Basic Earthworm Biology
Earthworms are nature's clean-up crew, aiding in the production of lush, humus-rich topsoil from spent plant and animal materials. These elegantly efficient organisms have been on earth for hundreds of thousands of years longer than humankind, largely untouched by evolution due to their nearly perfect adaptation to their role in nature.
Humankind has studied and learned to appreciate the talents of the earthworm, developing systems that capitalize on the natural role it plays in recycling organic matter back into humus. We now use earthworms for the remediation of organic “waste” materials, reducing the pressure on landfills and aiding in the regeneration of our valuable topsoils.
When beginning a foray into the operation of worm driven organics systems it is important to be clear on the intended goal of the project. Worm systems are typically managed for one of three reasons; waste management, production of worm biomass, and production of castings. While worms are being grown, organic materials are being processed, and castings are being generated in all worm beds, management methods will vary to some degree depending on the focus of the system.
Vermicomposting is defined as the practice of using concentrations of earthworms to convert organic materials into usable vermicompost or worm castings. These systems focus on the waste material and managing it so that it can be successfully and efficiently processed in a worm system.
Castings production systems are worm-processing beds that use feedstocks specially blended so that castings have a specific nutrient value, chemical characteristic or cross section of microorganisms. The focus of these systems is on end product value.
Vermiculture systems focus on producing the maximum level of worm biomass possible in a given space.
The Amazing Earthworm
Researchers have identified and named more than 4400 distinct species of earthworm, each with unique physical and behavioral characteristics that distinguish them one from the other. These species have been grouped into three categories, endogeic, anecic and epigeic, descriptive of the area of the natural soil environment in which they are found and defined to some degree by environmental requirements and behaviors.
Anecic species, represented by the common nightcrawler (Lumbricus terrestris), build permanent vertical burrows that extend through the upper mineral soil layer, which can be as deep as 4-6 feet. These species coat their burrows with mucous that hardens to prevent collapse of the burrow, providing them a home to which they will always return and are able to reliably identify, even when surrounded by other worm burrows. When deprived of this burrow environment anecic worms will neither breed nor grow.
Anecic worms feed in decaying organic matter and are responsible for cycling huge volumes of organic surface debris into humus.
Endogeic species build extensive, largely horizontal burrow systems through all layers of the upper mineral soil. These worms rarely come to the surface, spending their lives deep in the soil where they feed on decayed organic matter and mineral soil particles. While most people believe all worms eat soil, it is only the epigeic species that actually feed on significant volumes of soil itself.
These worm species help to incorporate mineral matter into the topsoil layer as well as aerating and mixing the soil through their movement and feeding habits.
Epigeic earthworm species, represented by the common red worm (Eisenia fetida), are found in the natural environment in the upper topsoil layer where they feed in decaying organic matter. Epigeic worms build no permanent burrows, preferring the loose topsoil layer rich in organic matter to the deeper mineral soil environment. Even in nature these worms are found in highest concentrations in the forest duff layer or in naturally occurring drifts of leaves and organic debris rather than in soil. We can duplicate the preferred environment of these worm species in bin culture, and it is largely for this reason that it is epigeic worms only that are used in vermicomposting and vermiculture systems.
Oxygen requirements
Earthworms are oxygen-breathing animals that absorb oxygen directly through their skin. Oxygen is dissolved into mucous coating the worm's skin and the dissolved oxygen passes through the skin and the walls of capillaries lacing the skin where it is picked up by hemoglobin in the worm blood and carried throughout the body.
Moisture requirements
Moisture is critical to the survival of all earthworm species because it is moisture within the worm's body that gives it shape, enables it to move, and aids in the worm's ability to absorb oxygen. To facilitate the absorption of oxygen the skin is very thin and permeable, meaning that the moisture within the body cavity is easily evaporated off, particularly in dry environments. The moisture range for most worm species is from 60-85%, which ensure the worm can absorb as much moisture as may be lost.
