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bottom of the tube and generates a current that, depending upon the animal, flows either from front to back, or from back to front.

Suspension versus Substrate Feeders

A feeding burrow in stinking mud hardly seems like a dream house—it smells, the food is bad, and it's stuffy. It is easy to look down upon the humble creatures that inhabit these burrows as backwaters of evolution, animals that are primitive and have been left behind in the race to become—us. Why else would they live in mud—or eat mud for that matter?

Simple digestive physiology seems, at first glance, to support this idea. Mud can be used as food, because it contains a lot of bacteria and other goodies, but, really, it is hardly a high-quality diet. Mud is mostly silica and other mineral: the nutrient value is high in a thin bacterial film coating the grains of silt. Bacterial films themselves are quite high-quality food, but what degrades the nutritional value of mud is the cost of getting at those films. Mud is viscous and heavy, and the costs of transporting it through the gut are high. Also, the more finely divided the clay, the more resistant the bacterial films are to digestion. Finally, an aerobic animal living in an anaerobic mud might not be able to avoid "doing as the Romans do": living in an anoxic layer means there will be little oxygen to support aero bic respiration, which forces the animal to use inefficient anaerobic pathways to extract energy from food. In short, the habitat of feeding burrows and the available food would seem to force on the inhabitants a sluggish and sedentary life style.

But anyone who visits an estuarine mud flat will be struck by just how abundant these creatures are. Population densities of a common burrowing worm, Arenicola, also known as the lugworm, can run as high as several hundred individuals per square meter. Other, smaller burrowers can exist in densities of thousands of individuals per square meter. So, despite their reputation for being sluggish and metabolically challenged, there seem to be an awful lot of them.

The quandary is deepened by taking a detailed look at what these animals are eating. This is easily done: you simply open up the gut and see what is inside. You would think that a substrate feeder living on bacteria on mud should have in its gut only mud and bacteria, but quite frequently they have lots of other things there, too. For example, substrate feeders' guts often contain abundant diatoms, small nematodes, and small arthropods that do not inhabit the mud they eat. Even more strange, the bacteria that are ingested frequently pass through substrate feeders' guts unscathed.

Things become even curiouser when you look at the chemical composition of these animals' diets. An animal's meal is a package that contains some quantities of nutrients, like carbohydrates, fats, proteins, and minerals. As this package passes through the gut, some nutrients are absorbed. Consequently, the remains of the food, excreted as feces, should have fewer nutrients in them than the original food. Many substrate feeders, though, seem to defy this elementary demand of the First Law of Thermodynamics. Lugworms, for example, deposit fecal pellets that are actually nutritionally richer than the mud they are derived from. Obviously, there is some not very straightforward digestive physiology going on here. Key to this unusual physiology is the burrows the organisms build.

Conveyor-Belt Feeding in Arenicola Worms The seemingly bizarre dietary physiology of lugworms has a rational explanation, but it requires looking beyond the worm itself to how the worm and the structure it builds interacts with the redox potential gradient in the sediments in which it burrows. The extra nutrients in the lugworm feces actually are produced using this source of potential energy.

Lugworms build J-burrows. The worm sinks a vertical shaft, the tail shaft, in the sediment, and then extends a short gallery horizontally from the bottom of the shaft. It lies head down in the burrow, with its head extending into the tail of the J. The worm ingests the sediment, extending the gallery as it feeds. The mud ingested passes through the gut and out through the worm's anus, located near the top of the tail shaft. There the feces piles up in coiled fecal casts. A lug-worm will extend several horizontal galleries from a single tail shaft—viewed from above, the overall effect is a rosette of horizontal galleries extending from a single vertical shaft.

Animals that live in both I- and J-burrows commonly nourish themselves by a method known as conveyor-belt feeding. Among lugworms, it works in this way. The worm uses fleshy paddles on its body to drive a current through the burrow, drawing water in through the opening of the tail shaft and forcing it to percolate upward through the sediments above the worm's head. As the water percolates through the sediments, it stirs them up and causes them to collapse into a funnel-shaped head shaft. Meanwhile, the sediment particles in the developing head shaft sort by size, with the fine, and usually nutrient-rich, particles at the top filtering downward into the gallery. There they are ingested by the worm. As the worm feeds, the head shaft funnel deepens, until it is finally tapped out. Once this happens, the worm extends a gallery in another direction from the tail shaft and begins the process again with another column of mud.

For some time, the enrichment of lugworms' feces was explained away as a side effect of conveyor-belt feeding. The extra goodies, so the explanation went, were carried in with the water the animal pumped through its burrow. The animal's meals, therefore, were supposedly a mixture of both mud and the planktonic organisms drawn in by the feeding current. But, again, deeper analysis knocks the props out from under this explanation—protozoans and other small organisms do show up in substrate feeders' guts, but not the kinds that would be floating around as plankton. Rather, the most abundant protozoans in the guts are mud-dwelling types. This might seem perfectly logical—the lugworms are eating mud, so of course there would be mud-dwelling organisms in the diet. The truly puzzling observation is how much more abundant these mud-dwellers are in the guts than in the sediments that supposedly make up the meal.

Getting Something for Nothing?

Lugworms, as far as we know, have not received an exemption from the First Law, so the extra energy in the lugworm gut must come from somewhere. So it does: from the redox potential gradients that span the RPD layer. In the fine-grained sediments inhabited by lugworms, the RPD layer is very shallow, only about a few millimeters below the surface. The lugworm's burrow, on the other hand, is 5-8 cm deep. With the burrow breaching the metabolic Great Wall, the flow of electrons no longer must make their way through the convoluted cartels of bacteria that span the RPD layer. Now, because of the ventilatory flow of water through the burrow, there is a continual supply of highly oxidizing electron acceptors—specifically sulfate, nitrate, and oxygen—into the sediments below the RPD layer. The burrow, therefore, is analogous to a short-circuit across the RPD layer.

The shortcut sets in motion a complex series of events in the sediments surrounding the burrow (Fig. 6.8). In undisturbed sediment, the microbial community below the RPD layer is dominated by the acetogenic bacteria and the methanogens. These sediments are therefore rich in methane, as well as in the feedstocks for the methanogens: acetate, carbon dioxide, and hydrogen gas. More powerful electron acceptors used by some anaerobic bacteria, like sulfate, are relatively scarce, in part because they are not replaced as they are used up. Seawater is fairly rich in sulfate, though, and when lugworms ventilate their burrows, a stream of sulfate is reintroduced, providing anaerobic sulfate reducers the electron acceptors they need to grow. The increased growth of sulfate reducers thus diverts electrons away from the acetogenic fermenters and methanogens and toward sulfate (Fig. 6.8). Oxygen is also introduced with the ventilatory currents through the burrow, which supports the growth of aerobic bacteria, most notably those that use hydrogen, carbon monoxide, carbon dioxide and methane as their food molecules.

The end result is a flourishing microbial community that spans the short-circuited redox potential gradient around the burrow's walls. This community channels electrons away from the existing cartels of anaerobes to power a prolific growth of aerobic bacteria. The bacteria are in turn eaten by protozoans, mostly predatory diatoms, which are in turn consumed by other predatory diatoms and nematode worms (Fig. 6.8). These are ingested by the lugworm, along with the bacteria in the mud.

As the enriched mud passes through the lugworm's gut, some is digested, but there is enough residual energy in the mud to power the continued growth of this

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