April/March 2015For Wildlife Watchers: Earthwormby Rob Simbeck, photo by Dan Garber
Earthworms are sold as fish bait and pet food, and for composting and gardening.
Many plants and animals we take for granted weren't part of the landscape that greeted the first Europeans who settled in America. Dandelions, honeybees, starlings and kudzu, just for starters, are all imports. But perhaps none is as surprising as this one: many species of earthworm, including the redworm and the common earthworm, or nightcrawler, aren't native. They came in the soil around imported plants and in the dirt used for ballast in ships, filling a very real vacuum in some parts of the continent.
"Earthworms were lost from northern latitudes during glaciation," says Dr. Tim McCay, chair of the Department of Biology at Colgate University. During the last Ice Age, glaciers reached as far south as the Ohio River Valley. "Following the recession of glacial ice, earthworms have been recolonizing, but very slowly," he adds.
There are still regions without them. "In rural upstate New York, for example, we find no earthworms at about one-third of the forested sites that we sample," says McCay. Today, about a third of the earthworm species in North America are imports, although in some cases a species' status as native or exotic is presumed rather than known as fact.
In fact, says McCay, "I am amazed by the magnitude of what we do not know about earthworms in their natural settings. Of the almost two-hundred species of earthworms found in North America, only about a half-dozen have received much attention from scientists."
A checklist of earthworms found in South Carolina published in the journal Megadrilogica lists thirty-four species, just fourteen of which are considered native. The most common was Amynthas diffringens, an energetic Asian import sometimes called the "crazy worm." Others included Dendrodrilus rubidus, the red wiggler, and Lumbricus rubellus, the red worm commonly sold as fish bait.
The importance of earthworms is difficult to overstate. Aristotle called them "the intestines of the earth." Ancient Egyptians banned their export. Darwin kept them in pots of dirt in his study and made them the subject of his last scientific book, saying he wanted to write about them "before joining them."
Many of us knew them first as robin food or fish bait, then as garden denizens and formaldehyde-soaked specimens in biology class. Inside the ring-like segments that make up an earthworm's body is a marvelous digestive system, perfect for turning, say vegetable scraps, into compost. From a tiny mouth, food moves through the pharynx into the esophagus, where calciferous glands essentially neutralize acid. Then it's on to a crop and a muscular gizzard that uses sand and grit to grind up leaves, soil and organic material, which emerge as nutrient-rich castings remarkably dense with substances made more usable to plants. The worm's own body turns much of what it eats to protein; worms are eighty-two percent protein, compared with three percent for cow's milk, forty percent for soybeans and fifty percent for lamb.
(Common species include Lumbricus terrestris, common earthworm or nightcrawler and Lumbricus rubellus, red earthworm)
Description: Three to twelve inches. Brown or reddish in color. Comprised of more than one hundred ring-like segments.
Range and Habitat: Throughout continental U.S. and sub-Arctic Canada except in pockets in northern forests and in deserts; throughout South Carolina.
Reproduction: Hermaphroditic individuals exchange sperm. Egg case secreted by clitellum and deposited in soil. Development typically two to four weeks before hatching.
Viewing Tips: Dig. A spade and a good patch of garden soil or compost should turn some up. Many emerge at night.
Earthworms tunnel as they dine. Each of those ring-like segments has bristles that can serve as anchors for one section as another's muscles expand or contract. Blood cells and hemoglobin produced by blood glands near the head flow through a network of vessels and capillaries, pumped by five heart rings wrapped around the esophagus, gizzard and stomach. A second system of fluid-filled chambers provides firmness and structure for these boneless creatures. Direction for all this comes via cerebral ganglia that are as close to a brain as an earthworm gets. Earthworms breathe through their skin. They have no eyes or nose, but the prostomium, a flap that covers the mouth, helps in sensing light and vibration.
Earthworms transform about a third of their weight in soil a day, mixing it and carrying plant material to lower levels, aerating it and providing channels for water drainage. Moisture is a must for earthworms; direct sunlight can quickly dry them out. Many emerge at night to feed, hence the name "nightcrawler." They may emerge during the day if it is raining - not because they are in danger of drowning, but simply to move elsewhere.
There are more than 6,000 species, including a six-footer found in Australia, though only about 150 are widely distributed. They have adapted to many conditions, with some non-burrowing types eating decomposing organic matter and others habituated to the seashore or the mud in streams, with only the driest deserts and polar regions completely unsuitable.
They serve as food for many species of birds, amphibians, insects, reptiles and mammals, including moles, foxes and even bears.
Earthworms are hermaphroditic, each containing male and female sex organs. A pair of worms will exchange sperm, which travels from two male openings to two sperm receptacles. Each worm has ovaries and a clitellum, an enlarged ring visible in mature worms at about the 15th segment that secretes an egg case filled with protein-rich albuminous fluid. The worm in essence crawls forward out of it, with the egg case picking up eggs at the female pore and stored sperm at the male pore. The egg case, containing the fertilized eggs, remains in the soil until the eggs emerge after two to four weeks of development as miniaturized versions of their parents. The eggs of some species overwinter, and eggs can survive up to three years in dry soil.
While some soils contain no worms, especially rich soil can contain a million or more per acre, with moisture, pH and the amount and quality of organic matter all factors. The biggest dangers they face these days are manmade.
"Insecticides and fungicides can seriously reduce populations of earthworms," says McCay. Even tilling disrupts burrow systems and can reduce populations.
Earthworms are sold as fish bait and pet food, and for composting and gardening. On the other hand, they can spread plant diseases and carry parasites that can affect their predators, and they sometimes damage seedlings. When humans introduce them - often by dumping unused fishing worms, which some wildlife agencies discourage - they can disrupt new environments.
If you're wondering if there is room for whimsy in the world of professional worm watchers, just talk to Dr. McCay: "My favorite earthworm is Amynthas hupeiensis, a recent immigrant from Asia," he says. This species is dark green to black, smells a bit like funky chocolate, and twists itself into knots when you pick it up."
I don't know about you, but I've got to find one.