Temperature requirements
Specific temperature requirements and tolerances vary from species to species, though the ideal range for most epigeic worm species is between roughly 60-80° F. The worm's ability to tolerate temperatures outside of ideal is highly dependant on the level of moisture in the system, with hot, dry conditions being the most lethal combination.
Nutritional requirements
Earthworms lack teeth and sufficient digestive enzymes of their own, relying instead on microorganisms to begin to rot and soften organic matter so it can be ingested, then relying on naturally occurring bacteria and fungi in their gut to digest their food. In the process of taking in this biologically active predigested organic matter the earthworm also ingests small particles of sand and soil, which lodge in their gizzard. As the organic matter and microbial life coating it move past this gizzard they are ground against the gritty particles lodged there and fragmented into smaller pieces, making them easier for the gut organisms to digest.
Researchers have recently learned that it is not from the organic matter itself, but from the bodies of the microbial life rotting the organic matter that epigeic earthworms derive the bulk of their most vital nutrients. Once thought to be detritus (debris) feeders, we now understand that the earthworm is actually a predator of microbial life, relying on microscopic bacteria, fungi, protozoa and algae as their major sources of nutrition. Thus, anything that will support microbial activity, that is, anything that rots, is potentially suitable food for earthworms. Materials that support the greatest level of earthworm activity are those that support the greatest and most diverse populations of microbial life.
PH Requirements
As microorganisms break down organic matter it goes through a series of naturally occurring changes in pH. Because earthworms thrive in environments rich in decaying organic matter they are adapted to tolerate these pH fluctuations with little or no change in their activity levels. In nature worms are found in environments with a pH range from 4-9, with processing and reproductive rates being no different at an acidic 4 than they are at an alkaline 9. In fact, all things being otherwise equal, earthworms actually prefer an environment with a pH of 5 to 5.5, contrary to the popular belief that they prefer a neutral pH.
With a pH tolerance this wide it is highly unusual for pH to be a limiting factor in any worm system. Further, the radical and artificial adjustment of the pH through the addition of buffering agents like lime can actually have a detrimental effect on the system. The organisms present in a given environment of organic debris are there because they are suited to that environment and whatever fluctuation may naturally occur through the process of decay. When the nature of the system is suddenly and radically altered it forces many of these organisms into dormancy and sometimes kills them outright, thus reducing the availability of nutrition to the worms and potentially slowing the processing rate of the organic matter.
The addition of lime to any worm system is generally discouraged except in those extremely rare circumstances where the pH has dropped well below the worms' level of tolerance.
Ultra-Violet Light Response
All earthworms are photophobic to some degree, meaning they react negatively to bright light. The severity of the reaction depends on the species of worm, how bright the light and the level of light to which the worm is accustomed. For example, earthworms accustomed to some light exposure will react less negatively to sudden bright light than will worms accustomed to complete darkness. Some species of worm react negatively to bright light but are actually attracted by dim light.
Earthworms sense light through photoreceptive organs along their back and on the prostomium (sensitive lobe of tissue overhanging the mouth that the worm uses to probe and sense its environment).
Reproduction
Earthworms are hermaphrodites, meaning each worm possesses both male and female reproductive organs. Some earthworm species can be self fertile, meaning they can fertilize their own ova to produce young, and some species are parthenogenic, meaning fertilization of the ova by sperm is not necessary to produce young. Most earthworm species, however, require that two worms exchange sperm in order to produce young.
When worms mate they lay side by side with their heads pointed in opposite directions, making close contact along the upper segments of their bodies. They excrete a mucous that coats both worms and binds them together, preventing them from being easily pulled apart and ensuring environmental conditions like rain or dew do not interfere with the exchange of sperm.
