Locomotion in False Bay
The head end of a polychaete worm (Family Nereidae).
As you walk around False Bay, you will quickly notice that little of the fauna is visible. The soft substrate (sand and mud) provides protection and food for the animals. It is therefore quite difficult to observe locomotion. But if you try to catch some of these animals, such as clams, you will find that many are efficient burrowers. Rapid burrowing is important for infaunal organisms in case they become exposed to harsh environmental conditions or to predators by wave action or other disturbances.

The way in which organisms have adapted to a burrowing lifestyle is varied.  Some organisms burrow and construct a tube only once, remaining sedentary for the remainder of their lives. Others construct burrows that must be maintained, and these organisms are quite active within the sediment. Below are four representative species from False Bay and their methods of burrowing. There is also an example of one of the few above-surface polychaetes that you may see while out on the bay. At the end of the page are links to other types of invertebrate locomotion as well as a suggestion for how you can observe infaunal locomotion.

Burrowing locomotion

The principles of burrowing by most organisms are straightforward. In the first step, a part of the body, for example a proboscis or a foot, is used to extend into the sediment. In order to push sediment out of the way, the other end of the organism must be anchored to avoid any backslipping. The part of the body expanded to prevent slippage is known as the "penetration" anchor, because it keeps the body in place while the other part moves forward, penetrating the sediment. The second step in burrowing occurs when the portion that has penetrated the sediment becomes a "terminal" anchor so that the rest of the body can be pulled forward. Burrowing is achieved with the coordinated repetition of these two movements. (Ruppert and Barnes, 1996)

Polychaete worms

Most actively burrowing polychaetes such as worms in the family Lumbrineridae use a specialized method of burrowing known as peristalsis. You may have observed this movement in earthworms.  Peristalsis is achieved by alternate contraction of longitudinal and circular muscles, creating a bulge in the body that can push against the sediment as the bulge moves backward along the body.  When the circular muscles contract, the body elongates, pushing into the sediment. Contraction of the longitudinal muscles and simultaneous relaxation of circular muscles shortens that portion of the body, widening it to form an anchor. The rest of the body can then be moved up to the anchor by partial contraction of the longitudinal muscles. This quick video can give you an idea of how peristalsis works.

Worms in the family Arenicolidae use similar burrowing mechanics. While many worms use peristalsis over the entire body for movement, in the Arenicolidae this movement is primarily confined to an eversible part pharynx. The process begins with repeated eversion of the pharynx to push surface sand aside. Force is minimal because the only anchor is the weight of its body. Once an anchor can be established within the sediment, true burrowing can begin. With the body acting as the penetration anchor, circular muscles contract, the pharynx elongates and penetrates the sediment. Fluid flow into the pharynx further widens it, creating the terminal anchor. The remainder of the body can then be shortened by longitudinal muscles and brought up to the pharynx, completing the cycle. Arenicolidae are particularly effective at burrowing because the septa within their bodies are not complete. This allows for flow of body fluid to evert the pharynx and form an anchor. (Treuman and Ansell, 1969)  A video of this motion is provided here.

Bivalves, Macoma and Clinocardium

A similar method is used by bivalves, such as the clam Macoma and the cockle Clinocardium, which use a muscular, blade-like foot.  Digging at the surface with the foot eventually provides enough penetration to hold the bivalve upright. The shell can then form the penetration anchor by pressing its valves against the sand and the foot can make a push into the sediment. This is followed by a dilation of the foot using hydrostatic pressure to form the terminal anchor. At this point, the valves of the shell close and the pedal retractor muscles pull the shell down to the foot. The process is then repeated.  Using this method, some bivalves are surprisingly rapid burrowers.

Crustaceans, Neotrypaea

A champion burrower, the ghost shrimp Neotrypaea spends much of its life under the sediment in mucous-lined burrows. These animals have been known to burrow down to 1 m (Dumbauld et al., 1996).  Along with the mud shrimp Upogebia, ghost shrimp use a very different method to form their burrows as compared with worms and molluscs. The legs of the shrimp are specially adapted to burrowing.  The chelate first and second legs loosen the sediment and pull it backwards. The third legs then work to accumulate the material around the third maxillepeds. Carrying its load of sediment, the shrimp then crawls backwards into a turn-around chamber, reverses itself, and climbs to the surface where the sediment is deposited. Piles of sediment typically mark the mouth of these burrows. (Kozloff, 1996)

The mud shrimp Upogebia, showing legs that are specialized for burrowing.
Non-Burrowing Locomotion

Polychaete worms, Nereis

The motion of the nereid worms is very different from that described above and is used mainly for crawling on the sediment surface and at times even for swimming.  Here is a video of a nereid crawling, exhibiting the characteristic sinusoidal motion.  Locomotion is achieved using the longitudinal muscles. There is an alternating motion of the two sides of the body, as the longitudinal muscles on one side contract, the other side relaxes. The parapodia in crawling worms such as the nereid are usually well-developed and provide a force against the sediment similar to that provided by legs. Movement of parapodial appendages is in coordination with the sinusoidal movement of the body, so that the power stroke of a single parapodia is at the crest of the wave.  Notice that the wave motion moves from posterior to anterior (how does this allow the animal to achieve forward movement?). As you can see from the video, each pair of parapodia is exactly out of phase, as one is in power stroke, the other is recovering. This sinusoidal motion allows the nereid and others like it to swim as well. The motion is not very powerful and is probably only used for short durations to escape a predator (Brusca and Brusca, 1990), although nereids in the reproductive epitoke form are found as plankton in surface waters.

Other Organisms

Nereid worms are not the only epifauna that you will see at False Bay. There are several types of crabs that scurry over the surface and hide under rocks. Bubble snails make their way slowly across the sand within standing pools of water. And if you take some time to look under the Ulva in the westernmost area, you will come across small invertebrates such as amphipods and isopods that quickly squirm away under the algae. To read more about how these other organisms move around, check out the invertebrate locomotion pages from Cattle Point, Argyle Lagoon, Plankton, and Floats.

Be the first on your block to have a worm farm!

Because most of these organisms are "infaunal," meaning that they live under the sand, it is difficult to visualize their movements in their natural environment. If you are really curious about what goes on under the sediment you may want to create a worm farm.