Breathing Air Underwater

For insects and spiders, reliance on diffusion for gas distribution has imposed a stringent limitation: it commits them irrevocably to breathing air. Should the tracheoles become flooded with water, for example, the insect could no longer enjoy the very high rates of diffusion that prevail in air. Consequently, insects and spiders that have returned to water have had to cobble together devices and structures that enable them to continue breathing air.

Despite being committed to breathing air, insects have evolved many clever ways of living underwater. Mosquito larvae, for example, use their colons as snorkels, hanging suspended from the surface of the water, held in place by surface tension. There, the larvae open their anuses to the air, breathing through their bums, so to speak. Another clever solution is to use plants as snorkels. Many "aquatic" plants are in fact physiologically terrestrial, with the same requirements for "breathing" air, mostly to supply oxygen to the roots. These plants modify some of their vascular tissues, which normally carry fluid between the roots and leaves, and fill them with air. This gives the roots easy access to oxygen in the atmosphere. In a nice double-cross, some types of beetle larvae tap into these air-filled tubes and use them for snorkels. Still others have "gone native," so to speak, developing so-called spiracular gills, in which the spiracles are permanently closed and the air-filled tracheal system is separated from the water by a thin membrane across which diffusion can move gases quickly. In some instances, the spiracle's cover is elaborated into a gas exchange organ consisting of air-filled tracheae. One of the more "deep-tech" (no pun intended) solutions is exemplified by diving bell spiders and aquatic beetles—these creatures simply take a bit of air down with them underwater.

would expect that scientists, with characteristic hard-nosed insistence upon evidence and proof, would be immune to this failing. They are not. In fact, there is among scientists a sort of cult of simplicity, which traces its origins to the fourteenth-century writings of a Franciscan monk, William of Occam. Brother William's legacy to us is a philosophical rule—which has come to be known as Occam's razor—which supposedly differentiates the one "true" explanation from many possible explanations. Put simply, Occam's razor asserts that the explanation most likely to be true is the simplest one, the one that requires the least in terms of special assumptions, rules, or exceptions.1 Take, for example, the apparent motion of the planets in the sky. There are two competing explanations for these motions, each corresponding to models of the solar system that place either the Earth or the Sun at the center. The geocentric, or Earth-centered, model explains the apparent motion of the planets just fine, but it requires the planets actually to move in some rather complicated ways. For example, it depends on various combinations of rotational and circular movements, unexplained accelerations or decelerations of the planets, and other special rules. A heliocentric, or Sun-centered, solar system requires only that the planets move in elliptical paths, obviously the simpler explanation. Occam's razor would lead us to select the heliocentric model of the solar system as the one most likely to be "true."

Like any useful tool, Occam's razor is prone to abuse. One of the more common abuses is the assumption that it should apply equally to all scientific endeavors, whether they be in the realms of physics, chemistry, or biology. This is probably a pretty safe assumption for sciences like chemistry and physics: I am less certain of its usefulness for biology. Among the assumptions one must make to use Occam's razor is that the universe is a simple place, for which simple

1. Occam's razor, from the horse's mouth, as it were, is stated thus: Pluralitas non estponenda sine necessitate. This translates into English as "Plurality must not be posited without necessity."

explanations suffice. Biology, mixed up as it is with a complicated evolutionary history and the often commented upon opportunistic nature of natural selection, offers every reason to believe that the simplest explanation will, in fact, often be the wrong one. Indeed, biology is so full of seemingly claptrap solutions to rather simple problems that I would not simply reject Occam's razor as a useful tool in biology. I would go further and pose a new philosophical rule, that the more complicated the explanation, the more likely it is to be true. We can call it, for lack of a better name, Goldberg's lever, after the cartoonist Rube Goldberg and his ingeniously complicated solutions to simple problems.2

The Case of the Bubble-Carrying Beetles Let us apply Occam's razor to the diverse uses of bubbles by beetles. Do we require two explanations for the two kinds of aquatic beetles, one for those that periodically come to the surface and one for those that do not? In the late nineteenth century, Occam's razor provided a nice way of judging this question: beetles used bubbles for buoyancy. Certainly, so the story went, the bubbles might play some role in respiration for the beetles that regularly come to the surface, but it does not explain why beetles that never come to the surface should carry them. If all beetles use bubbles for buoyancy, though, two explanations may be rejected in favor of one: if some beetles use their bubbles for buoyancy, then all must.