The worms exchange sperm, storing the received seed in a pore on the skin surface just above the clitellum (the differently colored or thickened band that encircles the worm body). Once they exchange sperm, a process that may take hours, the worms move apart and eject their own ova into a pore on their skin surface near the sperm pore. They secrete a thick mucous around the clitellum, which hardens on the outside but remains sticky underneath, forming a band out of which the worm backs, drawing the band over its head. As the band passes over the pores holding sperm and ova they are picked up and held on the sticky underside. Once the worm has backed completely out of the hardened mucous band the ends close forming a cocoon with sperm and ova inside where fertilization takes place. Each worm will continue to produce cocoons until they have used all of the sperm received from their mate.
The length of time it takes for the baby worms inside the cocoon to mature and “hatch” out, and the number of young in each cocoon depend on the worm species and environmental conditions.
Contrary to popular belief, worms are a closed species, meaning they can produce viable young only with sperm from members of their own species. They cannot be hybridized . In those rare circumstances when two worms from differing species have attempted to mate, the result was either no young being produced or, in rare circumstances, babies that were always sterile.
The worm cocoon is an incredibly tough structure, designed to protect the young inside from environmental extremes and even ingestion by other animals. Cocoons can be frozen, submerged in water for extended periods of time, dried and exposed to temperatures far in excess of what can be tolerated by adult worms without damage to the young worms inside. The cocoon can even be eaten by other animals, provided it can make it past the teeth, surviving the digestive process and passing out of the animals body in the manure! In areas of climatic extremes it's likely that the adult members of epigeic worm species do not survive, but the cocoons do, repopulating the environment when environmental conditions return to a range that can support worm activity.
Earthworm cocoons are easy to spot in the worm bed. They are roughly the size of a large grape seed and similarly shaped, with one end rounded and the other drawn out to a point. When first dropped from the body of the parent the cocoon is a creamy, pearlescent yellow, darkening to a cola brown as the young worms within mature and prepare to emerge.
Earthworm Species used in Vermiculture
While earthworm taxonomists have identified thousands of individual worm species, only six have been identified as useful in vermicomposting systems to date. These species were evaluated based on their ability to tolerate a wide range of environmental conditions and fluctuations, handling and disruption to the worm bed, and for their growth and breeding rate. Earthworm species with a short generation time, meaning a relatively short life span and rapid growth and reproductive rate, have been identified as most effective due in large part to the high concentration of juvenile worms present in their populations. Juvenile worms, like human teenagers, are voracious consumers, keeping the processing rate of the system high and ensuring an ongoing succession of young worms.
The growth and reproductive rates of each worm species listed below are the maximum identified under ideal conditions. These rates decline the further environmental conditions within the system shift from ideal.
Please note the Latin name of each earthworm species. Common names can be very misleading and often vary between different regions of the world and even regions within a country. It is very difficult to be sure which species of worm is being discussed unless the Latin name is being used. Professional worm growers should know and use the Latin names of the worms they culture.
Eisenia fetida* / Eisenia andreii
(common name, Red Worm)
There are two worm species listed here because in virtually all cultures of E. fetida, E. andreii is present. E. andreii so closely resembles E. fetida in behavior, environmental requirements, reproductive and growth rate, and appearance that the only way to distinguish between the two is through molecular scanning . There is no external difference between the two species. For all intents and purposes these worms can be considered identical. Eisenia fetida is generally the only worm mentioned because the two are so closely associated and because fetida is typically the more populous of the two.
Eisenia fetida/Eisenia andreii are the worm species identified as the most useful in vermicomposting systems and are the easiest to grow in high-density culture because they tolerate the widest range of environmental conditions and fluctuations, and handling and disruption to their environment of all species identified for this purpose. E. fetida/E. andreii are also common to virtually every landmass on earth, meaning there is no concern over importing potentially alien species to an environment where they might cause damage.
While this worm species is considered the premier worm for most applications, it is a small worm, not always suited for use as bait.
Temperature range: Minimum; 38° F, maximum; 88° F, ideal range; 70° F-80° F.
Reproductive rate: Approximately 10 young per worm per week under ideal conditions.
Average number of young per cocoon: Approximately 3.
Time to emergence from the cocoon: Approximately 30-75 days under ideal conditions.
Time to sexual maturity: Approximately 85-150 days under ideal conditions.
