|On the Segmented Worm Page the point is made that if an interplanetary
naturalist should visit Earth studying life here, he, she or it might choose annelids, or
segmented worms, as "average Earth animals." That's because in terms of animal
complexity about half of all animal species on Earth are less complex than
segmented worms, while about half are more complex. Of course we humans are the
most complex. With regard to earthworms, one point for backyard naturalists to keep in
mind is that earthworms are by no means "average Earth annelids." As the picture
of an earthworm head (or is it a tail... ?) at the right makes clear, among the annelids,
earthworms are very special because they are super-streamlined, stripped-down,
no-nonsense, fairly highly evolved critters.
For instance, the front ends of "average annelids" such as those found in the oceans generally bear mouthparts, eyes, and antennae-like structures. On the other end there may be another set of antenna-like projections, and, in between, each segment may bear a pair of foot-like appendages. Earthworms bear none of these typical annelid accessories. In fact, earthworms, at first glance, appear to be no more than long, segmented cylinders tapering to both ends.
ARE EARTHWORMS IMPORTANT?
Certainly what earthworms do isn't simple in ecological terms. The great naturalist Charles Darwin, after making a careful study of them, wrote this:
"History of the world," he said!
One important thing that earthworms do is to plow the soil by tunneling through it. Their tunnels provide the soil with passageways through which air and water can circulate, and that's important because soil microorganisms and plant roots need air and water just like we do. Without some kind of plowing, soil becomes compacted, air and water can't circulate in it, and plant roots can't penetrate it.
One study showed that each year on an acre (0.4 hectare) of average cultivated land, 16,000 pounds (7200 kg) of soil pass through earthworm guts and are deposited atop the soil -- 30,000 pounds (13,500 kg) in really wormy soil! Charles Darwin himself calculated that if all the worm excreta resulting from ten years of worm work on one acre of soil were spread over that acre, it would be two inches thick (5.08 cm).
This is something we should appreciate because earthworm droppings -- called castings when deposited atop the ground -- are rich in nitrogen, calcium, magnesium, and phosphorus, and these are all important nutrients for healthy, prospering ecosystems. In your own backyard you might be able to confirm that grass around earthworm burrows grows taller and greener than grass just inches away.
THE PROBLEM WITH NON-NATIVE EARTHWORMS
The earthworm situation isn't always black and white. When non-native worm species are introduced in an ecosystem they can actually cause big problems.
For example, in Canada about 20 earthworm species are recorded, but 18 of those have been introduced from Europe. It's a similar situation in the US's northern states. Any native earthworm species these northern areas may have had apparently were killed off during the last ice age.
Without worms, fallen leaves decompose slowly, creating a spongy layer of organic "duff." The duff isn't really soil, but many organisms depend on duff being there for their survival. Invading earthworms eat the duff. Karen Schik, an ecologist with the environmental group "Friends of the Mississippi River" in Minnesota, writes us:
"Not only are we losing our native woodland flora, but when the earthworms cause their demise and subsequent bare soils (which causes erosion), it opens the door for non-native invasive species such as garlic mustard, buckthorn, honeysuckle and narrowleaf bittercress, leaving little opportunity for the native species to recover, along with the many native wildlife species that rely on them. Our trees have what one researcher refers to as 'tree gingivitis' -- the root crowns are exposed due to the removal of several inches of duff and soil compaction (yes compaction) by earthworms."
More information is available in an online Scientific American article.
HOW ARE EARTHWORMS CONSTRUCTED?
Here are some visible and invisible adaptations to admire about earthworm physique:
WHEN CONDITIONS GET ROUGH....
When the soil gets very cold, very hot, very wet or very dry, it's hard on an earthworm. Survival strategies vary from species to species. A typical behavior is to tunnel deeper into the soil, to where conditions aren't so extreme. Some species, especially during hot, dry periods, do what's shown at the right. At a fair depth they roll themselves into balls and excrete mucous around themselves, creating a kind of cocoon. In such a state their body functions slow down drastically. They go into a kind of suspended animation, just waiting for soil conditions to improve. When an animal goes into such a state in order to survive summer's hot, dry conditions, it's said that they estivate or "go into estivation." I found the earthworm shown above one day as I worked in the very dry soil of a garden. The earthworm was inside a very hard, dry clod of soil. While the picture was being taken it began moving, so this worm was very much alive, just "resting" until a good rain.
HOW DO EARTHWORMS TRAVEL?
The secret to earthworm travel lies in two things you can't see just by looking.
Here's how these two features enable earthworms to travel:
Let's say that a worm in its burrow wants to move forward. First, using its complex musculature, it makes itself long. Then it anchors the front of its body by sticking its front setae into the soil. Now it pulls its rear end forward, making itself short and thick. Once the rear end is in place, the front setae are withdrawn from the soil, but setae on the rear end are stuck out, anchoring the rear end. Now the front end is free to shoot forward in the burrow as the worm makes itself long and slender. Then the whole process is repeated.
HOW DO EARTHWORMS REPRODUCE?
This matter is as complex as it is interesting, so put your mind in low gear and hold on... Here's a drawing to help you keep oriented:
First of all, not every earthworm segment bears sex organs. Counting from the front, the worm's male sex cells lie in segments 10 and 11. From here the sperm pass through sperm ducts to two male genital openings on the bottom of segment 15. On segments 9 and 10 there are two minuscule sacs called sperm receptacles, or pores, where, during earthworm sex, sperm are deposited. However, this is not where eggs are produced. The egg-producing ovaries reside in segment 13, from which eggs are released through the female pores into egg sacs in segment 14. Finally, there's a rubbery, arm-band-like thing covering the worm's body from segments 31 or 32 to 37, and this is called the clitellum.
Now, when two earthworms mate, they line up next to one another with their "heads" pointing in opposite directions. The clitellum of one worm lies opposite segments 9-11 of the other, which, you'll remember, contain the male parts. The worms now secrete tremendous amounts of mucus, until each is enclosed in a slime tube extending from segment 9 to the rear end of the clitellum at segment 37. Now sperm are ejaculated from segment 15 and carried backward in tubes formed by grooves in the body touching the slime tube and the sperm pass to the sperm receptacles on segments 9 and 10 of the other worm.
Then the earthworms go their different ways. Sex is done. However, that is not to say that eggs have been fertilized, because they haven't. It's just that now, in each worm's sperm receptacles, there are sperm from the other worm.
The next step in producing baby earthworms comes when the earthworm is by itself. Its clitellum secretes a second mucous ring that slides forward over the worm's body. When the ring passes the openings in segment 14, several ripe eggs leave the body and stick to the ring. The ring keeps moving forward until it passes the sperm receptacles in segments 9 and 10, and here sperm come into contact with eggs. Finally, within the mucous ring, the eggs are fertilized.
Now the ring containing the fertilized eggs slips off the worm's "head," seals at both ends, and becomes a sort of cocoon, which is left lying in the soil. Ultimately the eggs hatch and tiny worms escape from the mucous ring, into the soil.
On the Web you may get a kick out Mother Earth News' HOW TO RAISE EARTHWORMS (FOR FUN AND PROFIT).
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Conrad, Jim. Last updated . Page title: . Retrieved from The Backyard Nature Website at .