Mudworms

Those Wonderful Worms

Written by:
Charles D. Drewes, PhD
Department of Zoology and Genetics
Iowa State University
Ames, IA 50011

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Science fiction writers often tantalize their readers by conjuring up creatures with bizarre combinations of biological traits and powers. Similarly, teachers often muse longingly for the "perfect" animal for their classroom. Imagine for a minute an animal with transparent skin and red blood pulsating in a heart that is as long as the creature's body. This creature can endure being cut up into many pieces without bleeding or dying and can create a new head or tail, or both, from severed body pieces. It swims, without fins or appendages, by making twisting, corkscrew-like motions of its whole body, and it uses its tail to detect an approaching shadow. Conveniently for you, it lives just beneath the water's surface at the edges of murky ponds and marshes-perhaps even near your home or school.

Figure 1The California blackworm, Lumbriculus variegatus.

Impossible, you say? Not at all. In fact, the creature I've described is a common freshwater annelid, Lumbriculus variegatus. Also known as a California blackworm, or mudworm, Lumbriculus variegatus. (Fig. 1) is a member of the Order Lumbriculida, a small subgroup of oligochaetes that includes neither earthworms nor freshwater tubifex worms. About 1 to 2 inches in length, blackworms are found in sediments and submerged organic debrisespecially along the shallow marginsin ponds, marshes, and lakes throughout North America. Despite being widely distributed and having many interesting features, blackworms seem to have escaped the attention of most biology teachers and researchers.

Blackworms are exceptionally hardy and easy to raise at home or in your classroom, making them readily available year long. With a little patience and imagination, keen eyes, and insights from this article, you and your students can discover and share many intriguing aspects of biological organization and behavior. This worm should provide a variety of new insights and simple, enlightening investigations for your class or lab. No extensive training or expensive equipment is needed; in fact, all of the following laboratory activities have been done with little or no special apparatus or dexterity.

Mature blackworms, usually composed of about 150 to 250 segments, are hermaphroditic (that is, they contain both male and female sex organs). Sexual reproduction, presumed to be rare, involves direct embryonic development within a cocoon. Asexual reproduction by self-fragmentation is common. Under laboratory conditions, this seems to be the worm's sole means of reproduction.

Raising and Handling
California blackworms can be cultured and easily maintained in a small aquarium or deep pan filled with 2 to 3 inches of springwater (or aged tap water). At room temperature in the laboratory, populations double in about 3 to 4 weeks or less. Using a disposable plastic pipette, transfer a few dozen, undamaged, healthy worms into the aquarium. Never attempt to handle or transfer worms with forceps or hooks. They are easily injured by these instruments.

Next, add enough strips of brown paper towel to just cover the bottom of the container. The towel serves as a fibrous substrate of decomposing material, both for the worms and for numerous microscopic organisms that may cohabit the culture, such as bacteria, protozoans, rotifers, and ostracods.

Add sinking fish-food pellets as the primary food source for this simple aquatic ecosystem. Start by adding one or two pellets. After a few days, add one or two more, but only if the others have been consumed. Do not overfeed, since decomposition of uneaten food may contaminate the aquarium and cause a mass die-off of worms. Worms are not harmed, however, by irregular feeding or long periods of starvation.

Replace water lost to evaporation by adding springwater (or distilled water).I recommend continuous, gentle aeration, and this becomes increasingly important as biological decomposition of the paper occurs and as the worm population increases.

As the paper towel disintegrates and waste residues accumulate, replace the culture water regularly (about every two weeks) by slowly decanting it down a drain. Be careful not to lose remaining paper and worms at the bottom. After rinsing the paper and worms again with springwater, and decanting, refill the aquarium to the original level and add new pieces of towel. I suggest the occasional "harvesting" of surplus worms; these can be used for classroom experiments, as live food for fish, or for starting duplicate cultures. I strongly advise the maintenance of at least one duplicate culture. If you follow these procedures, the worms reproduce continuously by asexual reproduction (fragmentation), and cultures may be sustained for years.

Handling the worms is easy, but you should follow certain important precautions. Capture and handle the worms (or worm fragments) while they are immersed in water. Using an eyedropper or plastic disposable pipette, simply suck one or two up, along with a little water. Never pick up or handle the worms with forceps, hooks, or metal probes because even a slight injury by pinching or poking causes them to self-fragment (autotomize). However, if blackworms do become "dismembered," you need not discard the piecesjust save them for further experiments.

Figure 2(a) Image of newly regenerated head and tail ends from a 16-segment-long fragment.

Figure 2 (b) Enlarged image of newly regenerated head.

Regeneration
Blackworms are ideal organisms for studying segmental regeneration, developmental pattern formation, and developmental transformation of original body segments (a process called morphallaxis). Small fragments of the worm, some only a few segments in length, can be easily amputated, isolated, and stored using only basic lab supplies, such as a razor blade, filter paper disk, disposable plastic pipette, and storage containers (for example, capped centrifuge tubes or multiwell culture dishes).

