A lugworm and its burrow therefore mobilize the energy that exists in the redox potential gradient in un disturbed mud. This is accomplished partly by the straightforward introduction of oxidants into sediments below the RPD layer, where they are not normally present. The worm also biases the movements of material across the burrow lining, thereby altering the mix of oxidants and nutrients in the sediment around the burrow. The overall result is a stimulation of growth in the sediments that has been dubbed by some zoologists as "gardening," although I think "ranching" is probably a more appropriate term. The stimulation of growth is impressive: sediments with feeding burrows and active lugworms in them mobilize energy at nearly three times the rate that undisturbed sediments do.
It is worth asking just how the worm benefits from this enormous mobilization of energy. After all, the worm does all this work to construct a burrow and then pump oxidant into the sediment. Most of the energy that is mobilized in fact goes to benefit other organisms, not the worm. This is evident from a comparison or the energy consumed in respiration by the worm and by the various organisms in its surroundings. In feeding burrows of Nereis, another polychaete worm, the worm itself accounts for only about 10 percent of the total energy consumption. The rest is consumed by the vast community of other things mooching off the oxidant introduced by the worm: roughly 30 percent goes to things living in the lining of the burrow (mostly nitrogen fixers and sulfide oxidizers) and roughly 60 percent goes to things living in the sediments surrounding it (sulfate reducers mostly). So, what's in it for the worm?
The worm is benefiting by playing a game of leveraging. If a lugworm has a metabolic requirement of X joules per day, and its food has an intrinsic energy content of Y joules per kilogram, the worm's energy needs could conceivably be met by ingesting X/Y kilograms of food per day. In fact, it will have to ingest more, since there will always be some inefficiency in absorption and digestion. If its digestion is 10 percent efficient, for example, the worm will have to eat 10X/Y kilograms of food daily to meet its requirements.
The efficiency of digestion is related in part to the quality of the food. "Low-quality" food might require a lot of energy to obtain or process, or it might contain chemicals or other materials that impede its digestion. The muds that animals like lugworms inhabit are, as has already been noted, low-quality foods. Although a lot of energy is present in anaerobic muds, it is in a form that can fuel the growth mainly of bacteria—animals are not constructed to do all the strange metabolic things bacteria do. Fueling bacterial growth alone does substrate feeders no good at all, because most substrate feeders are incapable of digesting bacteria. All the extra bacterial productivity, provided courtesy of the lugworm, just passes right through the lug-worm's gut. Stimulating bacterial growth does, however, benefit microbial predators, like predatory diatoms or nematodes, that can digest the bacteria. The lugworm leverages this secondary production to its own benefit because it is able to digest these types of organisms.
So lugworms essentially are using the redox potential gradient in an anaerobic mud to power the conversion of low-quality food (bacteria and mud) into the high-quality menu of diatoms and nematodes. This is just what ranchers do, of course. Ranchers feed low-quality grass, grain, and straw to livestock, which convert it to meat and milk. Just as ranching operates on an energy efficiency of about 10 percent (only 10 percent of the energy in cattle feed ends up as cow), so too, it seems, do worms like Nereis, which divert about the same portion of the energy flowing through the sediment to its own consumption. Even with this small profit margin, though, mud flats along with their substrate feeders rank as one of the richest ecosystems on the planet. Such is the power of mud.
Man is certainly stark mad; he cannot make a worm, and yet he will be making gods by the dozens.
— MICHEL DE MONTAIGNE
If creationists are right and God did design the living world, then God must be a trickster. I say this because of the quandaries one is continually being led into by comparing what we believe God to be with the nature of His works. Let me offer a trite example: it is a common religious doctrine that God is perfection. It follows that God's works must reflect His perfection. Once we accept this, though, we soon are confronted with the manifest imperfections of the world, ranging from the profound (why is there famine and war?) to the trivial (why do so many kids need orthodontia?—a particular source of worry for me right now). This is an old, old conundrum, of course, and at least since Augustine, Christian doctrine has had an answer that satisfies it—our imperfection results from our original sin and our subsequent alienation from God.
Well, that explains nicely why people and their works are imperfect, sometimes even absurd. However, the rest of the living world is full of confounding creatures that fly in the face of any doctrine that the living world is a rational, well-designed place. Take, for example, this Zen-like riddle: why is an earthworm? Let me phrase it more prosaically: why are earthworms earth-worms? Earthworms, as we shall see, have no business living where they do, because they are physiologically quite unsuited for terrestrial life. Yet there they are, digging happily away, and flourishing in the bargain. How do they manage that?
In this chapter, I explore how the burrowing activities of earthworms benefit these physiological strangers in a strange land. This will require a new look at some commonly known facts about earthworms from an unusual perspective. For example, it has been known for ages that earthworms' burrowings build, aerate, and fertilize soils. Currently we are (finally!) reawakening to the fact that these activities are very advantageous for us, because earthworms are integral parts of productive agricultural ecosystems. But earthworms don't do all this work to promote our well-being (unless, given our common destiny as worm-meat, it is they who are cultivating us): they do it to benefit themselves. By burrowing as they do, I shall argue, earthworms co-opt the soils they inhabit and the tunnels they build to serve as accessory kidneys, ensuring their survival in an essentially uninhabitable environment.
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