I sometimes ask my graduate students what they think moves science forward. Because most of them spend enormous effort chasing down financial support for their work, it is not surprising that most answer

2. Occam's razor has been expressed as various aphorisms, such as "The simpler the explanation, the better the explanation" and so forth. In the hope that Goldberg's lever might catch on, I offer the following aphorisms rendered into Latin by my colleague Jim Nakas. The essence of Goldberg's lever is Quanto implicatius, tanto verius est ("The more complicated it is, the truer it is"). A more succinct aphorism might be Mirationem meam nihil moveat ("Nothing would surprise me").

"money." Often, I find I have to agree with them, even though my idealistic side doesn't want to—I still want to believe that science, even in our careerist age, is still primarily an intellectual endeavor divorced from the crass scramble for money and prestige. I usually am succored in my faith by examples of how an individual with a good idea, a healthy dose of skepticism, and only sparse resources can change the course of a science.

One of my favorite examples of this, even if it is a rather prosaic one, concerns the problem of the bubble-carrying beetles. The hero of the story is a German biologist, Richard Ege, who took on this problem in the years just prior to World War I. Two things made Richard Ege's work remarkable. First, the problem he chose exemplified nicely how Occam's razor got in the way of a simple experiment that would have provided a good answer. Why do an experiment, after all, when the answer is obvious (how many times have I seen that on my grant proposal reviews?). Second, his work showed how the relentless rejection of the simple can open up marvelous vistas of new biology to explore.

Richard Ege used in his work several species of aquatic beetles, including the main character in the story to follow, Notonecta. These beetles normally can stay submerged for five or six hours, after which they must come to the surface or drown. Ege had the ingeniously simple idea of testing whether the bubble had a respiratory function by altering the composition of the bubble gas, confining the beetle underwater, and seeing how long it survived. If the bubble was carried strictly for buoyancy, its gas composition would have no effect: a bubble of nitrogen will float a beetle as well as a bubble of air. If, however, the bubble had some respiratory function, then replacing its air with nitrogen should shorten the beetles' survival times. Sure enough, that was what Ege observed. When beetles were confined underwater with air-filled bubbles, they survived on average about six hours, but when they were confined with only nitrogen in the bubble, they suffocated in only five minutes.

So, the bubbles carried by Notonecta clearly were acting as a source of oxygen. There the matter would have rested for most people, and Ege's work would have been uninspiring, certainly to me and perhaps others. However, one of the marks of a great experimental biologist is the unwillingness to take a simple "yes" for an answer. Ege happened to be a great experimental biologist, and so he went on to do a second experiment, from which an extraordinarily puzzling result emerged. Let us go through the reasoning behind Ege's second experiment.

• Perhaps the beetle is using the bubble as an oxygen store: that is, it captures in the bubble a certain quantity of oxygen (it would be about 21 percent of the bubble's total volume), uses it up, and then returns to the surface to get a fresh supply.

• A beetle confined with a bubble of nitrogen should not survive as long as a beetle carrying a bubble of air, simply because the quantity of oxygen in the bubble's initial store is smaller. This was shown by experiment to be true.

• The converse should also be true: a beetle using a bubble enriched in oxygen should survive longer than a beetle carrying a bubble of air.

