Earthworms of the Amazon (Taxon of the Week: Urobenus buritis)

This week's Taxon of the Week post has been delayed a little: getting hold of some of the reference for it required me to enter a real library and locate an actual journal physically printed on paper. Always an experience.

Urobenus buritis (no pictures of this one, I'm afraid) is an earthworm only recorded from the region of Manaus in the state of Amazonas in northern Brazil. This may or may not be significant. Perhaps not surprisingly, collections of earthworms around the Amazon have been somewhat scattered; most of our knowledge of the area (including U. buritis itself in Righi et al., 1976) can be credited to the work of one researcher, Gliberto Righi. Nevertheless, what we do know suggests that many species in the area have very localised ranges (James & Brown, 2006). Earthworms are poor dispersers and many of them are very particular in their habitat preferences, making them highly vulnerable to disturbance.

The genus Urobenus possesses three pairs of calciferous glands in segments VII-IX (many other glossoscolecid genera possess only two*) and can be distinguished from the similar genus Rhinodrilus by the shape of the glands. In Rhinodrilus, all three pairs of glands are tubular; in Urobenus, the first two pairs are tubular and the third pair (in segment IX) is sac-shaped (Righi, 1985). Urobenus buritis has three pairs of spermathecae, delicate anterior septa and the male pores opening at segments 20-21 (U. brasiliensis, which U. buritis was originally described as a subspecies of, has them at segments 19-20).

*Offhand, this is something to consider when thinking about how vertebrate diversity compares to invertebrate diversity. Two externally quite similar taxa, that the vast majority of people would regard as both being 'just worms' and pretty much identical, may actually differ in something as seemingly basic as how many organs they have.

I haven't seen the original description of Urobenus buritis, so I can't say whether there is any information on its lifestyle, but the closely related U. brasiliensis is an epigeic (living above the ground) species inhabiting leaf litter (James & Brown, 2006). Many of the earthworms around the Amazon have cyclical life histories to deal with the contrast between wet and dry seasons. Tuiba dianae migrates towards drier forest as flood waters rise, maintaining a distance of at least five metres from the water's edge. Other species, such as Andiorrhinus tarumanis, climb up the nearest tree during the wet season and take up residence in patches of leaf litter trapped in the forest canopy while the ground is flooded. Whether Urobenus buritis indulges in such behaviour is currently unknown.

REFERENCES

James, S. W., & G. G. Brown. 2006. Earthworm ecology and diversity in Brazil. In Soil Biodiversity in Amazonian and other Brazilian Ecosystems (F. M. S. Moreira, J. O. Siqueira & L. Brussaard, eds) pp. 56-116. CABI Publishing.

Righi, G. 1985. Sobre Rhinodrilus e Urobenus (Oligochaeta, Glossoscolecidae). Boletim de Zoologia 9: 231-257.

Righi, G., I. Ayres & E. C. R. Bittencourt. 1976. Glossoscolecidae (Oligochaeta) do Instituto Nacional des Pesquisas da Amazônia. Acta Amazônica 6 (3): 335-367.