Earthworm

 

Common Name: Earthworm, Night Crawler, Angle worm, Lob worm (Great Britain), Dew worm (Canada), Fish worm, Rain worm (calque of the German regunwurm) – The common name earthworm is self-explanatory.

Scientific Name: Lumbricus terrestris – The genus name is Latin for ‘worm’ and the species name is Latin for ‘of the earth’. The scientific name is therefore also ‘earthworm’.

Potpourri: The earthworm instantiates insignificance. Spineless, slimy and limpid, it has no eyes to see, no ears to hear and no lungs to breathe; a metaphor for defenselessness and craven behavior. Before the advent of science and its attendant observation, earthworms were widely viewed as useless and nettlesome pests that disrupted gardens and despoiled the ground with slimy pathways. The renaissance of the earthworm as the engine of soil fertilization is largely the result of the interest and astute intuitions of Charles Darwin in his sixth and last book The Formation of Vegetable Mould through the Action of Worms, published in 1881 just six months before his death (vegetable mold was the erstwhile British terminology for soil). His conclusion concerning the earthworm is as prescient as that of evolution, if less ecumenical: “The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.”  It is now known that the earthworm is benign in most habitats but also insidious in others. The earthworm in North American is also, for the most part, a non-native, invasive species.

Darwin’s encomium was appropriate for L. terrestris from the perspective of the soils of Great Britain, as it is a native species. It is less appropriate for the earthworms of North America. There are small, native North American earthworms (genus Bimastos) but they have been outcompeted and marginalized by the larger Eurasian earthworms that were transported unwittingly attendant to the import of plants in their native soil (L. terrestris is the most common of about 50 and one of 16 that do most of the damage). A large introduced earthworm can remove all of the leaf litter on the forest floor in a matter of weeks, which, for a native earthworm, would take years. This can decidedly impact on the other soil dwellers in profoundly influencing the overall ecological balance of the forest. But the real problem occurs in areas where there are no native earthworms. The extensive glaciation of the Pleistocene Epoch which ended 10,000 years ago scoured the land of all soil (and earthworms) from Canada south to about the 40th parallel of latitude, including the Great Lakes region and New England. The forestation that followed the glacial thaw generated an earthworm-free covering of decomposing humus, providing a nutritive repository for the germination and growth of an understory of seedlings to fill in the gaps of arboreal senescence. The duff was further enriched by the growth of mycorrhizal fungi essential for the robust growth of trees. Mycorrhizal (literally fungus – root in Latin) fungi evolved with trees in a mutualistic relationship in which they envelop tree roots to extract nutrients; in return the fungi provide water from their extended hyphal network and minerals that they extract from the soil with their complex, unique enzymes. The introduction of earthworms changes the nature of the forest – in as little as two years, the forest organic layer has been consumed. According to Michael Tennesen in a March 2009 Scientific American article entitled ‘Crawling to Oblivion’, “…some northern hardwood forests that once had a lush understory now have but a single species of native herb and virtually no tree seedlings. Evidently, earthworms change the forest soils from a fungal to a bacterial-dominated system, which speeds up the conversion of leaf detritus to mineral compounds and thereby potentially robs plants of organic nutrients.” The recently manifest problem is only now being studied. There is no good way to get rid of earthworms. One can only hope to contain them.

As a segmented worm, the earthworm is one of about 10,000 species (some references list 20,000) of the Phylum Annelida which includes leeches and the marine polychaetes. The efficient functionality of its structure belies the complexity of its physiology. The key structure to earthworm anatomy is the segmented coelom, an interior cavity that surrounds the digestive gut tube that extends from the mouth to the anus. The partitioned annulus contains coelomic fluid that is regulated by an organ called the nephridium in each segment; in essence the earthworm maintains its size and shape hydrostatically, like an elongated water balloon. Each segment has several small, chitinous bristles called setae that extend radially and individual circular muscles that extend and contract rhythmically to enable the characteristic vermicular motion. The complexity of sequencing as many as 150 segments in synchronization requires the coordination of a central nervous system; the earthworm has a brain and a nerve cord that extends from one end to the other. The wavelike flexing of the body segment muscles requires a method to transport oxygen for the energy that its chemical interactions provide; the earthworm has a circulatory system with five muscularized segments which are rudimentary ‘hearts’ that send blood to the muscles and the nephridia. There are no respiratory organs; oxygen is absorbed directly through the skin and the blood vessel walls. Earthworms are compendious; simple yet complex.

