Diversity of Freshwater Subclass Oligochaeta in Wisconsin

by Alyssa Roberts
BIOL/WATER 361, Fall 2013

Key taxa: Annelida, Clitellata, Oligochaeta

Aquatic freshwater invertebrates that belong to certain taxa have been found to be effective indicators of the health of their freshwater and benthic environments. One group of these "indicator species" falls under the subclass Oligochaeta and have been extensively studied in Wisconsin’s streams and rivers for decades. Depending on the dissolved oxygen levels, predator abundance, and other factors, oligochaete abundance and richness can vary; albeit still persist in a wide range of conditions throughout the states lakes, rivers, and streams (Smith, 2001). Because freshwater oligochaetes have been thoroughly studied in Wisconsin by select members of the scientific community, I was able to research published records containing specific taxa representing members of this subclass. After referring to the current taxonomic record, the overall diversity of Oligochaeta in Wisconsin can be represented by 21 genera under three families. Studying these types of benthic animals is important for understanding the state of specific ecosystems for management purposes, especially due to the negative impact humans continue to have on streams and rivers today (Hiltunen & Klemm, 1985).

Wisconsin is known for its abundance of rivers and streams that support many fish communities. The freshwater members of Oligochaeta in Wisconsin are among the sources of food for a number of species of fish, especially bottom-feeders. Unfortunately, scientists have difficulty monitoring which species are consumed by fish due to their soft-bodied nature and susceptibility to digestive acids (Howmiller, 1974). Luckily, certain members of Oligochaeta can portray other aspects of an ecosystem just by being present in certain conditions. Knowing the structure of the oligochaete population that persists in the water body of interest can greatly assist in making management decisions in fisheries and within other agencies.

Currently, there are about 24 families of the subclass Oligochaeta in North America, the largest being Tubificidae with 19 genera and 56 species (Stimpson et al., 1985). Recent documentation shows that in the Great Lakes alone, there are 100 species that fall under 36 genera (Smith, 2001). Diversity of Oligochaeta in Wisconsin is represented by three families (Tubificidae, Naididae, and Lumbriculidae) with the following 21 genera: Limnodrilus, Lumbriculus, Arcteonais, Dero, Haemonais, Nais, Ophidonais, Stylaria, Aulodrilus, Branchiura, Ilyodrilus, Peloscolex, Potamothrix, Tubifex, Chaetogaster, Amphichaeta, Paranais, Piguetiella, Vejdovskyella, and Rhyacodrilus. With the help of a dichotomous key and dissecting microscope, certain structures can be observed in order to identify oligochaetes down to the species level.

Members of the family Lubriculidae are commonly found in sheltered wetlands, temporary ponds, or small pools in caves (Swayne & Wetzel, 2004). There has been only one species recorded in the state of Wisconsin. With more scientific effort, there is a possibility that there are more members of this family to be found throughout the state.

One family that represents a high diversity in Wisconsin is Naididae. The members of this family are usually smaller than other oligochaetes and are harder to detect for this reason (Hiltunen, 1985). Luckily, Wisconsin has an extensive record of this taxon due to extensive research done in the state. Naidid populations have been found to be correlated with levels of pH, dissolved oxygen and alkalinity in a study done specifically in Wisconsin (Smith, 2001). Having this knowledge is beneficial in that these animals can be used to tell managers about the state of a specific water body at that specific time.

The largest most widely distributed family of this subclass is Tubificidae. This family consists of the greatest number of indicator species for water quality monitoring (Stimpson et al., 1985). A study done by R. P. Howmiller (1977) suggests that members of Tubificidae favor oligotrophic lake conditions over other types of lakes.

These animals can be easily sampled from their habitat using the latest benthic stream sampling and preserving techniques. In order to identify specimens down to the species level most specimens must be of sexual maturity for ideal detection of various body parts (Stimpson et al., 1985), except in the family Naididae (Howmiller, 1976). Observation and examination of the animal’s chaetae, gill structures, budding zones, and the shape and size of the prostomium, is important for identification (Smith, 2001). This requires the utilization of optimum preservation techniques, as well as a trained eye and acute observation skills. The primary structures used to identify oligochaetes are called chaetae. They are short hair-like structures that protrude from the worm’s body and can be various shapes and sizes, depending on the species. Other secondary structures can be used to identify specimens, but the chaetae are highly variable and relatively easy to see. In order to view the chaetae closely, one must use a resinous media to mount a specimen on a microscope slide to view under a dissecting microscope (Klemm, 1985). Oligochaete worms are known to be very difficult to identify down to the species level, so other orders of aquatic invertebrates are usually used to assess habitat quality.

The chaetae are important key structures for oligochaete identification to the species level. Chaetae usually are paired and can be shaped differently depending on the location on the body of the specimen. Chaetae are present either on the dorsal side of the animal, or the ventral side, or both. The chaetae can be long and bifurcate into hooked with "teeth" at the distal end, or short, simple and surround the genital area. In the family Tubificidae, some species require sexual maturation in order to be identified (Stimpson et al., 1985).

