Distribution of segment regeneration ability in the Annelida.

The annelids are an excellent group in which to investigate the evolution of regeneration abilities. They exhibit qualitative and quantitative variation in regeneration ability, including among closely related species, and their segmental body organization makes comparing results among species relatively straightforward. Here, I compile information on the presence/absence of segment regeneration ability across the annelids. The ability to regenerate posteriorly appears to be nearly universal in the annelids. It is almost certainly ancestral for the phylum and may have been lost only a few times. The ability to regenerate anteriorly is common but less widespread. It is absent in about a dozen groups, almost surely representing multiple independent losses of this ability. Several non-regenerating species are closely related to regenerating species, indicating very recent losses (or gains). Despite the fact that lack of this ability is unusual, there is a publication bias against reporting the lack of regeneration ability, and in many cases the judgment that a particular species is unable to regenerate is based on incomplete or unpublished data. Thus, in order to build rigorous frameworks for future comparative studies of annelid regeneration, there is a need for published studies clearly documenting the lack of regeneration abilities in annelid species. The review of regeneration data presented here is especially useful in highlighting annelid groups that possess both regenerating and non-regenerating representatives. Investigations of these groups may be particularly useful for elucidating the mechanisms leading to the loss (or perhaps gain) of segment regeneration ability.