Respiration in Aquatic Organisms
Most aquatic animals need to obtain O2 from the surrounding water in order to carry on cellular respiration. As we have seen, the amount of O2 in water is limited, and both O2 solubility and demand are correlated with temperature. At most, there is only about 15 mg of O2 per liter of water. In order to carry out the chemical reactions needed to maintain life and reproduce, aquatic organisms must be able to efficiently extract that 15 mg of O2 from the water.
The primary method of O2 transport is simple diffusion. Since all molecules are always in motion (except at 0 K), they will tend to move randomly. If they are highly concentrated in one spot, they will be least likely to move towards that spot, as opposed to moving to any of the other spots in the environment. If you divide a card deck into the red cards and the black cards, and randomly move two cards from each deck into the other, you are more likely to move red cards into the black pile, and black cards into the red pile, than you are to move red cards into the red pile or black cards into the black pile, at least until there are about equal numbers of red and black cards in both piles.
Because the speed with which O2 molecules move in water at normal temperatures is fixed, we can make some estimates over the distances at which simple diffusion can take place in both water and body fluids; that distance appears to be about 1 mm. If a cell is no more than 1 mm from water with sufficient O2, then no special adaptations are needed for obtaining O2. If the O2 concentration of the water is low, or if the cell is greater than 2mm in diameter, or if the organism is multicellular, with some cells buried inside the body, then special measures are necessary.
One of the simplest measures is cyclosis, the internal streaming of the cytoplasm of a cell. Cyclosis can help distribute O2 within a cell, but even it has limits, as we shall see. Consider what happens to a cell as it grows. If we are talking about a spherical cell, its volume grows according to the formula v = 0.5236d3 (d = diameter), while its surface area grows according to the equation a = 3.1416d2. The volume is an index of how many O2 requiring enzymes are present, the surface area represents the "gateway" through which the O2 must pass. If the volume increases faster than the surface area - and it does, with the volume increasing by a factor of d3 and the surface area by only a factor of d2 - than the cell will soon reach the point where O2 will not be able to enter the cell fast enough. Similar arguments are used to determine the capacities of rooms with various numbers of doors when writing fire codes.
Thus we see that the problem of respiration in aquatic systems is a combination of the amount of dissolved O2 present, the distance over which diffusion can take place, and the surface area/volume ratio of the organism to be served. Taken together, these factors suggest that life in water should be restricted to very small organisms where diffusion distances are short and surface/volume ratios are high. The fact that large organisms are common in water suggests that there is a way around these restrictions, and, in fact, a number of methods are employed by large aquatic organisms to obtain enough O2; we will turn to those solutions next.
Obviously, one solution is to avoid the problem altogether. There are two basic mechanisms to avoid the problems of O2 uptake in aquatic systems mentioned above, and many aquatic organisms use both to some extent. They are to stay small and to have a low metabolic rate. Small size avoids the problems with diffusion distances and surface/volume ratios mentioned above; low metabolic rates decrease the need for O2. Many aquatic organisms, including most larvae, are small enough that simple diffusion will suffice to supply O2. Low metabolism is also possible, and, particularly at low temperatures, almost unavoidable for small organisms. Unfortunately, warm conditions will raise metabolism rates, often above levels, which can be matched by O2 uptake; this, coupled with the decreased solubility of O2 at higher temperatures, may define upper temperature limits for many aquatic organisms.
Problems with small size include the inability to counter strong currents, susceptibility to predation, decreased utility of predation as an energy source (it's hard to feed on organisms as big as you are; smaller ones are better), and decreased ability to control internal conditions such as osmotic pressure and temperature. Natural selective pressures for larger organisms no doubt played a role in developing some of the other solutions to O2 uptake in water.
