Earthworms of the Amazon
Published 22 June 2010
This week’s post has been delayed a little: getting hold of some of the references 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.
Internally twisted glands
Published 6 May 2023
Earthworm taxonomy is a question of organs. With relatively few obvious external differentiators, distinguishing species often requires slicing them open and discerning features of the reproductive and digestive sytems. Because of the challenge this presents, there are relatively few who find themselves up to the task. Nevertheless, earthworms are more diverse than you may realise, and their slow life styles make for interesting geographic connections. Consider the Glossoscolecidae.
Jamieson (1988) characterised the Glossoscolecidae by the presence of the supra-oesophageal blood vessel along the oesophagus only, and of segmental testis-sacs. However, a molecular analysis of earthworm interrelationships by James & Davidson (2012) identified the glossoscolecids as polyphyletic and lead these authors to move many of the family’s erstwhile members into a distinct Rhinodrilidae (James 2012). The remaining true glossoscolecids encompassed just a handful of genera that could be distinguished from rhinodrilids by the structure of the calciferous glands. These glands open into the digestive system and release calcium carbonate. Their exact purpose has been disputed, whether they serve to regulate the pH of the digestive tract, function in excretion, or control levels of carbon dioxide in the earthworm’s system. In glossoscolecids sensu stricto, a single pair of calciferous glands is present within the eleventh or twelfth body segment. The internal structure of the glands is unique to the family, being comprised of a system of intertwined tubules (in other families, they are less complex). Glossoscolecids are also distinctive in the structure of the typhlosole (a longitudinal fold in the intestinal wall), and in having the opening pores of the male reproductive system (like other earthworms, glossoscolecids are hermaphrodites) macroscopic and connected to muscular ejaculatory bulbs.
With the removal of the rhinodrilids, the glossoscolecids are a strictly Neotropical affair. The majority of species are found in South America, with a few outliers in Central America and the Caribbean. The diverse genus Glossodrilus, with over sixty known species, is most diverse in high-elevation, tropical or subtropical rainforests. A small number of species are known from secondary grasslands in the North and South of the genus’ range (Righi 1996). Some of this habitat bias may reflect research bias: our knowledge of South American earthworm diversity was largely established by a single researcher, Gilberto Righi, and thus we primarily know about species in regions he investigated. It may be that further research could change our view.
James, S. W. 2012. Re-erection of Rhinodrilidae Benham, 1890, a senior synonym of Pontoscolecidae James, 2012 (Annelida: Clitellata). Zootaxa 3540: 67–68.
James, S. W., & G. G. Brown. 2006. Earthworm ecology and diversity in Brazil. In: Moreira, F. M. S., J. O. Siqueira & L. Brussaard (eds) Soil Biodiversity in Amazonian and other Brazilian Ecosystems pp. 56–116. CABI Publishing.
James, S. W., & S. K. Davidson. 2012. Molecular phylogeny of earthworms (Annelida: Crassiclitellata) based on 28S, 18S and 16S gene sequences. Invertebrate Systematics 26: 213–229.
Jamieson, B. G. M. 1988. On the phylogeny and higher classification of the Oligochaeta. Cladistics 4: 367–401.
Righi, G. 1985. Sobre Rhinodrilus e Urobenus (Oligochaeta, Glossoscolecidae). Boletim de Zoologia 9: 231–257.
Righi, G. 1996. Colombian earthworms. Studies on Tropical Andean Ecosystems 4: 485–607.
Righi, G., I. Ayres & E. C. R. Bittencourt. 1976. Glossoscolecidae (Oligochaeta) do Instituto Nacional des Pesquisas da Amazônia. Acta Amazônica 6 (3): 335–367.