WHEEL WEARERS: Cycliophora

PHYSICAL CHARACTERISTICS

Wheel wearers are microscopic animals that live in the mouths of Norway lobsters. Symbion pandora is the only fully described species. Scientists have found two other species, but do not have enough details to describe them thoroughly.

The main phase in the life cycle of wheel wearers is the feeding stage. Wheel wearers in this stage are about 0.01 inch (350 micrometers) long. The body is made up of a bell-shaped mouth funnel, a trunk, and a stalk with an attachment disk. The rim of the funnel is made up of hairlike fibers alternating with muscle cells. The fibers are used to collect food, and the muscle cells close the mouth when the animal stops feeding. The inside of the funnel is covered with hairlike fibers. The funnel connects to the trunk with a narrow, movable neck. The trunk is slightly egg shaped and contains a U-shaped digestive system that runs from the mouth through the stomach to the anus, which is close to the neck. The trunk narrows to a stalk that leads to the attachment disk.

GEOGRAPHIC RANGE

Wheel wearers live in the northeastern part of the Atlantic Ocean, including the Mediterranean Sea.

HABITAT

Wheel wearers live only on bristles in the mouths of Norway lobsters.

DIET

Wheel wearers eat particles of the lobster's prey, usually mollusks and crustaceans. Mollusks (MAH-lusks) are animals with a soft, unsegmented body that may or may not have a shell. Crustaceans (krus-TAY-shuns) are water-dwelling animals that have jointed legs and a hard shell but no backbone.

BEHAVIOR AND REPRODUCTION

When a lobster seizes its prey, food particles and nutrients are suspended in the water around its mouthparts. The hairlike fibers in the wheel wearer's funnel beat and make a current, which causes water containing food particles to flow into the funnel, where the food particles are grabbed by the hairlike fibers and moved toward the stomach.

Feeding-stage wheel wearers use internal budding to replace their body organs several times. While the organ replacement is going on, the wheel wearers also use internal budding to produce larvae that contain miniature feeding-stage animals. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults. When fully developed, these larvae leave the wheel wearer through the mother's anus (AY-nuhs) and settle on the lobster's mouth bristles. A bag develops, the larva (LAR-vuh, the singular of larvae) breaks down, and the miniature feeding-stage animal starts to grow inside the bag. After a short time, the funnel emerges through the bag, and the new wheel wearer starts to feed.

Sometimes feeding-stage wheel wearers produce either females or larvae that give rise to males. The male-producing larvae can move only short distances. Immediately after they are released, these larvae seek the closest feeding-stage wheel wearer with a developing female inside and attach to the wheel wearer close to its anal opening. One or two dwarf males then begin to develop inside the larva. When the female leaves through the anus of the wheel wearer, the dwarf males emerge from the larva. Being good swimmers, the dwarf males quickly find and mate with the female, which contains a single large egg. The female then settles on the lobster's mouth. After settling, the female starts to break down and form a bag in which a new larva starts to develop. Larvae in this stage have a dense layer of hairlike fibers on the belly and are much better swimmers than wheel wearers in any of the other stages of the life cycle. They swim to a new lobster or stay free in the water while the lobster is shedding its shell. When the larvae have settled, a bag forms, and a new feeding-stage wheel wearer starts to develop.

WHEEL WEARERS AND PEOPLE

Wheel wearers have no known importance to people. Because they do not harm their lobster hosts, wheel wearers are not a problem for people who make their living catching and selling lobsters.

CONSERVATION STATUS

Wheel wearers are not considered threatened or endangered.

OUT WITH THE OLD, IN WITH THE NEW

A bud forms inside a feeding-stage wheel wearer. Inside this bud, a new set of organs, including the digestive system, nervous system, and mouth funnel, starts to develop. When fully developed, the new organs slowly move forward and push out all the old organs, until the new funnel can emerge through the neck and replace the old one. The only parts of the old wheel wearer that are used again are the trunk and the attachment disk. A feeding-stage wheel wearer can repeat this self-renewal process several times, and scientists are uncertain why it is necessary. Some believe that because the feeding-stage animals do not have a waste-removal system, waste accumulates, and the self-renewal enables the animal to eliminate these wastes before they become toxic.

FOR MORE INFORMATION

Books:

Valentine, James W. On the Origin of Phyla. Chicago: University of Chicago Press, 2004.

Young, Craig M., ed. Atlas of Marine Invertebrate Larvae. San Diego, CA: Academic, 2002.

Web sites:

Walker, Dave. "A Lobster's Microscopic Friend: Symbion pandora: A New Life Form and a New Phylum." Microscopy UK.http://www.microscopy-uk.net/mag/indexmag.html?http://www.microscopy-uk.net/mag/articles/pandora.html (accessed on February 24, 2005).