The classification of earthworms is a subject that can be approached from several different directions. Classically, one would endeavor to classify a group of creatures with respect to their shared phylogeny; that is, the shared evolutionary ancestry of the group of species in question would dictate their overarching classificatory structure. In the case of earthworms, however, researchers have instead opted to adopt a lexicon based more on earthworms’ functional-ecological roles than on their genetic heritage (although we must note that the two often, though not always, can be found to coincide). To that end, we present a brief overview of the three principal categories of earthworms that constitute the dominant classificatory paradigm of today’s researchers: epigeic, endogeic, and anecic. These categories are closely related to earthworms’ habits within soil, as explained forthwith.
Epigeic earthworms are so defined by their residence in the surface of the soil (some authorities also include in these worms’ habitat the duff layer, or the region formed by fallen leaves and other plant detritus). Epigeic earthworms are characterized by their small stature, high levels of pigmentation, and ubiquitous presence on nearly every significant land mass on Earth. Their prolific range comes at the expense of a poor burrowing ability, which limits their niche within a given ecosystem to the outermost layers of soil. Eisenia fetida is a particularly fecund species of epigeic earthworm, with a range that spans nearly every variety of climate on the planet. Indeed, but for the remarkable adaptability of epigeic earthworms, there would be no entity responsible for converting surface debris to topsoil, a process critical to the continual renewal of countless ecosystems.
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Endogeic earthworms, in contrast to their epigeic cousins, live within the soil and utilize it for sustenance. Crucially, they burrow into the soil in a horizontal fashion preferring to remain within a given stratum as they move around in search of nutrients. To a limited extent, endogeic earthworms can reuse their own burrows, but in general they are noted for their horizontal mobility. In stark contrast to epigeic earthworms, endogeic earthworms tend to possess a pale complexion, exhibiting such colors as grey, pink, green or blue. Endogeic earthworm species number too many to give a representative account within this overview, but for further information the reader may wish to investigate Allolobophora chlorotica or Murchieona muldali.
Anecic earthworms possess traits that stand orthogonal to those of both epigeic and endogeic earthworms. Unlike endogeic earthworms, anecic earthworms make vertical (rather than horizontal) burrows into the soil, and of note, these burrows are more or less permanent (in contrast to endogeic earthworms, the migratory promiscuity of which we have already noted). Due to their ability to form vertical burrows, anecic earthworms possess a remarkable ability to acquire nutrients from a variety of strata within the soil. For instance, anecic earthworms may emerge on the surface of the soil in order to drag leaves into the depths of their burrows and feed on them. Apart from being among the largest varieties of earthworm, anecic earthworms are noted for their darkly colored head ends (which can be either red or brown) and their relatively pale tail ends.
Summary and Conclusion
For those interested in the classification of earthworms, the preceding three categories (epigeic, endogeic, and anecic) serve relatively well in providing a first cut at the functional differences between the over 4400 separate species of earthworm thus far identified. To recap, epigeic earthworms are those that inhabit the top layers of soil; endogeic earthworms inhabit the interior of the soil and make horizontal burrows; and anecic earthworms migrate between the various soil strata while maintaining permanent vertical burrows. This is of course a mere shadow of our current state of knowledge regarding the diversity of earthworms, but we hope that it will provide a sufficient springboard for those interested in the remarkable diversity of earthworm species.
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