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Surviving Against All Odds: Investigating the Adaptability of the Common Periwinkle

Natalie Intro

Click to enlarge.


My aim was to investigate the properties of animal adaptability in a polluted environment because I live near a heavily polluted body of water. The North West Arm is an aesthetically beautiful area near the city of Halifax, and many times I have thought about taking a refreshing dip. However, the North West Arm is not a safe environment for recreation according to the signs along its banks and the warnings parents give their children. As I grew older, I wondered how plant and animal life could survive in such an unhealthy environment. Perhaps people had exaggerated the potency of the materials found in high concentrations in the Arm, or perhaps the animal life had indeed suffered from the harsh conditions.

The main source of pollution for the North West Arm is fecal contamination from the 12 million liters of untreated sewage that are discharged into the Arm each day, as well as the overflow from Halifax Harbor, which receives 150 million liters of sewage a day (Halifax Harbor Solutions Project 2001). This contamination promotes the spread of enteric pathogens (salmonella and Norwalk virus). The main outlet for this material is at Point Pleasant Park; however, overflow sites exist at Jubilee Road and the Armdale Rotary.Other pollutants contribute to the contamination of the Arm, such as petroleum and polynuclear aromatic hydrocarbons. The Arm also has higher-than-normal levels of nitrogen, phosphorus, copper, zinc, lead, and mercury.

After my initial decision to conduct an investigation on the biological implications of a polluted environment, I chose to conduct my research on the gastropod Littorina littorea, otherwise known as the common periwinkle, because they appear frequently along the beaches. My first concern was with the care of specimens during the time I was observing them. I was initially unsure of what periwinkles ate, their ability to survive in an observation tank, and the amount of light they needed. However, after conducting some research, I discovered that periwinkles are low-maintenance.


North West Arm (Click to enlarge).

Collecting Samples
To catalogue the mutations (if any) in Littorina littorea from the North West Arm, I endeavored to find a sample of the same species in a pristine environment. Through research I discovered that Chester Basin is not only a healthy environment, but also contains the same geological features present at the North West Arm. The point of collection in this area was Graves Island. In this area I found 124 periwinkles along a 30-meter stretch of coastline. It was evident that the ecosystem was intact, as I also noted the presence of dolphins, a large variety of fish, sandpipers, herons, and jellyfish. I also noted that the periwinkles in this environment appeared on rocks far from the waterline and did not react to exposure to light. From this environment I collected five periwinkles, as well as two liters of water and 0.25 kilos of sand.

Graves Island (Click to enlarge.)

Periwinkle specimens were collected from sites in the contaminated North West Arm as well as from sites at Graves Island, a relatively pristine environment.

My first point of collection along the North West Arm was near the main sewage outlet at Point Pleasant Park, on a foggy evening in April. I combed the shores for two hours (at low tide) and found only four periwinkles. However, the abundance of periwinkle shells indicated that the population must have been more pervasive at other times. I returned the following day at high tide and found no presence of periwinkles, and even the broken shells had been washed out to sea. On both occasions I collected water samples in sterile containers, which I later tested for pH level, turbidity, and the level of chlorine present. I returned to this area on a sunny day when the amount of bacteria in the water is known to be lower. I found few traces of periwinkles at either high or low tide.

On a bright May evening one week later, I examined the beach near a secondary sewage outlet along Jubilee Road; this is also the location of a popular recreation area, the Saint Mary's Boat Club. At first, the only traces of periwinkles were scattered shell remains on the beach. After further investigation, however, I found 12 common periwinkles along a 30-meter stretch of beach. Along the same stretch of beach were several pounds of dehydrated black seaweed and an abundance of garbage. The 12 periwinkles were all found suctioned to the bottom of a large igneous rock two meters from the waterline. I collected three of these periwinkles, as well as one stalk of green seaweed, 0.5 kilos of igneous rock, 0.25 kilos of sand, and two liters of North West Arm water. I then arranged the contents in a 20-liter tank.

I returned later in the week, during inclement weather, to collect another sample of water. This was to document the changes in concentrations of fecal bacteria, metals, and PCBs to give a more accurate picture of the Arm's environment.

