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.
Figure 3 Freeze-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.
Rapid Escape and Shadow Reflexes
Figure 4 The worm's
tail at water surface
(side view).
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.
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.
