/kg), Ca+2 (0.413 gr/kg), K+ (0.387 gr/kg), HCO3- (0.142 gr/kg), Br- (0.067 gr/kg), and Sr+2 (0.008 gr/kg); other ions are present in trace amounts, including gold. Freshwater contains similar ions, but the amounts are highly variable and depend on season, amount of rainfall, type of rocks, etc.
Imagine water of two different salinities. We say that the more saline solution is hypertonic (or hyperosmotic) in relation to the other, or that the less saline solution is hypotonic (or hypoosmotic) in relation to the other. If the salinities were the same, they would be isotonic (isosmotic). Osmoticity, then, is simply a comparison of the salinities (or, more accurately, the number of dissolved particles, including non-ionic compounds) of two solutions. It is important to remember that, in considering osmoticity, that you must have two different solutions to compare - it is a relative term.
Organisms have, of course, internal osmotic levels which may differ from those outside the body. Basically, the same three possibilities mentioned above exist, and these are illustrated in Figure 2.
Figure 2. Three possible osmotic situations.
Organisms in marine environments tend to be isotonic in relation to the seawater. In this case, they do not have to regulate ion levels, and are termed osmoconformers. They are typically restricted to narrow ranges of salinity (no great handicap in the ocean, where salinity changes are not common), and are thus termed stenohaline. Many marine organisms, both invertebrate and vertebrate, while they may be close to isotonic, will vary somewhat and need to regulate to a small extent. These organisms will have one or more of the adaptations listed below.
Major exceptions to the above include sharks and marine tetrapod vertebrates. Sharks maintain an internal environment which is hypertonic to seawater. They raise their internal osmoticity by retaining urea in their blood. As a result of being hypertonic, they tend to gain water from the seawater through their gills and the lining of their guts. The excess water is excreted as a dilute urine.
Marine tetrapod vertebrates, which evolved on land, have blood which is hypotonic to seawater. Since they breathe from the atmosphere directly, there are no respiratory surfaces in contact with the seawater, thus reducing the surface area over which water loss can occur. Still, these organisms do lose water when excreting urea (mammals) or uric acid (turtles, reptiles, birds), and when breathing; and they gain salt ions whenever they eat or drink. The only way for these organisms to obtain water is metabolically from the breakdown of carbohydrates, and by drinking seawater. This still leaves them facing net water loss and ion gain. There are two basic solutions to this problem. Turtles and birds have special salt glands (concentrations of chloride cells) near their eyes which actively pump Cl- ions out of the body; Na+ ions follow. Thus, birds and turtles can drink seawater and pump the excess ions out of their bodies, retaining the water. Similar cells are located on the gills of those marine fish (or invertebrates) with hypotonic body fluids.
Marine mammals have some of the most efficient kidneys known. Their kidneys can resorb most of the water from the urine, leaving a very concentrated solution of urea and salts to be excreted. They also are very efficient at removing water from the rectum, so that food wastes pass out with a minimum of water. By minimizing water loss in this way, marine mammals are able to survive on metabolic water.
Freshwater organisms (and many estuarine organisms) are hypertonic in relation to the water and thus face a constant influx of water from the surrounding hypotonic medium; they can potentially lose important ions to that solution also. Therefore, the strategy among most freshwater organisms is to cover as much of the body as possible with an impermeable coat, and leave all water exchange to a relatively small number of cells. These cells will maintain the water balance, and the remaining cells are bathed in an isotonic solution. Cells can maintain osmotic balance by using ATP to pump Cl- ions into the cell actively. These are the same chloride cells found in the salt glands of marine turtles, they just run in reverse. The inside of the cell becomes negatively charged, and other ions, such as Na+ come in because of this. Water that flows into the body of a freshwater organism moves into the blood and excreted as a dilute urine. Freshwater organisms, because of this active manipulation of their ionic balance, are called osmoregulators and are frequently tolerant of a wider range of osmotic concentrations, in other words, they are euryhaline.
Finally, organisms in hypersaline environments such as the Great Salt Lake face problems similar to those of marine fish with hypotonic body fluids. They must actively pump chloride and other ions out of the body, and obtain water by drinking.
Chloride cells are used by both marine and freshwater organisms to pump ions. In fish (both freshwater and marine), they are located on the gills (Fig 3). Because respiratory structures must have permeable surfaces for gas exchange, they are also a common place to put chloride cells on a body which is otherwise impervious to water flow. Another popular place is in the gut and kidneys; in both places ion concentrations are manipulated to get water to flow where the organism wants it to. Amphibians in freshwater locate these cells on their skin to absorb ions from the water. Chloride cells are also more common on organisms in habitats with changing salinity, such as temporary pools or tide pools. Aquatic mayfly larvae, which must use chloride calls on the gills (mayfly gills may be more osmoregulatory then respiratory in function) to pump ions into the body, increase the number of chloride cells as they move into more dilute water and decrease the number as the ionic concentration of the water increases.
Figure 3. Diagrams of two hypothetical "fish," one in freshwater and one in saltwater. Main sites of ion exchange are the gills and the excretory organs (kidneys). Osmotic exchange also takes place across the lining of the gut (not shown here). The freshwater fish gains water but loses ions passively across the gills; to compensate, the gills actively pump in ions and the kidneys form a dilute urine. The saltwater fish gains ions and loses water across the gills; to compensate, water is ingested (along with salt), the gills actively pump ions out of the body, and a small amount of concentrated urine is formed.
Osmoregulation and the Transition Between Habitats
As a final note, let's consider the evolutionary history of how different groups of organisms have come to colonize all the available habitats in terms of osmoregulatory adaptations.
Life originated in the oceans, and early organisms were probably isotonic, stenohaline, osmoconformers. Terrestrial arthropods, including insects, probably arose from marine arthropods which had developed water-saving adaptations in tidepools and other marginal marine habitats. Some of these organisms, i.e. insects, were later able to move into freshwater. Terrestrial vertebrates, on the other hand, no doubt arose from fish which had invaded freshwater. The freshwater fish had developed internal fluids more dilute than the oceans as a means of minimizing their osmotic regulatory needs in dilute freshwater, and the first terrestrial vertebrates, the amphibians, retained these relatively dilute body fluids. Vertebrate specialization on land required increased ability to deal with lack of water, and these water-conserving methods were useful when certain groups - marine turtles, crocodiles, birds, and mammals - returned to the sea. The main point here is that it was the osmoregulating groups that were able to colonize land, and this flexibility later allowed members of these groups to move into nearly all conceivable habitats (Fig. 4).
Figure 4. Major groups of osmoregulating animals and the mechanisms evolved to allow them to control osmotic levels in their habitats. Amphibians excrete both ammonia and urea; reptiles both uric acid and urea; crocodiles (not shown) excrete all three.
The failure of certain groups to make these transitions is also very interesting. Crustaceans, molluscs, echinoderms and a host of other marine groups have failed to move onto land. Those crustaceans and molluscs which have invaded terrestrial habitats are limited to moist areas, a consequence of the failure to develop efficient terrestrial respiratory structures comparable to the tracheal system of insects or the book lungs of arachnids. For the vast majority of marine organisms, however, the real problem is even deeper - the inability of the larval forms to survive in water with an ionic concentration much different from seawater. Most successful terrestrial organisms have elaborate mechanisms to protect the relatively delicate embryos from osmotic stress, while similar mechanisms are lacking for most marine phyla.
Further Reading (Osmoregulation)