Shovel-headed Garden Worm

Introduction

The Shovel-headed Garden Worm, Bipalium kewense, was first discovered in the hothouses of the famous Kew Gardens in England (its scientific name means 'plate-headed worm from Kew'). It was formally described in 1878. Shovel-headed Garden Worms belong to the Phylum Platyhelminthes, a large phylum of worms known as flatworms because of their flattened bodies. Many flatworms are parasitic, but the Shovel-headed Garden Worm is a free-living terrestrial species.

Identification

In addition to its peculiar shovel-shaped head, the Shovel-headed Garden Worm has a long, soft, flattened body (a few millimetres in width) with five longitudinal dark brown stripes, which vary in their distinctiveness and width. One of these bands runs along the upper midline and there is a dark patch in the neck region. These worms are light ochre-yellow on the upperside and greyish white or pinky grey on the underside. As they move along, they excrete large amounts of slimy mucus from glands in the body wall, and wave their heads from side to side. They slide over this mucus carpet by the action of closely spaced, tiny hairs which are found in a strip on the underside. If they migrate onto plants, or other objects, they can lower themselves back to the ground by a string of mucus.

The Shovel-headed Garden Worm, like other flatworms, does not have a respiratory or circulatory system, a skeleton, or an anus. The mouth, which also serves as the anus, is found near the middle of the body on the lower, or ventral, side. It has a protrusible muscular pharynx that is used as a feeding organ; this is attached to a three-branched intestine. It has a cluster of nerves at the head end that serves as a brain, and this is connected to a ladder-like nervous system, circular and longitudinal muscles for movement, and a primitive excretory system.

Habitat

Because they avoid light and require high humidity, Shovel-headed Garden Worms are confined to dark, cool, moist areas, under objects such as rocks, logs, leaf litter and other debris. They also occur on the surface of the soil following heavy rains. They are found primarily in tropical and subtropical regions, although they can survive in cooler areas if their microhabitat is protected.

Distribution

As early as 1899, the Shovel-headed Garden Worm was thought to have a cosmopolitan distribution, supposedly transported to other parts of the world from England and becoming established in a similar way to the common garden snail of Europe. It now seems more likely that the Shovel-headed Garden Worm originated somewhere in Indo-China; its natural range extends from Vietnam to Kampuchea, possibly extending to Malaysia. It was probably sent to the Kew Gardens from its place of origin in a shipment of plants. They are now distributed widely as a result of horticultural practices, being dispersed in potted plants. From pot-plants, they can readily move into the adjacent environment if the habitat is suitably moist and humid. They are very common in the bush around Sydney.

Feeding and diet

Shovel-headed Garden Worms are carnivorous, feeding on earthworms, insects and insect larvae, and young snails and slugs. They detect their prey using chemical receptors found in a pit on the underside of the head. The head-waving allows the worm to hone in on the chemical cues emitted from its prey. Struggling prey are held close to the substrate and entangled in slimy secretions from the worm. The pharynx is protruded from the mouth and into the prey, which is reduced to small particles prior to digestion. The food particles are stored inside the body, and the worm can survive for many weeks, but shrinking in size, between meals. They are also capable of digesting their own tissues, such as the reproductive organs, for food when other reserves are depleted.

Other behaviours and adaptations

They hide during the day, emerging only at night to feed.

Life history cycle

Like other flatworms, reproduction takes place mainly by the Shovel-headed Garden Worm splitting into parts from the tail end. This method of asexual reproduction is called fission. The sides pinch in about a centimetre from the tip of the tail. The tail piece breaks off when it sticks to the substrate and the parent worm pulls away. The broken fragment can move about immediately, and a head begins to form within a week to ten days. A couple of fragments break from a parent worm every few weeks.

They can also breed by sexual reproduction. Sperm transfer involves copulation between two worms (rather than self-fertilisation) and is reciprocal; each worm has both a penis and a female gonopore, the receptacle for receiving sperm. Eggs are deposited in reddish cocoons, which later turn black and hatch after about three weeks.

Economic impacts

These worms are not harmful to humans, but can be pests capable of eradicating earthworm populations.

Predators

Shovel-headed Garden Worms have few enemies because their slimy secretions appear to be distasteful, if not toxic. They may, however, be eaten by other flatworms.

References

• Pope, E. C. 1972. When the Rains Come. In A Treasury of Australian Wildlife. Pp. 334-338. (Ed. D. F. McMichael.) Ure Smith, Sydney.
• Winsor, L. 1997. A revision of the cosmopolitan land planarian Bipalium kewense (Turbellaria: Tricladida: Terricola). Zoological Journal of the Linnean Society 79: 61-100.