Members of the genus Dero, in the family Naididae, have digitiform lobes on the anterior anal segments of the body which vary between species (Hiltunen & Klemm, 1985). These are of more benefit to the observer for identification purposes than the chaetae are for other genera. Members of this genus are also distinguished from other genera because they lack visible eyespots. In order to identify specimens of this genus down to the species level, the lobes, chaetae, and gill pairs are also examined (Hiltunen & Klemm, 1985). The more specifically a specimen is identified, the more we can understand about its benthic habitat and the waters it inhabits.

Freshwater oligochaetes are commonly found in the benthos of freshwater ecosystems in Wisconsin, including lakes, streams, rivers, groundwater, and even cave pools (Swayne et al., 2004), and commonly occupy the same niche as their terrestrial counterparts, the earthworms (Smith, 2001). They even can be found in many areas of the cold and deep Great Lakes due to their growing nutrient enrichment (Howmiller, 1974). In the past, only Lake Mendota had records of oligochaete occurrences for the state of Wisconsin, but now there are many more records being published and that number is still rising. One might wonder if the large diversity of oligochaetes is correlated with the continuation of human enrichment of our waters with organic pollutants.

Many people associate the presence of oligochaete worms with polluted waters. This is one reason they have been referred to as "sludgeworms." The truth is, that a certain presence of specific groups of species is indicative of either high, or low organic pollutions and dissolved oxygen (Howmiller, 1974). Oligochaetes, especially members of Tubificidae, can even withstand anoxic conditions, though they tend not to proliferate (Howmiller, 1977). Different oligochaete communities have varying degrees of pollution tolerance levels and dissolved oxygen requirements, and they are assumed to have differing tolerance indices. Members of the families Tubificidae and Naididae contain species that are pollution tolerant, and their biology is important to understand for water quality research. Not all species have been described enough in order to assign certain tolerance indices, however. Indices are great tools for initiating water resource management strategies in the state of Wisconsin. This is why a more thorough taxonomic list needs to be constructed with strict tolerance levels applied.

Predator abundance is another important factor in determining oligochaete abundance and diversity. One invertebrate of Wisconsin lakes that preys upon oligochaetes is a dipteran under the genus Chaoborus (Howmiller, 1977). This predator can be found in conditions similar to that of which oligochaetes can tolerate and tends to affect the abundance of Tubificidae in Wisconsin. Another predator on oligochaetes that is usually found in Wisconsin lakes and rivers are members of the family Chironomidae, also commonly known as midge larvae (Howmiller, 1977). It is unknown, however, if the presence of this species can directly affect the abundance of an oligochaete population in lakes.

Members of the subclass Oligochaeta, especially those under the families Naididae, and Tubificidae, are great indicators of the health of our water resources. Their diversity in Wisconsin is dependent on many factors, and occur in many different types of water bodies. Wisconsin is fortunate to have such detailed records of these taxa and their ecology relative to other parts of the United States, specifically the South and Southwest where the records are very limited (Smith, 2001). Knowledge of the diversity of Oligochaeta in Wisconsin is important for understanding the evolutionary history of the subclass and its families. Also, understanding of their abundance and distribution can greatly increase the productivity of fisheries and other water resource management in the future throughout the state and even the United States. They are complicated to identify specifically, but the knowledge of the extent of their diversity is rapidly growing with continued research. After all is said and done, with optimum information on the taxonomic distribution and diversity of these animals we can hopefully improve our aquatic resources and the organisms that depend on those resources for survival.

References Cited

• Hiltunen, J.K. & D.J. Klemm. 1985. Freshwater Naididae (Annelida: Oligochaeta). pp. 44-69. [in] Klemm, D.J. editor. A Guide to the Freshwater Annelida (Polychaeta, Naidid, and Tubificid Oligochaeta, and Hirudinea) of North America. Vol. 1. Dubuque (IA): Kendall/Hunt Publishing Company.
• Howmiller, R.P. 1974. Studies on Aquatic Oligochaeta in Inland Waters of Wisconsin. Wisconsin Academy of Sciences, Arts and Letters 62: 337-356.
• Howmiller, R.P. 1977. On the Abundance of Tubificidae (Annelida: Oligochaeta) in the Profundal Benthos of Some Wisconsin Lakes. American Midland Naturalist 97: 211-216.
• Klemm, D.J. 1985. Methods of Collecting and Processing; Museum Depository for Annelids In Klemm, D.J. editor. A Guide to the Freshwater Annelida (Polychaeta, Naidid, and Tubificid Oligochaeta, and Hirudinea) of North America. Vol. 1. Dubuque (IA): Kendall/Hunt Publishing Company.
• Smith, D.G. 2001. Annelida: Oligochaeta. pp. 276-287. [in] Pennak’s Freshwater Invertebreates of The United States, 4th edition.
• Stimpson, K.S. et al. 1985. Freshwater Tubificidae (Annelida: Oligochaeta). [in] Klemm, D.J. editor. A Guide to the Freshwater Annelida (Polychaeta, Naidid, and Tubificid Oligochaeta, and Hirudinea) of North America. Vol. 1. Dubuque (IA): Kendall/Hunt Publishing Company.
• Swayne, H., M. Day & M.J. Wetzel. 2004. Limnodrilus hoffmeisteri (Annelida: Oligochaeta: Tubificidae) in Pop’s Cave, Wisconsin, USA. Journal of Cave and Karst Studies 66: 28-31.