Gordian Worms

Introduction

Gordian worms belong to a small phylum, the Nematomorpha: a name that means 'form of a thread'. Their habit of writhing and contorting themselves into knots, with one or more worms tangled together, accounts for their common name, 'Gordian' Worm. This is after Gordius, King of Phrygia, who tied an intricate knot and declared that whoever untied it should rule Asia. Alexander the Great cut the Gordian knot with his sword.

Identification

Adult Gordian Worms are shaped like long, thin pieces of cord. They taper slightly at each end and reach lengths of about 22 cm to 30 cm. The diameter is usually not much more than a millimetre. They are brown or black in colour. There is no distinct head and the male differs from the female in having a forked tail. The males are often very active in their swimming, while females are more sluggish.

Habitat

Gordian Worms are found all over the world, in still water such as puddles, ditches, ponds and quiet areas in streams. Sometimes they get into domestic water supplies, such as toilets or bowls of water. They are usually found after rains, their sudden appearance leading to stories to explain their presence. Some communities believe that a 'rain of worms' has occurred, others that horsehairs have 'come to life' after falling into a pond or stream. Since they often appear in animal watering troughs and their appearance is not unlike that of horsehair, although they are thicker, it is not difficult to see how this explanation came about. This also led to other common names, such as 'Horsehair Worms' and 'Horsehair Snakes'.

Adult Gordian Worms are free-living in water, but the juveniles are parasitic in land-dwelling insects and spiders. Because these hosts are not found in water, the larvae must reach them by a two-step process. The adult Gordian Worms attach long egg-strings to waterweeds and other debris in water. From these eggs, tiny larvae hatch and sink to the bottom, but can only survive for a short time unless they find a host. Many animals, such as fish, snails and small crustaceans, get infected.

Distribution

Gordian Worms are found all over the world.

Other behaviours and adaptations

The Gordian worm larvae are thought to bore into the first host's tissues by means of an armature of spines on the proboscis (feeding structure, 'nose'). The larvae then form cysts (protective shells on their outer surfaces) in the body, but usually do not proceed further through their life cycle within these aquatic hosts. Many insects, however, have an aquatic, worm-like, larval stage (for example mosquito and dragonfly larvae), and if these animals are infected, then the Gordian cysts can be carried from the water when the insect larvae metamorphose into flying insects. When these insects are eaten by other invertebrates, such as grasshoppers, crickets, mantids, beetles, cockroaches and spiders, the Gordian Worm cysts can then break open and the larvae penetrate the gut of the new terrestrial host. Sometimes these hosts can be infected directly when they visit water, or live close to it.

A larva lives in the body cavity of its host, secreting digestive enzymes through its skin to gradually absorb the host's body contents. It grows and sheds its larval structures, such as the hooks and stylets of the proboscis, and gradually changes its skin to that of an adult. Larval development takes a number of weeks, or even months, during which time the contents of the host's abdomen are completely consumed and the larva completely fills the cavity. It then breaks out when the host is near water, and takes up a free-living existence until its sex organs mature. The worms don't feed at all in the adult phase of their life cycle, so they die when the stored foods in their bodies are depleted.

Life history cycle

The free-living adult is the reproductive stage of the life cycle. The sexes are separate (i.e., individuals are either male or female) and the entire body of an adult is filled with two cylindrical sex organs (the gonads) containing eggs or sperm. The worms mate in water. A single female can lay as many as 10 million eggs. The larvae hatch after two to four weeks. Adult Gordian Worms are not often seen as they live for only two to four weeks and the females tend to hide among rocks, sticks and small branches in the water.

A number of freshwater fishes feed voraciously on Gordian Worms in summer and their stomachs may be crammed with the brown, thread-like worms. The fishes are not harmed by the worms and they are still safe to eat.

There have been reports of humans and dogs excreting Gordian Worms. Accidental ingestion of the worms, perhaps in drinking water, is the most likely explanation for these observations. Although Gordian Worms can resemble some of the parasitic round worms (Phylum Nematoda), but there is no danger of humans becoming parasitised by them.

References

• Barnes, R. D. 1980. Invertebrate Zoology. Saunders College, Philadelphia, USA.