Aquatic Earthworms

Phylem: Annelida
Class:Oligochaeta

 

by Sarah Vogan

 

A generalized illistration of an oligochaete worm from Dr. Erseus web page. You can see the clitellum, it's the dark large segment to the left.

Index:
What is an Aquatic Earthworm?
What is the Anatomy of a Worm?
How do Worms Reproduce?
What do Worms Eat and What eats Them?
Where can Oligochaeta be found in Emigration Creek?
References

What is an Aquatic Earthworm?

First of all it may be better to answer the question of what is a worm. Worms belong to the Phylum Annelida. The Phylum Annelida includes earthworms, polychaete worms, and leeches. All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. Segmentation increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization (Myers, 1995). The oligochaetes include earthworms and a group of related, mostly freshwater, species of annelids. According to Pennack (1989), aquatic oligochaetes have the same fundamental structure as common terrestrial earthworms.

What is the Anatomy of the Oligochaeta Worm?

Arrangement of internal organs and the body wall of the earthworm from Biodidac

Segmented worms are constructed on a "tube-within-tube" plan. Segmented worms are made up of segments that are formed by subdivisions that partially transect the body cavity. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. The body wall is soft, muscular, and covered with a very thin cuticle. The body of oligochaetes has a swollen girdle-like structure, the clitellum, which serves an important function in reproduction. The specialized digestive tract has a terminal mouth and anus and is supported in the coelom by thin transverse septa that make the internal segmental divisions. True aquatic oligochaetes are much more delicately constructed and small, the usual length range being 1 to 30 mm. The body wall is thin, and the internal organs can be easily seen in living specimens.

Internal anatomy of the earthworm: Paired ventral nerve cords and the ventral blood vessel (bright red tube in center), the digestive tract lies below and the sub neural blood vessel lies underneath the nerve cord. Picture from Biodidac.

The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia. The nervous system includes a pair of cephalic anglia attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment (Myers, 1995)

Cross section: the outer edge indicates the epidermis, and the muscles line the epidermis. The digestive tube is the center circular feature.

 

How do Worms Reproduce?

Position of earthworm during mating.

Although some aquatic oligochaetes reproduce asexually, the majority are sexual, and all are hermaphrodites. At mating, two oligochaetes lie side by side so that the head of one is opposite the tail of the other. Sperm then pass reciprocally into small sacs, where they are temporarily stored. This transfer is more complex in the earthworms, however, because the respective male pores are not in direct opposition; each individual forms a temporary skin canal through which the sperm flow to their respective sacs for storage. After the eggs have matured, a mucous tube, secreted from the clitellum, slides along the body as the worm moves backward. The stored sperm are discharged into this tube, as are the eggs when the tube slides along the section containing them. As the worm literally passes out of the tube, a mucous, lemon-shaped cocoon forms around the now-fertilized eggs. This cocoon serves as a kind of primitive nest, in which the young hatch. (Encyclopedia Britannica, 2000). Cocoons containing embryos are deposited on rocks, vegetation, and debris. This generally occurs in late summer and early autumn (Pennack, 1989).

 

What do Oligochaeta Eat and What Eats Them?

What Oligochaeta Eats

Oligochaetes feed primarily on algae, diatoms, or miscellaneous plant and animal detritus. Detritus includes tree wood and leaves, all of which contains energy for the worms. These earthworms also cycle huge quantities of soil through their guts. This process speeds the turnover of nutrients in soil and increases productivity, and aerates the soil (Myers, 1996).

What Eats Oligochaeta

Aquatic oligochaetes are important food for fish and larger invertebrates (Myers, 1996). When the oligochaeta deposit cocoons full of embryos on rocks small fish eat many of them.

 

Where can Oligochaeta be found in Emigration Creek?

The oligochaeta that we were able to find were found by examining debris from under rocks. These locations tended to be found in the middle of the creek with little to no current present. During spring and summer after periods of high flood water, mud is deposited along the creek bank. Dense networks of worm trails are exposed in the mud after such events. These worm trails indicate worm movement in and out of the creek water. Quite possibly these could be indicating worm movement into the water to eat the leaves, algae or diatoms. The storm drains along Emigration Creek affects the amount and chemical quality of water that will flow into the creek.  As mentioned earlier, Physical conditions, and pulsing flood flow from the watersheds affects the presence and abundance of these worms.

 

References

Erseus, Christer. 2000. Oligochaeta (http://www.nrm.se/ev/research/oligo.html.en)

Morin, Antoine and Jon Houseman. 1999. Biodidac (http://biodidac.bio.uottawa.ca/Thumbnails/catquery.htm?Class=Oligochaeta&Sujet=animalia)

Myers, Philip. 1995. Phylum Annelida (http://animaldiversity.ummz.umich.edu/annelida.html)

Myers, Philip. 1996. Class Oligochaeta (http://animaldiversity.ummz.umich.edu/annelida/oligochaeta.html)

Pennack, Robert W. 1989. Freshwater Invertabrates of the United States. Protozoa to Mullusca. 3rd Edition The Ronald Press Company, New York, NY.

Reproductive Behavior. 2000 Encylcopedia Britanica. (http://www.britannica.com/bcom/eb/article/4/0,5716,119304+18+110426,00.html?query=annelida)

 

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