Eek! Earthworms on the Grinding Rock

March 25, 2019

Written by Mary Louise Flint

 Cleaning the grinding rock.

On March 4, I arrive at the Maidu Village to help Effie Yeaw Nature Center's Lead Naturalist, Brena Seck, clean the grinding rock, and there are worms everywhere. Why do we keep finding worms in the water-filled grinding holes?

 Earthworms on the grinding rock.

It rained a lot the night before, and the rain has flushed the earthworms out of their burrows. A close inspection of the area surrounding the grinding rock reveals little piles of soil everywhere. These are worm castings. Each pile represents the spot where a worm emptied its digestive system. Earthworms often clear their tunnels of water after a rain by swallowing it and bringing it up to the top mixed with castings.

Earthworm castings on the soil after a rain. 

 Close up of earthworm an casting.

There are probably several reasons why earthworms come out of their tunnels when it rains and reasons will vary with earthworm species. It was once thought that earthworms emerged primarily to avoid drowning when their burrows fill with water. However, more recent research indicates that common species can survive for several days submerged in water, although extensive waterlogging (and lack of oxygen) can kill them. Scientists now believe that another important reason for the rain-associated activity is dispersal.   

Earthworms require free water to be active.They absorb oxygen and release carbon dioxide through their skin (or cuticle) and require a moist surface for gas exchange.The cuticle is coated with slimy mucus, which allows them to breathe and also acts as a lubricant to help them move through the soil or on the soil surface.They cannot travel far on dry soil so they stay in their moist tunnels when it is dry. Rainy weather (or heavy lawn sprinkling) provides conditions that allow them to disperse to find new food sources, territory, or, in some cases, mates.

In the absence of strong light, earthworms willingly and enthusiastically climb wet surfaces, explaining why the worms travel up to the water-filled holes on the top of the grinding rock. They probably climbed up the rock during the rainy night and dispersed into the holes and couldn’t get out. Earthworms are very sensitive to light and can be killed by 3 hours or more of exposure to UV light. During the day, when the sun comes out, they retreat into their burrows or leaf litter.   Of course, predation by birds and other animals is a much greater threat after the sun comes out as well.

Earthworms swallow dead leaves, other organic material, microbes, and soil. They digest and grind these mixtures up in their compartmentalized digestive system and deposit excrement (called castings) in the tunnel linings or at the top of the burrow.  Earthworms are voracious feeders with some species ingesting and excreting 30% of their body weight daily. Earthworm activity improves soil quality by breaking down surface litter, accelerating incorporation of nutrients, facilitating vertical mixing of soil layers and aerating the soil.

 Cross section showing an earthworm pushing castings to the soil surface.

Earthworms are segmented worms (Phylum Annelida) in the order Opisthopora.  They are cylindrical with 100 or more segments, which provide structure for muscles. Small bristles on each segment help anchor the worm as it moves. The head end is narrow with a mouth and an anus is at the tail. Mature earthworms have a thick band called the clitellum toward the head end, which is used in reproduction. Earthworms are hermaphrodites and join at the clitella to exchange sperm and fertilize eggs.

 The earthworm body has a pointed head, a wider clitellum, and a blunt tail.

There is surprisingly little research about earthworms, and virtually none related to the species that are native to the Sacramento Valley. It has been estimated that there are more than 4000 species of earthworms worldwide and probably more than a hundred in California alone. Up to 100-500 worms can be found per square yard where conditions are favorable, usually with several species represented. Not all species of earthworms make burrows; many, such as the red worms used in vermiculture, are adapted to living in leaf litter or just under the soil surface rather than deeper in the soil. Moisture requirements vary as well. Some native California species are better adapted to dryer chaparral or oak woodland conditions.

Most of the earthworms found in disturbed, irrigated areas like cropland or landscapes such as our Maidu Village area are in the family Lumbricidae.  European in origin, these species are adapted to wetter, temperate conditions and are the species on which most earthworm research has been carried out. When temperatures are hot and/or soil moisture is low, these earthworms burrow deep into the soil (in some cases several feet) and enter a dormant state that can last for long periods. 

As the rainy season draws to a close, we’ll be seeing fewer earthworms on the grinding rock.  However, they will be beneath the soil, improving soil aeration, reducing leaf litter and benefitting the ecosystem.They are important food for birds, moles, opossum, snakes, skunks, frogs and toads, and many other animals.   Our trails are dotted with holes, large and small, left by animals digging for earthworms.


Author:  Mary Louise Flint, Docent, Effie Yeaw and Extension Entomologist Emerita, UC Davis

Photos:  By the author except #5 by Dwight Kuhn and #6 by Jack Kelly Clark, University of California Statewide IPM Program