To maintain a high metabolism, or to increase body size, other strategies must come into play. The most obvious is to increase surface area without increasing volume, and there are two basic ways to do this. The first is to alter the shape of the organism. We looked at what happens with spheres, where surface/volume ratios decrease as size (diameter) increases. The same relationship holds for other shapes, but it is somewhat diminished for long, thin shapes. For example, let us consider cubes and rectangular boxes. A cube has its surface area equal to 6*L2, where L is the length of the sides, and its volume is equal to L3. Obviously, when the length exceeds 6 units, the surface/volume ratio will be less than one. For a cube 10 units long, the surface area is 600; the volume is 1,000; and the surface/volume ratio is 0.6. Consider a rectangular box 100 units long by 10 units wide and 1 unit high. Its volume is the product of its length times its width times its height or, in this case, 1,000, the same as our cube. Calculation of the surface area of such a box is a little more involved. There are two sides with an area of 100*10 or 1000; two sides with an area of 100*1 or 100; and two sides with an area of 1*10 or 10. Multiply and add up all the areas and you get a total surface area of 2,220 and a surface/volume ratio of 2.22, well above the value of 0.6 for a cube of the same volume.
Organisms that avoid blocky, compact shapes such as spheres and cubes, and tend towards shapes with at least one dimension greatly elongated maximize surface/volume ratios while maintaining a constant volume. This in turn allows for efficient gas exchange even without some of the additional systems we will mention below, and, as a bonus, has the effect of minimizing diffusion distances. For instance, in our cube and box, a cell at the middle of the cube would have been 5 units from the nearest surface; in the box no cell would be more than 0.5 units from the nearest surface; if the units were mm, then all cells in the box would have been able to receive O2 by diffusion.
In aquatic systems, many organisms apparently optimize their shape in this way. There are flattened organisms such as the simple Mesozoa and Placozoa, Platyhelminthes, Ulva the sea lettuce, nudibranchs, and others. Many seaweeds and aquatic vascular plants such as Vallisneria have flattened blades or leaves; in fact, the leaves of terrestrial vascular plants are also examples of this phenomenon, although here the surface area is maximized to provide for light capture, not O2 uptake. Other aquatic forms with large surface areas include the myriad vermiform (wormlike) phyla. Simple gas exchange across the surface of the body, whether the body is spherical or wormlike, is called cutaneous uptake, and it is an important source of O2 for many organisms, even complex ones with more advanced systems as discussed below.
The other way to maximize surface area without increasing volume is to include numerous small protrusions, evaginations or invaginations on the surface of the body. This approach is also common in nature and found in the gas exchange structures of both terrestrial and aquatic organisms, and also in filtration systems, digestive tracts, and even the surface of the human brain.
Some aquatic organisms can get by simply by having bodies with numerous evaginations or invaginations. Sea anemones and jellyfish, for instance, have large spaces inside their bodies that are continuous with the outside water, and cells adjoining these spaces are thus able to absorb O2 directly from the water contained there. No specialized structures are needed to move the O2 around the body, and normal feeding movements circulate the water in the spaces. Most aquatic organisms, however, that rely on either basic body shape, small size, or simple evaginations or invaginations, are limited to a life with low metabolic demands. They will be sluggish, slow moving, and unable to support much in the way of nervous tissue, a notorious consumer of O2. It has recently been suggested that the dorsal nerve cord of vertebrates evolved in response to the need for high O2 levels in nervous tissues; the prototypic vertebrate may have been a bottom feeder with the ventral side constantly exposed to anoxic conditions; this would have favored migration of the nervous tissue to the dorsal side (Bieri and Thuesen, 1990).
The evolution of larger, more complicated aquatic organisms capable of high metabolic rates required additional solutions to those already mentioned, although, it should be noted, the basic principles we have already seen provide the basis for the solutions employed by more complex organisms. Many of these organisms employ specialized respiratory structures called gills. Gills are basically areas of the body modified for gas exchange by virtue of being either highly evaginated or highly invaginated. The large surface area allows for rapid gas exchange; the only problem that remains is getting that O2 to the tissues that need it. For this, many organisms have evolved circulatory systems that pump an internal fluid through the gills to obtain O2 and then transport the O2 to metabolizing tissue. Often pigments, probably derived from pigments used in the electron transport chain, are used to carry the O2 in the circulatory fluid, or blood.
