Earthworm adaptations

Adaptation is an evolutionary process whereby an organism becomes increasingly well suited to living in a particular habitat. Natural selection results in helpful traits becoming more common in a population. This occurs because individuals with these traits are better adapted to the environment and therefore more likely to survive and breed. The timeframes for different types of adaptation are variable – behavioural adaptation can be a very quick process whereas structural changes may occur over a very long period of time.

Adaptation is also a common term to describe these helpful or adaptive traits. In other words, an adaptation is a feature of an organism that enables it to live in a particular habitat.

Most species of earthworms share some common traits or adaptations, such as their streamlined body shape. However, different species of earthworms have adapted to different habitats and occupy different niches within the ecosystem. As a result, earthworm adaptations are many and varied. In all animals, types of adaptations can be grouped into three main categories: structural, physiological and behavioural.

Structural adaptations

Structural (or morphological) adaptations are the physical features of the organism. These include things you can see, like its shape or body covering, as well as its internal organisation.

These are some examples of structural adaptations of earthworms:

• Each segment on an earthworm’s body has a number of bristly hairs, called setae (sometimes written as chaetae). These hairs provide some grip to help the earthworm move through the soil.
• An earthworm has a streamlined body with no antennae or fins or arms or legs! This streamlined shape is an adaptation to living in narrow burrows underground and the need to move easily through the soil.
• An earthworm has circular muscles that surround each body segment. It also has longitudinal muscles that run the length of its body. These two groups of muscles work together to help the earthworm move.
• In order to get food into its mouth, an earthworm pushes its pharynx out of its mouth to grasp hold of its food. It then pulls the food back into its mouth and wets it with saliva

Physiological adaptations

Physiological adaptations relate to how the organism’s metabolism works. These adaptations enable the organism to regulate its bodily functions, such as breathing and temperature, and perform special functions like excreting chemicals as a defence mechanism.

These are some examples of physiological adaptations of earthworms:

• Many earthworms secrete a mucus (coelomic fluid) that helps them to move more easily through the soil. In some burrowing species, this fluid forms a cement-like substance that lines their burrows to help keep the walls from collapsing. In the New Zealand native species Octochaetus multiporus, the mucus may also be part of its defence system as it is toxic to soil bacteria O. multiporus has another special adaptation – its mucus is bioluminescent! When it is disturbed, the O. multiporus earthworm squirts mucus from its mouth, anus and dorsal (underside) pores, and the fluid emits a bright orange-yellow light that glows in the dark.

• When the environmental conditions in an earthworm’s habitat change, for example, the soil becomes too hot or too dry, many earthworms become inactive in a process called aestivation. They move deeper into the soil, coil into a tight ball, excrete a protective mucus and lower their metabolic rate in order to reduce water loss. They will remain like this until conditions become favourable again.

Behavioural adaptations

Behavioural adaptations are learned or inherited behaviours that help organisms to survive.

These are some examples of behavioural adaptations of earthworms:

• Earthworms cannot see or hear but they are sensitive to vibrations. Birds looking for food or humans collecting earthworms for bait stamp on or vibrate the ground in some manner, causing earthworms to move to the surface. Perhaps this is to escape from moles, whose primary food is earthworms. We don’t have moles in New Zealand, but some people think that earthworms carry a ‘memory’ of this predator and still respond by leaving the ground.
• Earthworms are sensitive to light. Most species spend their days in their burrows or in the soil or leaf litter. In general, you usually find them on the surface at night.
• Earthworms lose moisture through their skin. They move out of their burrows to migrate or reproduce when the ground is wet with dew – one reason why we may see them in the early morning.