Thursday, October 9, 2008

WormWatch - Ecology Reproduction (EXT)

This article was extracted from Worm Watch

Ecology - Reproduction

How does an earthworm eat?

Earthworms eat by pulling food into their mouth with their prostomium combined with the muscular pharynx which creates a very high suction (like a vacuum). The food is stored in the crop and then ground up into small digestible pieces in the gizzard. Earthworms need a gizzard because they do not have any teeth. The nutrients are absorbed into the body in the small intestine.

Earthworms probably feed while they are moving. They are not picky eaters as they will eat all sorts of dead things such as decaying plant and animal matter. They consume a great deal of matter in a short period of time - they can produce their own weight in castings (worm dung) every 24 hours!

View of the digestive system of an earthworm.

Where do earthworms live?

Earthworms are found all over the world. Australia, the Sahara Desert, Greenland and China are among only a few countries that have their own distinct indigenous species. Although several species live in various horizons (layers) of the soil or in the surface layer, others can be found in rotting logs, in the axils of tree branches (the upper angle between the branch and the trunk, sometimes up to 10 m above ground) or along the moist soil surrounding bodies of water (lakes, rivers, springs, ponds).

Despite this wide variety of habitats, there are still certain environmental conditions which must be maintained for an earthworm to survive. For example, all earthworms need an adequate food supply to be close at hand. Earthworms generally remain close to their food supply. Since earthworms breath through their skin (they have no lungs), it is important that their environment is moist to allow for respiration. Earthworms release internal fluids (like perspiration) which traps the dissolved oxygen. Too much moisture (heavy rainfall) however takes the place of the valuable oxygen dissolved in the soil (also needed for survival), which may cause the earthworms to crawl to the soil surface. Here at the soil surface, earthworms will be exposed to ultra-violet radiation (sunlight) which is lethal to earthworms in a short period of time.

Earthworms are light sensitive and prefer moist slightly warm soil to grow and reproduce. Since earthworms live and travel around in the soil they form burrows as they move. Some species make deep vertical burrows. These earthworms are anecic species. Other species burrow continuously to form a network of channels -some vertical and some horizontal in the rhizosphere and are called endogeic species. Some earthworm species are not strong burrowers and live in the uppermost layer of soil in the litter layer. These earthworms are called epigeic species and they form shallow vertical burrows where they temporarily escape from drought, heat and disturbances.

Though small, earthworms are fighters. They have developed certain survival strategies which help them cope with nasty environmental conditions. When the weather gets cold and the soil starts to freeze, earthworms move deeper down and overwinter in a state called aestivation. To aestivate, the earthworm generates a natural antifreeze and then curls up in a little knot. Earthworms also aestivate when conditions become dry or hot.

How do earthworms reproduce?

Of all the soil organisms, the earthworm has by far one of the most unique modes of reproduction. Earthworms are hermaphrodites, which means they have both male and female reproductive organs. This brings us to one of the oldest myths about worms: that earthworms can fertilise themselves. Though some earthworms can fertilise themselves (parthenogenesis), this is not the norm. Most earthworms require a mate of the same species to reproduce.

Reproduction between two Earthworms

You can tell when an earthworm is ready to mate because its’ clitellum will change colour from pinkish to red-orange. The two earthworms line up in a head to tail fashion and exchange spermatozoa (sperm). The sperm is stored in the spermathecae. Both worms do this at the same time. A slime tube then forms around the clitellum, which dries and fills with a fluid called albumin. The earthworm then wiggles out of the tube head first. While the tube passes from the clitellum to the prostomium, it passes over the female pore which deposits ovum (eggs) into the capsule, followed by the spermatheca pore (male pore) which releases the stored spermatozoa. Some earthworms mate on the soil surface and some earthworms mate in the soil. Given the dark soil environment, we think that earthworms produce a pheromone (chemical) that signals other earthworms in the area that it is ready to reproduce.

What do earthworms do?

Life on this planet would be a lot different without our little friends from down under. The quality of our soil depends heavily on the livelihood of earthworms. In the long run, healthy high quality soils will be the key to a sustainable environment and thus ourselves. The biological, chemical and physical properties of soil are essential for plant growth, regulating and partitioning of surface to ground water, and buffering, detoxifying and scrubbing of hazardous chemicals

In actuality, the soil is a reservoir of biological diversity that likely exceeds that of aboveground ecosystems.

The effects of soil quality on soil productivity.

Earthworms have some unique functions in the soil. Their large burrows allow rain water easy entry into the soil, increasing the infiltration rate of soils. This prevents water erosion and lets water enter the rooting zone where it can be used by plants. Their burrows also allow roots to move easily through the soil into new spaces. Soil that has been worked by earthworms has a stable crumb-like structure which is less likely to blow away in the wind.

Earthworms are considered very important in soil organic matter cycling. Certain species are responsible for burying surface residue, while other species are actively involved in the decomposition process, making available important nutrients for other living organisms in the soil - like plants.

The effects of soil productivity on food quality, health and environmental quality.

Earthworms have some negative traits as well, in certain areas introduced species have created competition for native earthworms. This makes it very difficult for native species to live successfully and even survive, native species frequently exist in small isolated areas. Many of the earthworms we think as common have been introduced from Europe. This is not necessarily a bad practice but has created some problems. An example is the middens associated with Lumbricus terrestris, in Western Canada were soils are dominated by calcareous parent material the deep burrowing Lumbricus terrestris creates middens of concrete. Their burrowing exposes the hard subsoil to drying and wetting cycles which makes this material very hard and difficult to manage in gardens, golf courses and lawns.

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