A basic circulatory system involves a heart to pump the blood and vessels to carry the blood. The vessels must branch enough to bring the O2 within diffusion distance of any cell in the body. Most organisms use an open circulatory system, where the blood is confined to vessels only part of the time, usually on the way to the tissues. After reaching the tissues, the blood drains back to the heart via sinuses. In a closed circulatory system the blood is always encased in vessels. The greater the demands placed on the circulatory system, the more likely it is to be a closed system; organisms with advanced nervous systems usually have such a system.
The pigments used to transport O2 are critical. Hemoglobin is the most familiar; it exists in a number of forms and is found throughout the animal kingdom in such taxa as vertebrates, echinoderms, molluscs, insects, crustaceans, annelids, nematodes, flatworms, and ciliates. Other pigments include hemocyanin, a copper-containing pigment found in molluscs, cephalopods, gastropods, crustaceans, and chelicerates; the iron-containing pigment hemerythrin found in sipunculans, polychaetes, priapulans, and brachiopods; and chlorocruorin, a third iron-containing pigment found in some polychaetes. These pigments generally bind O2 more strongly when O2 levels are high (in lungs or gills), and release it when O2 levels are low (in respiring tissues) (Fig. 1). All of the pigments exhibit the Bohr effect; they bind O2 more tightly under alkaline conditions and release it more readily under acid conditions. Thus O2 uptake is further facilitated in the respiratory structures where CO2 levels are low (and thus alkaline pH), and O2 release is facilitated at actively respiring tissues where there is excess CO2 and thus an acid pH. Some organisms, such as the midge Chironomus, use hemoglobin not so much as an O2 transport molecule as an O2 storage molecule, much the way myoglobin is used in vertebrate muscles.
Figure 1. Oxygen dissociation curves for human hemoglobin at three different pH levels. The S-shape of the curves is due to the fact that hemoglobin begins to absorb O2 rapidly when O2 levels are between 20 and 40 mm Hg. The Bohr effect is illustrated here by the shift of the curve to the right as pH decreases. Of course, pH levels are high in the lungs where CO2 escapes into the air; here O2 uptake is favored. In tissues, high CO2 levels lower pH, favoring O2 release from hemoglobin. Myoglobin and fetal hemoglobin, which must take up O2 from normal hemoglobin, have curves to the left of those shown here.
Organisms with very high metabolic rates, such as fish, go to even further lengths to optimize O2 uptake from the water. One of the most elegant adaptations is countercurrent flow (Fig. 2). Imagine a gill as a flat plate, with water flowing over the thin surface of the plate from front to back. Along the front edge of the gill is a vein; along the back edge is an artery. Oxygen-poor blood flows into the artery at the back of the plate and flows forward through the capillaries, counter to the current of water flowing back over the gill. Almost immediately, the blood will pick up O2 from the water, even though the water has already lost some O2 to the gill as it flowed over it. By the time the blood reaches the vein at the front of the gill the blood is nearly saturated with O2, yet the O2 does not leak back to the water because the water is saturated with O2. If numbers help, try it like this: the water at the front of the gill is 100% saturated, grading to 20% saturated as it reaches the back of the gill. Blood at the back of the gill is 10% saturated, grading to 99% saturated at the front of the gill. At every stage along the way, the blood has less O2 than the surrounding water, so O2 always flows into the blood, never out. If the blood flowed with the water current, the blood would at best become 50% saturated, for over 50% it would start losing O2 to the water. Countercurrent flows are the most efficient way to extract something from a fluid, and we will see them again in the heat-exchange mechanisms of aquatic vertebrates and the digestive systems of many organisms.
Figure 2. Countercurrent flow across a gill lamella in a fish. Deoxygenated blood flows from the artery at the posterior end of the lamella forward to the vein. As it moves forward, it encounters water moving posteriorly and will pick up O2 if the level of O2 in the water exceeds that of the blood. The countercurrent flow insures that this condition exists; the graph at the bottom shows the partial pressure of O2 in the blood and in the water at each point along the gill. Note that the O2 level of the blood is always lower than that of the blood, and is in the range where O2 uptake by hemoglobin is maximized (see Fig. 1). The dotted line shows where the O2 level of the blood would equilibrate if a countercurrent flow did not exist.
