Emigration Creek Project

Ecology 340

Westminster College, Salt Lake City, Utah

Tessa D. Roden

AQUATIC SNAIL

(possibly Pyrgulopsis)

INDEX:

What Are These Snails Called and Where Are They Found?

What Do They Look Like?

What Do They Eat / What Eats Them?

How Do They Move?

How Do They Reproduce?

Snail Anatomy

How Do They Breathe Underwater?

How Do Seasonal Changes in the Creek Ecosystem Affect Snail Population?

References


What Are These Snails Called and Where Are They Found?

We are uncertain of the identification of the snails found in Emigration Creek, but have tentatively identified them as watercress or springsnails. Information in Chamberlain & Jones (1929) identifies them as a species of watercress snail probably in the genus Pyrgulopsis. There are over 50 species of Pyrgulopsis found in the United States, mostly in the western states. P. kolobensis is the most likely species to be found in Salt Lake County (Hershler, 1998). There are a number of rare spring snail species found in isolated areas in Utah, one of which is the Bonneville Springsnail (Pyrgulopsis transversa) whose range is shown in the map below. It is unlikely that the Emigration Creek species is one of these rare taxa (Oliver & Bosworth, 1999). In Emigration Creek, on the Westminster College campus in Salt Lake City, UT, snails typically cling to the underside of rocks along the stream bottom.

http://www.rsgis.usu.edu/ucdc/

What Do They Look Like?

The snails of Emigration Creek are small, with a shell of 3 to 5 millimeters in length. The shell is dull brown in color. The shell appears smooth, but with close examination fine growth lines may be seen. The coils of the shell are called whorls, and the snail specimens we took from Emigration Creek had shells of 3 to 4 whorls.

What Do They Eat / What Eats Them?

These snails are vegetarians, feeding on diatoms found on rocks along the stream bed. The snails in turn provide food for leeches and trout in Emigration Creek. Occasional mallard ducks may also eat snails.

How Do They Move?

Snails move using several important muscles. The columellar muscle is attached to the shell internally and is used to withdraw the snail's body into its shell. The foot, which is mostly muscle tissue, is the main source of propulsion for the snail. The front end of the foot secretes a thin, flat ribbon of mucus for the snail to move along. When the snail is caught in vegetation or out of the water it may move by "hunching", or muscular contractions of the foot along with a jerky pulling forward of the shell (Pennak,1953).

How Do They Reproduce?

Springsnails have separate sexes, unlike some other species of aquatic snails which are hermaphroditic. The male copulatory organ, called a verge, is located at the base of the right tentacle. The female reproductive pore is located at the edge of the mantle (shell) cavity. Eggs are laid in a gelatinous mass usually anchored to the underside of rocks. Usually egg-laying occurs in early fall. The young snail develops within the egg mass, and by the time it leaves the egg mass it looks like a miniature adult (Pennak,1953).

Snail Anatomy

DIGESTIVE SYTEM:

Snails have sets of jaws inside their mouths used to cut off bits of food. Just behind the jaws the digestive tract is swollen to form a large buccal mass with muscles attached. This area is covered by the radula, the snail version of the human tongue. The radula moves back and forth very rapidly to grind up pieces of food. It wears away with use, but is continuously replaced since it is formed in a radular sac at the end of the buccal mass and grows constantly, like the human fingernail. The teeth are fastened to the radula in rows. A photo of the radular teeth of the Pyrgulopsis species may be seen below. Snails may have up to thousands of individual teeth, with tiny cutting points called cusps. The esophagus leaves the buccal mass and passes from the foot into the visceral mass within the shell to form a crop. A pair of salivary glands line the crop or esophagus. Behind the crop is a dilated stomach, which is followed by the long intestine, whose posterior end is dilated to form the rectum. The anus opens into the mantle cavity near the edge of the mantle and the shell. A large digestive gland called the liver empties into the stomach as well (Pennak,1953).

http://photo2.si.edu/150photocontest/r2_a.html

CIRCULATORY SYSTEM:

The heart of the snail is found on the left side and consists of one auricle and one ventricle. The ventricle pumps blood through an aortic trunk to all parts of the body through a series of arteries and capillaries. From the capillaries the blood passes into sinuses, or spaces in the tissues called the hemocoel. From the hemocoel blood passes into the veins and back to the auricle. The blood contains a dissolved compound called hemocyanin which transports oxygen, like hemoglobin in human blood does (Pennak,1953).

NERVOUS SYSTEM:

The greater portion of the gastropod nervous system, called the brain, consists of nine large ganglia, eight of which are paired. They are connected to each other by commissures and are found around the esophagus just behind the buccal mass. Large branching nerves originating in these ganglia innervate all parts of the body, while several small ganglia are associated with sense organs (Pennak,1953).