*Note: The spelling ‘ fetida ' was changed a few years ago to ‘ foetida ' then subsequently changed back for reasons clear only to a few earthworm taxonomists. The different spellings do not denote different species. Information on this species can be found under both spellings, though the correct spelling is ‘ fetida '.
Eudrilus eugeniae
(common name, African nightcrawler)
This worm is a semi-tropical species, meaning it cannot easily tolerate cool temperatures and is usually grown indoors or under temperature controlled conditions in most areas of North America. E. eugeniae is a large species, well suited for use as a bait worm, but does not tolerate handling or disruption to its environment.
This species is used in some vermicomposting systems around the Mediterranean region and in some areas of eastern Asia.
Temperature range: Minimum; 45° F, maximum; 90° F, ideal range; 70° F-80° F.
Reproductive rate: Approximately 7 young per worm per week under ideal conditions.
Average number of young per cocoon: Approximately 2.
Time to emergence from the cocoon: Approximately 15-30 days under ideal conditions.
Time to sexual maturity: Approximately 30-95 days under ideal conditions.
Amynthas gracilus
(common name, Alabama or Georgia jumper)
A. gracilus is another large worm species well suited for use as bait. It is also a tropical species with a poor tolerance for cold temperatures. This worm tolerates handling and disruption to the worm bed as well as does E. fetida and is generally considered an easy worm to culture provided appropriate temperatures can be maintained.
A. gracilus is used in a few vermicomposting systems in Malaysia and the Philippines.
Temperature range: Minimum; 45° F, maximum; 90° F, ideal range; 70° F-80° F.
Reproductive rate: Undetermined, though believed to be similar to E. eugeniae.
Average number of young per cocoon: Undetermined, though believed to be similar to E. eugeniae .
Time to emergence from the cocoon: Undetermined, though believed to be similar to E. eugeniae
Time to sexual maturity: Undetermined, though believed to be similar to E. eugeniae
Perionyx excavatus
(common name, Indian Blue worm)
Perionyx excavatus is a beautiful worm with an iridescent blue or violet sheen to its skin clearly visible under bright light. It is a very small worm, poorly suited as fishing bait, but has an impressive growth and reproductive rate far in excess of the other species grown in bin culture.
This is another tropical worm species with a very poor tolerance for low temperatures, fluctuations in the bin environment, handling or disruption to the system. P. excavatus is often referred to as “the Traveler” for its tendency to leave the bin en masse for no apparent reason.
Due to it's temperamental nature this species is rarely used in vermicomposting systems in North America, though it is naturally occurring at low population levels in systems in contact with the soil in the southeastern US and most tropical regions of the world.
Temperature range: Minimum; 45° F, maximum; 90° F, ideal range; 70° F-80° F.
Reproductive rate: Approximately 19 young per worm per week under ideal conditions.
Average number of young per cocoon: Approximately 1.
Time to emergence from the cocoon: Approximately 15-21 days under ideal conditions.
Time to sexual maturity: Approximately 30-55 days under ideal conditions.
Eisenia hortensis
(European nightcrawler)
E. hortensis is a large worm species well suited for use as a bait worm. Its ideal temperature range is a bit cooler than is that of E. fetida and it requires higher moisture levels than do the other species tested for use in bin culture and vermicomposting, but the species tolerates handling and disruption to its environment, and environmental fluctuations very well.
Because this worm has a very low reproductive and growth rate, relatively speaking, it is considered the least desirable species of those tested for either bin culture or vermicomposting systems. It is used in a few vermiprocessing systems in Europe for the remediation of very wet organic materials.
Temperature range: Minimum; 45° F, maximum; 85° F, ideal range; 55° F-65° F.
Reproductive rate: Just under 2 young per worm per week under ideal conditions.
Average number of young per cocoon: Approximately 1.
Time to emergence from the cocoon: Approximately 40-125 days under ideal conditions.
Time to sexual maturity: Approximately 55-85 days under ideal conditions.