Students soon discover that survival rates of body fragments are excellent, and the regeneration of missing parts occurs quickly. For example, development of a head (usually 8 new segments in length) and/or a new tail (ranging from 20 to 100 or more segments in length) is usually completed over 2 to 3 weeks (Fig. 2). Using a stereomicroscope, students can count and readily distinguish new segments from the older, original segments in the fragment.

Remarkably, the entire process of head and tail regeneration by a small body fragment can be played out within a small, tightly sealed container with as little as 1 to 2 mL of water and no food. I have continually kept isolated worm fragments alive under these conditions for more than 6 months. Such fragments gradually diminish in size, new segments readily regenerate. Because of their small size and pale color, the new segments can be easily distinguished from the older ones.

Locomotion
Like their terrestrial relatives, blackworms move by means of circular and longitudinal muscle contractions acting on the worm's hydrostatic internal skeleton, that is, the fluid-filled body cavity (coelom) in each segment. However, blackworms are quite acrobatic. Several very different forms of locomotion are possible, depending on the sensory cues present in the worm's immediate environment. For example, if you place the worm on wet filter paper and stroke the tail lightly with a hair, it crawls forward by reflexive peristalsis. In contrast, if you stroke the head lightly under these same conditions, reverse peristaltic crawling moves the worm backwards.

Figure 3Freeze-frame video images of clockwise (a) and counter-clockwise (b) twist of body during corkscrew swimming in a small specimen (arrow indicates direction of worm's forward movement). Elapsed time = 0.1 sec. 10x actual size.

To see very different movements, place the worm underwater on a flat, bare glass or plastic surface, such as the bottom of a dish containing at least a centimeter of clear water. Now, prodding the worm's tail initiates several cycles of forward swimming. To accomplish this, the worm suddenly and repeatedly twists its body into a helical or corkscrew-like shape. Each helical twist then passes rearward, as a coordinated wave, thus propelling the worm forward and away from the stimulus. Each wave cycle alternates between clockwise and counter-clockwise orientations (Fig. 3).

Prodding the worm's head while it is underwater initiates an unusual reversal behavior in which the body suddenly coils and then unfurls so that the head and tail ends rapidly reverse their original positions. Since the worm can't swim backwards and has no means of traction, this seems to be a novel but practical way for the worm to make a 180 turn and get its head removed from a menacing stimulus.

Rapid Escape and Shadow Reflexes
When the blackworm occupies natural sediments, it prefers to protrude its tail vertically out of the bottom debris and toward the water surface. This allows the head to continue feeding or probing in the bottom debris while the tail extends up into the more oxygen-rich water column.

Figure 4 The worm's tail at water surface (side view).

If the water is shallow enough, the worm stretches its tail all the way up to the surface where it actually breaks the surface tension of the water (Fig. 4). While doing this, the worm bends its tail at a right angle with the dorsal surface facing skyward and exposed to air. Although this is an optimal position for gas exchange, it also makes the worm's tail especially vulnerable to predation.

To offset this problem, the worm uses its rapid escape reflexes, in which the tail end rapidly shortens in response to the sudden onset of threatening stimuli. Some stimuli that readily elicit this response include direct touch, substrate vibration, or the abrupt onset of a shadow. In fact, photoreceptors used to detect a shadow are present in the worm's tail!

To produce the escape reflex, these sensory inputs initiate electrical impulses in rapidly conducting, giant nerve fibers found within the worm's ventral nerve cord. These impulses, in turn, trigger the synchronized motor outputs and muscle activity needed for rapid tail withdrawal.

Figure 5 (a) Image of pulsation waves in dorsal blood vessel.

Figure 5 (b) Image of lateral branches of the dorsal vessel.

Blood Flow and Pulsations
Place a worm on moist filter paper and view it under a stereomicroscope. You notice immediately many large and small blood vessels coursing through its body. The two largest vessels, running lengthwise along the worm, are the dorsal and ventral blood vessels. As in their terrestrial relatives (namely, earthworms), the dorsal blood vessel of blackworms is pulsatile, and contraction waves in the wall of this vessel pump blood from the tail toward the head (Fig. 5). The small, segmentally arranged lateral branches of the dorsal vessel are a unique and diagnostic feature of blackworms. Found in each segment, these branches are also pulsatile and seem to act as auxiliary pumps.

Pulsations of the dorsal vessel help to deliver blood throughout the entire body after it is oxygenated in the worm's protruding tail. A closer look at the dorsal blood vessel in tail segments reveals several important anatomical and physiological adaptations for gas exchange, including an expanded volume of the dorsal vessel, rapid pulsation rates, and close contact between the vessel and the dorsal epidermis. You can devise many novel experiments to measure pulse rates and the velocity of pulse waves under various conditions, in either whole worms or worm fragments.

The freshwater blackworm, Lumbriculus variegatus, is a "user-friendly" creature with an unusual combination of biological features and functions that can be easily observed, in either whole worms or worm fragments. With so little previous research done on this organism, there are many potential opportunities for students to make original observations and contributions regarding its development, behavior, physiology, and ecology.