Aquatic vertebrates, other than most fish, must surface to use their respiratory organs, which are adapted for O2 uptake from the air. Many fish in stagnant waters also utilize a highly vascularized portion of their gut (the precursor of the lung) to supplement O2 uptake from their gills. Common aquarium catfish that regularly rush to the surface are doing this; if you notice them rushing often to the surface you may have a problem with the O2 level in the water. Amphibians probably obtain as much O2 through their porous, thin skin as they do from their lungs; in fact, there is a family of salamanders, the Plethodontidae, which do not have lungs at all! Frogs and salamanders can remain submerged for long periods of time in water. Turtles also can remain underwater for extended periods by obtaining O2 cutaneously, sometimes even pumping water in and out of vascularized regions in the throat and rectum to increase O2 uptake. This O2 uptake is sufficient to meet metabolic needs in cold weather, such as when the animals hibernate and metabolic activity is low. Reptiles and amphibians often have ways of shunting blood away from the lungs, which are useless underwater. Diving birds and mammals must maintain high metabolic rates, but have adaptations to ensure that critical organs, such as the brain, get enough O2. These adaptations are even present in humans, especially human children, and explain why it is not uncommon for people to survive extended periods underwater in cold water (which slows metabolism).
One last group deserves special mention at this point, primarily because it contradicts many of the statements made so far. It is not a trivial matter, either, because this group is the most successful of the animals, the insects. While most insect larvae utilize evaginations of the body surface - gills - to respire in water, aquatic adult insects are much different. All adult insects utilize tracheae, air tubes that branch throughout the body and deliver gaseous O2 directly to every cell. The tracheae are connected to the outside air through spiracles, holes in the body sometimes equipped with a closable flap. Tracheae are tiny, and, because they are not well ventilated, would not function if they had to deliver O2 in H2O - in other words, they must stay relatively dry to function.
How, then, do adult aquatic insects do it, that is, breathe, under H2O? Basically, they cheat by covering their body with hydrophobic hairs that prevent water from reaching the body or the spiracles. They trap a thin layer of air near the bodies, and thus the tracheae remain full of air also. Oxygen can diffuse into this air space and thus be delivered to the spiracles and tracheae. This arrangement is known as a physical gill or plastron (Fig. 3). The problem with this system, which is also used by aquatic spiders, is that as O2 is used up the bubble shrinks. The bubble can be maintained only in O2 saturated water; a few tiny beetle species living in cool, fast streams are the only insects that can maintain their bubbles indefinitely. All other insects must surface periodically to replenish their bubbles, and large, actively swimming insects in relatively stagnant waters do so relatively frequently. In fact, for a few species, observations have shown that such surfacing is so regular that experienced naturalists can guesstimate the O2 content or temperature of a body of water by observing how often certain species surface to replenish their air. Other insects obtain O2 by extending tubes through the water surface (mosquitoes, water scorpions), or even into aquatic plants.
Figure 3. Operation of a physical gill at the surface and at 1 m deep. Near the surface, the bubble initially resembles the gas composition of the surrounding water. As O2 is used by the beetle, the partial pressure of N2 increases, rising above that of the surrounding water and beginning a slow decrease in size of the bubble as N2 diffuses into the water. At one meter, the partial pressure of N2 is even greater and thus the N2 diffuses into the water faster. The beetle must return to the surface as the bubble shrinks. Because O2 is removed from the bubble by the beetle, additional O2 always diffuses into the bubble. CO2 always moves readily from the bubble into the water.
Because insects use a tracheal system to move O2, this burden is removed from the circulatory system. The circulatory system of insects is thus decidedly crude when compared to such groups as the crustaceans or molluscs with similar metabolic demands. Insects posses an open circulatory system which does not have any O2 carrying pigments; their circulatory system functions mainly to distribute food and heat.
Finally, in our discussion of respiratory systems we have ignored the problem of CO2 release. For aquatic organisms, this is rarely a problem since CO2 readily goes into solution and is carried off in the water. If there is enough surface area for O2 uptake, there will certainly be enough for CO2 dispersal. Of more critical importance is the osmoregulatory burden imposed by respiratory systems; every bit of surface area available for gas exchange is a surface that is also open to ion or water exchange; a real problem for organisms which are not isotonic in regards to the surrounding water, and the next subject we will take up.