SENSES:

Snails have well-developed eyes at the base of their tentacles. Their sense of hearing is centered in two tiny sacs called statocysts, which contain fluid in which bodies called statoliths are suspended. The snail uses its "hearing" more as an ability to detect vibrations and maintain a sense of equilibrium. Taste is centered in the mouth region, where Semper's organ is the organ of taste (Pennak,1953).

How Do They Breathe Underwater?

Respiration in snails is aquatic and occurs through an internal gill, or ctenidium. The ctenidium consists of a series of narrow, flat leaflets well supplied with blood and arranged like the teeth of a comb. The ctenidium is located on the surface of the mantle cavity of the body whorl (first whorl of shell). A small amount of oxygen may also be obtained through the general body surface (Pennak,1953).

How Do Seasonal Changes in the Creek Ecosystem Affect Snail Population?

Snail population in the creek varies seasonally due to a variety of causes, including availability of food sources, physical and chemical characteristics of the stream, and seasonal changes in the watershed. Snail populations vary accordingly with diatom populations. As diatom populations increase, more snails may inhabit the creek. The diatoms which the snails in Emigration Creek feed on are more plentiful in early fall due to increased amounts of minerals (nitrogen and phosphorus) in the chemical composition of the stream from decomposing leaves that fell from surrounding trees. However, in spring and late fall, diatoms in the stream are affected by runoff pulses from snowmelt or storms. Urban stormdrains allow pulses of runoff through the creek, changing many physical characteristics of the stream. Snails have adapted to live in stable springs and streams, so disturbances like scouring from runoff may reduce snail population. The creek water runs faster and deeper, which disturbs the rocks that provide a habitat for snails and the diatoms on which they feed. Scouring and deposition of silt along the creek bed, which occur during runoff pulses, cause diatoms to be covered in silt. When scouring and deposition occur, diatoms die off, leading to fewer snails in the creek during periods of runoff pulses.

In addition to food, snails need high amounts of dissolved salts, especially calcium carbonate, in the water they live in. These materials are essential for shell construction. Fortunately, there exists an abundance of limestone rocks in the Emigration Creek watershed, providing a wealth of carbonate materials in the creek. High concentrations of dissolved oxygen in water are also necessary for the snails. Pollution limiting the amount of oxygen available causes snails to disappear from an ecosystem (Pennak,1953).


References

1. Chamberlain, Ralph V. and David T. Jones. 1929. A Descriptive Catalog of the Mollusca of Utah. University of Utah, Salt Lake City, UT.

2. Hershler, R. 1998. A systematic review of the hydrobiid snails (Gastropoda:Rissooidea) of the Great Basin, western United States. Part 1. Genus Pyrgulopsis. Veliger 41:1-132. (http://www.si.edu/ofg/staffhp/hershler).

3. Hershler, R. 1998. A systematic review of the hydrobiid snails (Gastropoda:Rissooidea) of the Great Basin, western United States. Part 2. Genera Colligyrus, Eremopyrgus, Fluminicola, Pristinicola, and Tryonia. Veliger 42:306-337. (http://www.si.edu/ofg/staffhp/hershler).

4. Hershler, R., M. Mulvey and H.-P. Liu. 1999. Biogeography in the Death Valley region: evidence from springsnails (Hydrobiidae: Tryonia). Zoological Journal of the Linnean Society 126:335-354. (http://www.si.edu/ofg/staffhp/hershler).

5. Hershler, R., H.-P. Liu and M. Mulvey. 1999. Phylogenetic relationships within the aquatic snail genus Tryonia: implications for biogeography of the North American Southwest. Molecular Phylogenetics and Evolution 13:377-391. (http://www.si.edu/ofg/staffhp/hershler).

6. Oliver, George V. and William R. Bosworth III. 1999. Rare, imperiled, and recently extinct or extirpated mollusks of Utah[:] a literature review. Publication number 99-29. Utah Division of Wildlife Resources, Salt Lake City. (http://www.rsgis.usu.edu/ucdc).

7. Robert Hershler, Curator of Mollusks, National Museum of Natural History, Smithsonian Institution, Washington, D. C. (http://www.si.edu/ofg/staffhp/hershler).

8. Pennak, Robert W. 1953. Fresh-Water Invertebrates of the United States. The Ronald Press Company, New York, NY.

9. Smithsonian Institution: Office of Imaging, Printing and Photographic Services. (http://photo2.si.edu/150photocontest/r2_a.html).


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