Oligochaeta: Biological Indicators
by Sheila Behling
BIOL/WATER 361, Fall 2013
With such great diversity, the subclass Oligochaeta of phylum Annelida can be argued to be the most valuable water quality indicator among the benthic invertebrate populations. Most of these can be generalized as segmented worms that lack setae and possess a clitellum (Carter et al., 2006). Of the 5000 valid species of oligochaetes, about 1700 are aquatic (Martin et al., 2008). Found everywhere from extremely productive to severely eutrophic freshwater systems, Oligochaetes offer a full index of values for water quality assessment. In replacement of sampling and testing water for a number of variables, we can look at the diversity and density of benthic organisms to tell us more about the environment at hand with fewer steps and variables to analyze. Depending on the body of water and its intended use, water quality is a relative term (Abassi & Abassi, 2012). The use of oligochaetes usually refers to a test for organic pollution. Conveniently, oligochaetes are not only found in benthic environments of freshwater, estuaries, and marine environments, but also terrestrial habitats. This makes them not only a water quality indicator, but an overall valuable biological indicator.
This subclass has almost five thousand described species spread across the world. The families are generally divided into two groups, megadriles, and microdriles, differentiated at about two centimeters in length (Martin et al., 2008). Most aquatic oligochaetes are within the microdrile group, with a few exceptions. When talking about oligochaetes being water quality indicators, most often you will see members of the Tubificidae family of Clitellata, and leeches and branchiobdellids are generally excluded. Containing over 1000 aquatic species of which over half are freshwater (Martin et al., 2008), the tubificids dominate the list of useful indicators of water quality indices (Martins et al., 2008). However, it is important to note that there has been an ongoing increase in number of recognized species due to the non-stop discovery of oligochaetes. When considering the value of this family as a water quality indicator it is important to understand where it has radiated from. Potentially paraphyletic, the basal branch of Clitellata is thought to be the clade Capilloventridae (Martin et al., 2008). This shows the beginning of the class to be of aquatic origin and makes the terms Oligochaeta and Clitellata one and the same. This gives me grounds to believe that clitellates have even greater value as water quality indicators with their long history among aquatic habitats. Phylogeneticly these worms are perfect for the job, and it just so happens they are geographically appropriate as well.
When considering a resource as important and vast as water, it is important to have a tool that is just as widespread. The distribution of oligochaetes is very extensive, but somewhat concentrated in the northern Palearctic region of Asia and Africa and throughout Eruope. It is estimated that of the 616 recognized species of this area, eighty percent are endemic (Martin et al., 2008). This leads to a distinctive difference in the list of species and number of species used as water quality indicators for the southern regions. There are almost double the number of species found in the Holarctic region compared to all the rest of the regions combined (Martin et al., 2008). However, this may only be the case due to a lack of interest and study of this group of organisms in the southern hemisphere. Rhyacodrillinae is the only subfamily of this group that is found within all of the regions, and with exception of the Antarctic region, Naidinae and Tubificinae are quite widespread as well. On the other hand, the Phreodrilidae are secluded to the southern regions. It is hypothesized that these sporadic concentrations of specific species is due to continental drift (Martin et al., 2008). To date, these species are now separated by vast oceans and mountain ranges. Found across the world, in a variety of environments, methods associated with the use of oligochaetes as water quality indicators varied as well.
When an ecosystem accumulates a high level of nutrients, whether it’s fertilizers or sewage, plant life begins to explode. When this happens, the level of dissolved oxygen beings to plummet (Abassi, 2012). Starting with the species with the highest oxygen demand, life begins to die off, making room for other species with lower requirements. This is when species of Oligochaeta begin to move in, slowly taking over the vacant niches within this changing environment (Carter, 2006). It is acceptable to associate high densities of oligochaetes with degraded environments, but it does not mean that this ecosystem has always been that way. These species not only prevail due to their high tolerance for hypoxia, but also because of lower competition and predation rates (Balian, 2008). While some assessments may use the overall abundance of oligochaetes as an indicator of organic enrichment, others use the relative abundance of a particular species, usually from Tubificidae. It’s also a common practice to take inventory of the most prevalent species, and take into account the ecological demands of each of these species individually. These are then compared them to indicate an overall ecosystem quality (Howmiller and Scott’s Index) (Martins et al., 2008). Martins et al. (2008) used a combination of the density of Tubificidae, percentage of Limnodrilus hoffmeisteri, and a modified version of the Howmiller and Scott Environmental Index (TC) to evaluate the level of organic pollution in the São Pedro stream environment. While finding a significant difference in both the density of Tubificidae and the percentage of L. hoffmeisteri, they did not find a significant difference in the TC. With the number of variables the index takes into account, it is not surprising to see that it did not show significance. However, it is likely that the significant differences they discovered in two other areas could mean the stream is in its first stages of becoming eutrophic. Verdonschot (2006) concluded that this bias can be shown when limiting identification of benthic organisms to only the family level. This shows the importance in each species individually. So no matter the method, the species themselves can be a direct reflection of the environment in their own way.
The most well-known group of low oxygen tolerant species belong to the family Tubificidae. They are greatly associated with low levels of oxygen resultant from high levels of organic matter (Sundic & Radujkovic, 2012). One can use the density of species within the family, or the percentage of a particular species within the family to pinpoint the pollution type or severity. The percentage of Limnodrilus hoffmeisteri can give some insight into the level of pollution the water is at (Dafoe et al., 2011). When looking at the tolerances of the species of Oligochaeta, it is considered the most tolerant of pollution (Chapman & Brinkhurst, 1984). Whereas, L. udekemianus is tolerant of low oxygen levels but not pollution. This would be a good indicator for systems that are naturally have low levels of oxygen. When this species is present it is safe to say there is no direct pollutant. When there are high densities of other benthic organisms, the species Tubifex tubifex is not very abundant because of its low tolerance for competition (Martins et al., 2008).
Other factors that affect the overall water quality index value of an oligochaete species includes substrate type, the density of other taxa, heavy metal content, water velocity, water body depth, and temperature (Verdonschot, 2006). No single factor can explain the presence of a single species. There is a very complex system of influences always acting in combination on each species. This is why it is so important to know the specific tolerances and tendencies of each species. It is also true that these tolerances and tendencies may vary by geographical location. While older studies report that oligochaetes are indicators of pollution and eutrophication, newer studies are showing that some species prefer polysaprobic and oligosaprobic conditions (Verdonschot, 2006). Coming to an accurate conclusion of water quality is not only important for the current use of the water, but also for future recovery or restoration plans to come.
Water itself is invaluable and can be very complex. With water being constantly used and abused, it is important to have a solid system to track the quality of this resource. By knowing the strengths and weaknesses of the organisms that depend on this resource, we can properly assess not only the chemical components of water, but the biological components as well. Although oligochaetes show variability in their value as indicators of water quality, they hold an important role in assessing ecosystems on many different levels. Oligochaetes go to show that they are the most appropriate group of species to represent water quality based on their phylogeny, geographical spread, and their wide range of specific tolerances.
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