• The world beneath our feet: Earthworms

Barry Muir

Earthworms are wonderful little critters, admired by gardeners everywhere: if you have lots of worms the soil is considered healthy. But what are earthworms? They are tubular, segmented worms that eat living (microbes, fungi, etc) and dead organic matter such as decaying plant material and dead animals. There are over 1,000 named native species in three families in Australia and probably lots more to be discovered, especially in Tropical North Queensland where very little work has been done. They breathe through their skin, so must keep it moist; have a circulatory system with red haemoglobin (same as us); and a central nervous system with a ‘brain’ surrounding the mouth, with nerves running the length of the body. They have special sensory organs near their mouth which allow them to detect food, and a gut with a grit-filled stomach called a gizzard. The gut runs the whole length of the body. Muscles around each segment and along the body allow them to move, sometimes quite speedily if they feel threatened. Touching an earthworm causes both a pressure response and a response to the toxic salt on human skin, and elicits a reflex causing the writhing movements observed when we pick up an earthworm. This response does not require the animal’s central nervous system: it is purely a defensive reflex and does not indicate pain. The sudden movement is probably intended to scare off a predator. Generally, the number of segments in the body is consistent within a species, and individuals are born with the number of segments they will have throughout their lives. Earthworms are normally hermaphrodites, each carries male and female sex organs, but they need to mate with another worm to produce eggs. However, there are a few common species that are mostly parthenogenetic, meaning that growth and development of embryos can happen without fertilisation – very handy if there are not many worms about! Mating occurs on the surface of the soil, most often on damp nights. After mating, long after the worms have separated, the clitellum (that fatter group of segments about one-quarter of the way down the worm from the mouth) secretes material that forms a ring around the worm. The worm then backs out of the ring, and as it does so, it injects its own eggs and the other worm’s sperm into it. As the worm slips out of the ring, the ends seal to form a cocoon in which the baby worms develop. Baby earthworms emerge fully formed, lacking only their sexual structures which develop in about 60 to 90 days. They attain full size in about one year. Scientists predict that the average lifespan of a worm under field conditions is four to eight years, while most garden varieties live only one to two years.

In soils, earthworms play a major role in the conversion of large pieces of organic matter into rich humus, thus improving soil fertility. This is achieved by the worm’s actions of pulling deposited organic matter, such as fallen leaves or manure, below the surface, either as food or to plug its burrow. In addition to dead organic matter and fungal fragments, the earthworms ingest minute soil particles into their gizzards, wherein those minute fragments of grit grind everything into a fine paste that is then digested in the intestine. When the worm excretes this in the form of ‘worm-casts’, deposited on the surface or deeper in the soil, minerals and plant nutrients are changed to an accessible form that fungi can then pass to plants. Investigations in the United States show that fresh earthworm casts are five times richer in available nitrogen, seven times richer in phosphates, and 11 times richer in potassium than the surrounding soil. The earthworm’s burrowing also creates a multitude of channels through the soil and is of great value in maintaining the soil structure, improving aeration

and drainage, and forming pathways along which mycorrhizal fungi can grow their mycelium. Permaculture co-founder Bill Mollison pointed out that by sliding in their tunnels earthworms “act as an innumerable army of pistons pumping air in and out of the soil on a 24-hour cycle (more rapidly at night)”. Thus, the earthworm not only creates passages for air and water to pass through the soil but also modifies the vital organic component that makes a soil healthy. In Australia, the application of toxic superphosphate on pastures and a switch from sheep and cattle to grain farming has had a devastating effect on populations of native earthworms. Globally, as a result of the use of synthetic fertilisers and pesticides, many earthworm species are endangered. Additionally, about 80 species have been introduced to Australia from overseas, some as worm-farm inhabitants which are then released when people get tired of looking after them. We have no idea what impact these feral worms are having on native species, but like most introductions of animals (cane toads, rabbits, cats), they are probably causing extinction of native species. There are at least two species of earthworms in the Cairns Botanic Garden, a bright red one about 12cm long and a pale pinkish one that gets up to 20cm or more in length. I have observed both in the Flecker Garden on the surface after heavy rain, but there are probably many more species in the rainforest area and in moist soils surrounding the Freshwater and Saltwater Lakes. Wouldn’t it be nice if we had some 2m long bright blue ones as thick as garden hoses, like those found in the hills behind Cairns and in the Daintree area?

Inside an earthworm head end. You thought they were simple, didn’t you?

From: homework.sdmesa.edu Lateral nerve

Pharynx Brain Buccal cavity Prostomium Second left aortic arch Seminal vesicles Esophagus Epidermis Gizzard

Mouth Circular muscle

Longitudinal muscle Dorsal vessel

Intestine

Left circumpharyngeal connective Seminal receptacle Testes Sperm funnel Ovary Egg funnel and oviduct Nerve cord

Subneural Nephridium

Ventral vessel

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