Wigglers, crawlers and swimmers: The surprisingly complex lives of worms and leeches

It may be doubted if there are any other animals which have played such an important part in the history of the world as these lowly organized creatures. — Charles Darwin

Pop quiz: When first published, which of the following three books by Darwin was a best seller? “On the Origin of Species” (1859), “The Descent of Man” (1871), or “The Formation of Vegetable Mould Through the Action of Worms” (1881)? You guessed it, the worm book. Suffice it to say, the British love their gardens.

Of course, all gardeners know the value of worms. As a boy with a fishing pole in hand, I appreciated worms as well. Back then, I knew nothing of their scientific classification.

Intro to Annelida

Approximately 20,000 species of segmented worms fall under Phylum Annelida. These include everything from marine sand worms and tube worms to the subjects of this article, earthworms and leeches. They can range in length from .5 mm to nearly 10 feet.

The Annelids’ segmented design allows for complex behavior and movements as well as segment independence. You see, each segment can function on its own. When it comes to Annelids, their motto should be “United we stand, and divided we still stand!” Or maybe that should be “United we burrow, and divided we still burrow.”

Let’s move on to our local representatives.

Red wigglers

Speaking of local, red earthworms (Eisenia fetida), or wigglers, are thought to have hitched a ride with European settlers to North America during colonial times. Since they thrive in manure, compost, and rotting vegetation, it’s not too difficult to figure out where they were stowaways.

These worms are now naturalized across much of the United States. They are known by different common names in different locales, such red wigglers, manure worms, panfish worms, trout worms, etc.

Time for a vocabulary worm word: peristalsis. This describes the expansion and contraction of their muscles that powers them through the soil. Tiny bristles, called setae, aid in pushing them along. These compost guzzlers stay close to the surface and reproduce quickly. Not surprisingly, they are popular with vermicomposters, or folks who use worms to convert organic waste into fertilizer.

Nightcrawlers

Our next European immigrant is the nightcrawler (Lumbricus terrestris). They burrow much deeper, around six feet, in the soil and reproduce more slowly than the red wigglers. Both earthworms are hermaphrodites, meaning all worms have both male and female reproductive organs.

An indicator of a mature worm is the clitellum, an unsegmented, thickened, glandular section of the body. It is located near the head and plays a crucial role in reproduction by secreting a lemon-shaped cocoon for eggs. The clitellum varies in color from white to orange-red.

During earthworm reproduction, both sets of sex organs are used by both worms. If all goes well, the eggs of each of the mates become fertilized. As you might expect, this is a highly efficient way of ensuring the survival of the species.

The cocoon can contain anywhere from four to 20 eggs that will hatch in roughly two to three weeks. The newborn worms will hatch and emerge from the cocoon into the soil. If they are nightcrawlers, they will start burrowing down into the earth, enhancing soil aeration and structure for the roots of nearby plants. This cycle of reproduction can happen every seven to 10 days, another reason earthworm populations can grow quickly.

As mentioned previously, earthworms are able to reproduce by themselves, if need be. They can regenerate new segments if they lose a few to a shovel blade or trowel. Most earthworms are better at regenerating tails than heads, but some can.

The following description comes from Amy Steward’s 2004 book “The Earth Moved”: “Generally speaking, you can cut a few segments of the tail of a worm and it will grow back in a couple of weeks. The wound will remain open at the end, with new segments forming a slender tail that expands to its full width only after all the segments are in place.” She states that wigglers are much better at this manner of regeneration than nightcrawlers.

Blood-seeking swimmers

Let’s step away from Darwin and Steward and dip into a pond for our final segmented Annelid, leeches (subclass Hirudinea). Unlike the previous two worms, these guys lack setae since they move their ribbonlike bodies by swimming rather than burrowing. Obviously, they feed on blood rather than soil.

There is a sucker at either end of their elongated bodies, and the suckers are often used for locomotion. The mouth sucker is usually at the narrower head end of the leech. About a quarter of the way down from the head is our old friend the clitellum, which, as with the earthworms, becomes more noticeable during times of reproduction.

As with earthworms, leeches possess the remarkable ability to regenerate segments of their bodies, including neural connections. Interestingly, there is a terrestrial variety of leech that feeds on their fellow Annelids, earthworms.

Understanding leech and earthworm regeneration may provide insights into regeneration in more complex organisms, including humans. As Amy Steward writes, “Why is it that a worm can regrow most of its body, but we can’t replace so much as a finger? I am left with the troubling conclusion that the worm’s survival may, in the grand scheme of things, be more important than my own.”

Perhaps, it was not such a leap for Darwin to move from natural selection to earthworms.

• Mark Spreyer is the executive director of the Stillman Nature Center in Barrington. Email him at stillmangho@gmail.com.