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DISPLAYING POSTS TAGGED: earthworms (1)

Bug of the Month - the earthworm

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by Tim Blackburn
Publish date
1 June 2012
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June's Bug of the Month is certainly not a bug, but the integral role that the earthworm plays in many terrestrial ecosystems is why I've selected it this month. The famously influential Charles Darwin studied earthworms at great length. His 1881 book, The Formation of Vegetable Mould Through the Action of Worms, With Observations on Their Habits, sold more copies than On the Origin of Species. Darwin commented, "...it may be doubted if there are any other animals which have played such an important part in the history of the world as these lowly organized creatures."

Earthworms belong to the phylum Annelida which incorporates all the segmented worms, including the marine worms and the leeches. More than 3,000 species of earthworm, ranging in length from one centimetre to two metres, are found right across the planet in a diversity of habitats – including Melbourne Museum's gardens.

Earthworm An earthworm, showing its long segmented body.
Image: Tim Blackburn
Source: Museum Victoria

Earthworms inhabit moist, rich soils and emerge at night to feed on decomposing organic matter. They possess bristle-like hairs called setae which form a ring around most body segments. The setae help the worm to sense its environment and to grip the soil as the earthworm moves around. They do not have a skeleton, per se, but possess a fluid-filled body cavity (a coelom) against which their muscles contract. A swollen band towards their anterior (front) end, called a clitellum, secretes an egg-filled cocoon soon after mating.

Earthworm The bulge visible toward the anterior end of this earthworm is the result of the peristaltic (wave-like) contraction of its muscles against its hydrostatic skeleton. The swollen, orange band around the body is the clitellum.
Image: Tim Blackburn
Source: Museum Victoria
 

earthworm clitellum A close-up of the earthworm's clitellum
Image: Tim Blackburn
Source: Museum Victoria
 

Earthworms play an important role in stabilising soil structure and maintaining soil fertility. They are instrumental in the decomposition of organic matter and the associated replenishment of soil nutrients. Charles Darwin estimated that earthworms add a 5mm layer of nutrient-rich soil to English pastures each year.

Earthworms have a lower optimal body temperature than most invertebrates and prefer damp soils, since they must keep their cuticle moist to be able to respire through it. Earthworm activity will be high in Melbourne during June as temperatures plummet and evaporation decreases. I found a dense population of earthworms while digging up one of the garden beds in the Milarri Garden this week. The worms seem to be breaking down much of the leaf litter that accumulated during autumn, thereby returning nutrients to the soil.

Trees in autumn The current view of Carlton Gardens, looking out from the Millari Garden. Earthworms and soil microorganisms will break down the autumn leaves within a matter of months.
Image: Tim Blackburn
Source: Museum Victoria
 

The benefits that earthworms provide for soil are due to their burrowing habits and their methods of feeding, digestion and excretion. They swallow much of the soil and organic matter they encounter and deposit it as nutrient-rich faecal casts. The waste products and mucus secretions of the worms provide nutrients for many microorganisms, which improve soil fertility through further decomposition. Earthworms' burrowing actions also aerate and drain the soil, preventing it from becoming compacted and waterlogged.

These animals are essential components of both natural and human-dominated ecologies, and they've also influenced human history. For example, the migration and settlements of early humans were limited by the productivity and fertility of soils. The role that earthworms have played in the burial of ancient buildings over millennia was studied at length by Charles Darwin, a phenomenon which illustrates just how closely human societies are intertwined with earthworms. The world's diverse ecologies, agricultural systems and expansive cities owe much to the largely unnoticed action of earthworms below ground.

Links

Infosheet: Giant Gippsland Earthworm

Via Darwin Online: The Formation of Vegetable Mould Through the Action of Worms, With Observations on Their Habits

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