Well Scott, your response did send me back to do some searching to see if perhaps my ever dwindling memory banks were failing me...but no I found plenty to confirm my initial response.
Some of my findings (lots of good reading available)...
Types of Earthworms
There are approximately 4500 species of worms in the world. Of those about 2500 are earthworm species. Earthworms are either earthmovers or composters. Earthmovers tend to be solitary species which tunnel through the earth, aerating, decompacting, and mixing soil strata and thus making surface nutrients available to plant roots at lower levels. Composters live en masse in organic matter on the soil surface, where they consume bacteria present in dead vegetation, animals and manure, turning it into humus.
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.
Which type of worm is best for composting?
Eisenia fetida, or red wigglers, are the hardiest, most preferred species of earthworms for composting purposes. Eisenia fetida are very prolific, thrive at high population densities and have voracious appetites
Eisenia fetida (Savigny) (Lumbricidae)
Manure Worm, Compost Worm
â€¢ Body cylindrical, 35-130 x 3-5 mm
â€¢ Color purple, red, dark red, brownish red. There are unicolored forms and forms that have alternating bands of red-brown on dorsum with pigmentless yellow intersegmental areas.
â€¢ Epigeic habit; detritivorous; very little soil consumed
â€¢ Principal earthworm for composting kitchen wastes, animal manures, etc.
â€¢ Live a maximum of 4-5 years, 1-2 more frequent
â€¢ Reproduce sexually, up to about 900 eggs per year per worm
â€¢ Little tendency to burrow into mineral soil
There are two main categories of earthworms: the earthmover and the composter.
Earthmovers (like nightcrawlers) move soil through their systems extracting their valuable nutrients. These worms are usually solitary so you won't find a large number in a given area.
Composting worms work as a group to break down organic matter into valuable worm castings and vermicompost. These worms do not like soil and make their living eating organic waste (while also living in it!). Composting worms will eat anything that was once alive (even your cotton T-shirt!).
Of the composting worm species, we recommend and sell the redworm (Eisenia Fetida) because they are extremely hardy, easy to grow, reproduce well, have a large temperature range in which they can live comfortably, and make excellent fine-grained castings. We've tried growing and vermicomposting with several species of worms and settled in on the "work horse" composter: the redworm.
The red wiggler is the most common type of composting worm. It can process large amounts of organic matter and, under ideal conditions, can eat it's body weight each day. It also reproduces rapidly, and is very tolerant of variations in growing conditions. Other names for red wigglers include Tiger worms, Garlic worms, Manure worms, and Brandling worms.
The redworm is a very good composting worm. In sunlight, it is a very active wriggler and is thought by many fishermen to be irresistible to fish. Redworms are very effective at aerating and mixing the soil, and consume a large amount of organic material, although less than red wigglers. These worms are commonly found in decomposing animal manure and compost piles. Other names include Red worms, blood worms, and red wiggler (but not the same as the worm described above).
Nightcrawlers are one of the most common types of worms normally found in your yard and garden. These worms are popular bait worms because they are large and easy to place onto fishing hooks, are relatively easy to raise and are relatively tolerant of variations in growing conditions. Nightcrawlers are not a particularly good worm for use in a vermicomposting bin, since they like their burrows undisturbed and prefer to eat things found on top of the soil.