The alimentary system of the earthworm is an evolutionary marvel of efficiency and singular purpose, one of Darwin’s “endless forms most beautiful and wonderful.” The pathway from the mouth to the anus through the pharynx, crop, gizzard and intestine is a bioengineered disassembly line; the earthworm is in essence a peristaltic engine. Engorging through the undisturbed soil, everything in its path is ingested without regard to composition. Mineral particles, decayed plant detritus, and, most importantly, the decaying organisms from which nutrients are extracted are all swallowed with random abandon. The earthworm consumes its own weight in soil every twenty-four hours, consolidating the ingesta in the crop, grinding the admixture using sand and small pebbles in the gizzard, passing the slurry through the intestine. Three glands in the digestive tract insert calcium carbonate (CaCO3). The resultant admixture is excreted at the anus, and is euphemistally called a cast as in ‘cast off.’ The cast is a recognizable tubular consolidation distinct from its surroundings in both its physical and chemical properties. In passing through the body of the earthworm, soil is physically comminuted into fine particulate, the aerated topsoil ideal for the roots of germinating plants. The soil is altered chemically by the addition of PH raising calcium carbonate which renders the cast less acidic then its source soil. According to James Nardi in Life in the Soil “earthworm casts therefore contain around 50 percent more calcium, nitrogen, phosphorous, potassium, and bacteria than the surrounding soil.”

Earthworm behavior in response to external stimulus affords some insight into their surprisingly complex sensory capabilities. While they have no eyes, they are sensitive to light, seeking the palladium afforded by their epigeal burrows against diurnal predation. While they have no ears, they are sensitive to vibration. Earthworms are well known to vacate their protective subterranean habitats with alacrity in large numbers in broad daylight when the ground is struck so as to produce a particular vibration. Throughout the southeastern United States, individuals known colloquially as worm grunters have refined this general knowledge to practice. They hammer a stake into the ground and use what is called a rooping iron to induce the worm’s Pavlovian vibration. The vermicular exodus that results is collected and sold as the mainstay bait of the angler. It had long been thought that this radical behavior must be related to flight from predation; moles are prodigious consumers of earthworms, their primary diet. An eastern American mole (Sealopus aquaticus) eats its body weight in earthworms every day, about 20 worms a day and 7,000 worms a year. Even Darwin made mention of the connection between worms and moles, but it was not until the 21st Century that it was proven in a field test. In a March 2010 Scientific American Article entitled Worm Charmers, Ken Catania reports on the conduct a series of experiments to test the mole-worm hypothesis that consisted of putting moles in test beds filled with worms. “As the moles dug tunnels in various directions, the worms came spilling out of the soil in apparent panic … it was clear the worms were escaping from a feared predator as they came out at full (worm) speed.” Worms flee from moles based on the vibrations of their tunnel boring. Case closed. Darwin was right.

While it is abundantly clear that earthworms lack anything approaching mammalian intelligence, they are most assuredly smarter than they look. This is manifest in their feeding habits, an evolutionary adaptation that minimizes their epigeal vulnerabilities. A leaf is chosen from those in the immediate vicinity of the wormhole and dragged downward into it as both a means of blockage of predators and as a ready source of food. This is not a random act; earthworms meticulously feel around the edges of the leaf to select the pointed, petiole end to pull from, thereby facilitating the curling of the leaf as it is pulled downward. Darwin noted this behavior and commented on their possible mental qualities as a matter of conjecture. He opined that “It may be doubtful whether they suffer as much pain when injured, as they seem to express by their contortions.” This is a point to ponder when next you are drawn to the fine art of angling and the procurement of a cache of worms to use for bait. The earthworm is the only animal that is routinely and ruthlessly impaled while still living on the end of a sharply barbed hook, their writhing an additional attraction to the target fish. The brutal world of earthworms made worse.

In this the age of gay rights, earthworms merit consideration as a totem for the LGBT movement; they are hermaphroditic, producing both eggs and sperm on the same individual – the zoological equivalent of the perfect flower. Unlike the perfect flower, however, earthworms cannot inseminate themselves (the scatological epithet is not possible for annelids either); they require a hermaphroditic partner. While it is not clear what signal one worm sends to another to indicate the desire to mate, it must presumably be met with a willing rejoinder (foot tapping is not physiologically possible). Earthworm copulation is generally a nocturnal surface activity. Held together with a mucilaginous secretion, the earthworms exchange sperm. Once copulation is complete, the worms separate for the fertilization step of the reproductive process. It is at this crucial juncture that the structure called the clitellum plays a key role. The clitellum (Latin for packsaddle) is a thickened glandular section of the body wall of the earthworm. It is readily discernible as the pinkish region that extends roughly between segments 35 and 45 (depending on the species). The clitellum secretes what is essentially a cocoon that extends around the worm’s circumference. The gelatinous ring cocoon detaches and moves forward to pick up the eggs at segment 14 and the sperm at segment 9 before slipping over the head for fertilization. Baby worms emerge about two weeks later. Earthworms are the epitome of evolutionary adaptation. They can eat almost anything, live almost anywhere, and reproduce with anyone.