Of the 4,400 earthworm species, the common ones we see in our gardens or pavements exhibit certain behaviors, such as coming out after rainfall or escaping moles.
For a while, scientists and enthusiasts, including myself, wondered what made worms come out of the soil after the rains. Do they do that to prevent drowning or they can breathe underwater?
The conclusion was that earthworms can breathe underwater but not all species can live underwater. Since they breathe through their skin, they draw oxygen from the air and water has less oxygen than the air. If they stay underwater for too long, most of them drown.
How do earthworms breathe?
Earthworms breathe through their skin by a process called cutaneous respiration. This process occurs on their moist skin which allows for the exchange of gasses (oxygen in, carbon dioxide out) since they don’t have lungs or gills like other animals.
Earthworms have a thin layer of mucus on their skins through which gasses such as oxygen and carbon dioxide diffuse. These gasses are exchanged with the air and water around the worm as it moves through the soil or water.
The thin layer of mucus on their skins should be kept moist at all times to allow for the exchange of gasses. The worm will suffocate if the skin dries up. If you see a dry worm on your pavement, it’ll likely be dead or dying.
Can earthworms breathe underwater?
Earthworms can comfortably breathe underwater. The process they rely on for breathing (cutaneous respiration) works both in water and on land. However, given that water has less oxygen than the air, they will not draw as much oxygen as they need in the water.
This can lead to suffocation if they stay in the water for too long, or they’re not adapted to such conditions. Earthworms can drown if they’re not adapted to breathing underwater. Most of the earthworms found in swamps and other water-logged areas are either adapted to breathing under such conditions or they come out every once in a while to take a breath.
Why do earthworms surface after rain?
You’ve probably seen the earthworms in hoards after rainfall. Why do they come out? Are they drowning?
I see them on my pavement after each period of rainfall and always wondered what sent them above-ground. After some research, I found out the following:
1. Avoid suffocation and drowning
Although earthworms breathe through their skin and can stay for several weeks underwater, most species need to come to the surface and take a breath. When it rains, the soil becomes clogged with water with most of them moving to the surface.
This, however, isn’t the only reason they do so given that most small worms remain in the soil.
2. Escape predators
If you’ve been fishing like I do every other weekend, you know earthworms are some of the best types of fish bait. To get some, you send vibrations to the soil called “fiddling” and then they come out.
Moles make similar vibrations and, when the earthworms feel them, they move out of the way and end up on the surface.
Rain also makes vibrations similar to the moles which may bring the earthworms to the surface.
3. Migration
Naturally, earthworms migrate through the soil as they colonize new areas and find mates. This keeps them away from the high surface temperatures, predators, and dryness on the surface. Moving through the soil, however, is quite slow since it involves breaking up and burrowing through it.
When it rains, it becomes easier for the earthworms to move to new areas on the surface given the moist and slippery nature of the surface. They also move faster and, often, there are fewer natural predators like chameleons and bearded dragons on the surface after the rains.
4. Finding food
Since earthworms are detritivores (they eat decaying organic matter), they always go where there is such decaying matter. When it rains, the water stimulates microbial activities on the surface, availing more decaying matter. The worms will then move to the surface to feed on the newly available organic matter.
5. Reproduction
The moist environment after rainfall is conducive to the reproduction of earthworms. Besides availing more worms for mating, laying eggs in new areas is easier on the surface where movement either through water or the surface is much easier.
Earthworms might look vulnerable when it rains. However, they’re quite clever and you’ll rarely see them for too long after it rains. In most cases, they’ll burrow back into the soil as soon as the surface starts drying up.
Can drought affect earthworms?
Earthworms are used to moist or wet conditions which maintain the mucus on their skin to allow for respiration. If the soil dries up too much such as during a drought, most worms will be affected as follows:
1. Dehydration
Since worms have a soft skin which needs to remain moist for respiration, drought conditions will likely dehydrate the worm to the extent that the skin dries up and the worm can no longer breathe.
2. Reduced movement
As the soil dries up in drought conditions, it becomes hard and compact, making it hard for the worms to move around. This inhibits their ability to find food and mating partners.
Eventually, prolonged drought leads to a lack of reproduction and a reduced worm population. At times, the soil may dry so much that it kills the worms due to high temperatures and low moisture levels.
3. Low food availability
Earthworms feed on decaying organic matter which becomes scarce during a drought. The lack of moisture kills microbes responsible for the decaying process, leading to little to no decaying of the leaves and other organic matter.
As the amount of food for the earthworms reduces, most will starve off and die.
4. Exposure to predators
Worms may come to the surface of the soil during drought periods to look for moisture when burrowing deeper doesn’t work. As they do so, they expose themselves to predators like moles and worms since they would be easier to spot and move much slower in the dry conditions.
5. Reduce soil quality in the long-term
Earthworms are among the most important constituents of the soil as they enhance the soil structure, decompose organic matter, and cycle nutrients. Charles Darwin even called them “nature’s ploughs” for this.
When they die off and can’t reproduce in drought conditions, they also affect the quality of the soil negatively.
Even with these aspects, there are many earthworm species with various adaptations. Some of them can live under water for extended periods while others never even leave the water. Still, there are those that live in damp but not wet conditions, and others adapted to dry regions such as deserts.
My neighbor’s kid calls them wiggly worms owing to their wiggling motions. She likes them but doesn’t know how important they are to our soils and ecosystem as a whole. Next time you see one, don’t rush to kill it since it means no harm.