Evolutionary Aspects of Annelid Regeneration


Annelids, like many other invertebrate animals, replace lost body parts in a process called regeneration. However, the ability to regenerate lost segments is present in some groups and not others; for example, leeches do not regenerate lost segments. Anterior and posterior regeneration involves the formation of a bud containing stem cells that differentiate into the new head or tail segments. Annelid regeneration also involves remodelling of surviving body fragments. The ability of annelids to regenerate tail segments appears to be nearly universal among species capable of regeneration. The ability to regenerate head segments, although common, is not universal and can depend on the number of segments lost. The absence and presence of regeneration across annelid groups, including closely related species, suggests that regeneration ability may be an ancient trait that has been lost in some species during annelid evolution.

Key Concepts

  • Annelids vary in their capability for regenerating body segments, including among closely related species.
  • The ability of annelids to regenerate posterior segments appears to be nearly universal.
  • The ability of annelids to regenerate anterior segments, although common, is not universal and is often limited depending on the number of segments lost.
  • Annelid regeneration may involve both epimorphic and morphallactic mechanisms.
  • Multiple losses and gains of regeneration ability have likely occurred during annelid evolution.
  • Why regenerative ability among annelids varies extensively remains unclear.
  • With development of new techniques for genetic analysis and microscopy, annelids are becoming important model systems for the study of regeneration.

Keywords: annelid; polychaete; oligochaete; leech; regeneration; epimorphosis; morphallaxis; segment; nervous system

Figure 1. Phylogenetic variation in annelid segmental regeneration. Annelids are thought to have evolved from a burrowing marine ancestor that likely was able to regenerate lost body parts. The ability to regenerate posterior segments is widespread while anterior regeneration varies across the phyla. AR – anterior regeneration; PR – posterior regeneration. (Reproduced with permission from Bely et al. (2014) © UBC Press.)
Figure 2. Regeneration in the oligochaete worm . Following injury and wound healing, a blastema is formed on both the anterior and posterior ends of the fragment. Over a 2‐week period, head and tail buds are generated from blastema‐derived stem cells in a regenerative process called epimorphosis. Concurrently, tissues of the original body segments undergo a reorganisation of their physiology and anatomy, in the absence of new stem cell differentiation, in a regenerative process called morphallaxis. Anterior epimorphosis in this species produces a head of only 7–8 segments. Thus, segments of the posterior fragment isolated from the injured animal on the left become anterior segments of the new animal on the right.


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Martinez‐Acosta, Veronica G, and Zoran, Mark J(Oct 2015) Evolutionary Aspects of Annelid Regeneration. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0022103.pub2]