Now here is the remarkable part. Ege confined his beetles with bubbles of pure oxygen. By all rights, they should have survived longer than the beetles carrying bubbles of air, because they had more oxygen to start with. Beetles carrying bubbles of pure oxygen survived for only 35 minutes, however, longer than the 5-6 minutes beetles carrying nitrogen bubbles lived but one-tenth the survival time for beetles carrying bubbles of air.

The Ege Effect

Clearly, something strange was going on with the bubbles carried by Notonecta. Ege went on to document this curious phenomenon in several different species of aquatic beetles, so extensively that it came to be known as the Ege effect. Understanding the Ege effect is the key to understanding how insects and spiders use bubbles as accessory gas exchangers. It is also the looking glass that helps us find novel structures that might also serve as gas exchangers.

The first step is to understand how gases move between a bubble and the water surrounding it. To do so, we must be able to explain the following phenomenon: an ordinary bubble, drawn from air, will gradually shrink until it disappears. The shrinkage indicates that the gases contained in the bubble are leaving it and dissolving in the water. This movement can only occur if the gases in the bubble are at higher partial pressure than the gases in solution, that is:

where the subscripts b and s refer to the gases in the bubble and in solution, respectively.

To understand why a bubble shrinks, we must explain where this difference in partial pressure comes from (Fig. 8.2). The partial pressures of the gases in solution are easy to understand: at equilibrium, a gas in solution is at the same partial pressure as the gas in the atmosphere above it. To illustrate, let us assume the atmosphere is similar to air, a mixture of 79 percent nitrogen and 21 percent oxygen:3 for dry air at sea level (atmospheric pressure = 101 kPa), pO2(s) is about 21.2 kPa and pN2(s) is about 79.8 kPa. Most importantly, these partial pressures are independent of depth: they are the same at the surface as they would be at 10 cm or 10 m depth.4

Pressures in the bubble are another matter, because

3. I am ignoring the trace gases that collectively make up about 1 percent of atmospheric air, as well as the water vapor it contains.

4. Strictly speaking, this is only the case if there are no sources or sinks for oxygen in the water. If the water contained microorganisms that consumed oxygen, the oxygen partial pressures would decline with depth.

they are affected by how deep the bubble is and by how big the bubble is. A bubble held at depth is squeezed by the water's hydrostatic pressure, which increases at a rate of about 100 pascals per centimeter of depth: this is the pressure you feel in your ears when you dive to the bottom of a swimming pool. If a beetle takes down a bubble of air from the surface at atmospheric pressure, the bubble will be squeezed, and as a result the partial pressures of all the gases in the bubble will be elevated. For example, a bubble derived from air at atmospheric pressure of 101 kPa and carried down to 5 cm depth would have a total pressure of 101.5 kPa, an increase of roughly 0.5 percent. The partial pressures of all the constituent gases would increase commensurably. Thus, pO2(b) would now be roughly 21.3 kPa (100.5 percent of 21.2 kPa), and pN2(b) would be roughly 80.2 kPa (100.5 percent of 79.8 kPa). Both are higher than their respective partial pressures in the water, and so both gases will be driven from the bubble to dissolve in the water. As the gases leave, the bubble shrinks until the partial pressures in the bubble and water equilibrate (Fig. 8.3). At this point, the bubble's size should stabilize, because there are no longer any partial pressure differences to drive gases out of the bubble. Remember, though, that the bubble does not stabilize—it continues to shrink until it collapses. Some additional force therefore must be compressing the bubble and keeping the gas partial pO^ = 21.2 kPa pN2(b) = 79.8 kPa pO2(a) = 21.2 kPa pOm = 79.8 kPa

Figure 8.2 Forces acting on a bubble of air at depth d and their effects on the partial pressures of the various gases contained within it. Partial pressures are given for gases in the air (a), in the bubble (b), and in solution in the water (s).

hydrostatic pressure (<^d)

hydrostatic pressure (<^d)

/pO2(b) > 21.2 kPa \pN2(b) > 79.8 kPa, pO2(S) = 21.2 kPa " pN2(s) = 79.8 kPa

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