The environments around the polluted habitat and the control habitat are essentially geologically identical. Both collection points are situated along bays on the eastern coast of Nova Scotia bordering the Atlantic Ocean. The underlying rock is from the Mefima Group, and the primary deposits are slate, schist, and migmatite, which formed approximately 450 million years ago. The thin layer of topsoil in the surrounding area is the result of glaciation that occurred 10,000 years ago. The soil along the shore is formed from decomposed lichens. As a result of similar soil conditions, both environments support the same species of trees. Examples include the black spruce, white spruce, red maple, red oak, and white pine. After surveying both areas, I found that concentrations of these species within a 10-meter by 10-meter enclosure were essentially the same. In both environments, coniferous trees were more dominant than deciduous.


Testing Water Quality
The water samples I used to conduct my tests of water quality were obtained at low tide, about three meters from the waterline, on a sunny day when the air temperature was approximately 20°C. To make sense of my results, I obtained the U.S. Environmental Protection Agency's requirements for a recreational environment. I found the following:

  • Temperature of water should not exceed 18°C
  • pH should be within the range of 7.0-8.5
  • Dissolved oxygen should not exceed 8 milligrams per liter
  • Exposure to over 0.06 mg/L of ammonia is harmful to fish
  • Exposure of over 0.37 mg/L of chlorine is deadly to aquatic life
  • Turbidity levels should not exceed 4.5 NTU (Nephelometric Turbidity Units)

To obtain the average pH of the water in which I found my specimens, I tested two samples collected from the Jubilee Road outlet on a sunny day and two samples collected on a rainy day, in order to find the average pH in the Jubilee area. I determined the pH using a very precise pool-testing kit for commercial pools. The pH was determined to be 8.7. To compare this with my control, I tested two water samples collected from the Graves Island site on a sunny day and two samples collected on a rainy day. After analysis, I found the Graves Island water to have a pH of 8.0. 

I also measured the chlorine levels found in the polluted water, as this substance is extremely toxic to aquatic life. Utilizing two water samples collected at low tide on a sunny day and two samples collected on a rainy day, I used the same commercial pool-testing kit to determine the amount of free active chlorine. I used a series of three reagents and compared my results against a chart measuring chlorine levels from 0.05 to 1.0 mg/L. The average result determined for the North West Arm water was 0.4 mg/L. Interestingly, this is higher than the level (0.37 mg/L) the U.S. Environmental Protection Agency deems possible for the survival of fish. I compared this with results from the Graves Island control site and found that the amount of chlorine in this area was less than 0.05mg/L.

According to the "Pollution Control" part of the Halifax Harbor Report, "high concentrations of nutrients have led to decreased oxygen levels." This supports a hypothesis I formed after reading that dumped sewage contributes high concentrations of nitrates and phosphates to an area. These nutrients may initially stimulate the growth of aquatic plants, but the excessive amounts in 12 million liters of sewage a day promote the uncontrolled growth of algae and waterweeds, which block the waterway and expend large amounts of oxygen. This in turn creates a high level of carbon dioxide in the water. The low level of oxygen and the high level of carbon dioxide kill aquatic life.Using a filter apparatus, white filters, and a standard color chart to measure turbidity, I found the turbidity of North West Arm water to be 52 NTUs. This compares to the Graves Island control site, which had a water turbidity of 10 NTUs.


This drawing shows the common periwinkle,Littorina littorea, anatomy. The shells of specimens collected at the North West Arm site displayed less variation in color and were thinner than those collected at the control site.

Comparing Periwinkles
The most prominent and easily recognizable feature of a periwinkle is its shell, which is comprised of many whorls. I classified the periwinkles I found as common periwinkles, Littorina littorea , because of the size, color, and texture of their shells. The shells of the periwinkles collected from the North West Arm did not have the diversity of shell color and number of ridges of those collected from the Graves Island control site.The diameter of the shells collected from the Arm ranged from 10 to 35 millimeters, while the specimens collected from Graves Island were consistently larger in diameter, ranging from 26 to 50 millimeters. The thickness of the North West Arm shells was thinner than those collected from Graves Island. The thickness of shells from the North West Arm site ranged from 1 to 2 millimeters, while the shells collected from the control site were between 1.5 and 3 millimeters. 

The shells I examined displayed whorls that increased in diameter along a center point, referred to as the columella. The whorls become increasingly smaller in diameter as they reach the summit of the spire. The largest whorl contains an oval opening that houses the foot and the head of the periwinkle. The operculum is a piece of tough skin that acts to close off the aperture when the foot and head are inside the shell. The foot is located in the ventral region and is used to attach to surfaces for locomotion. The head is located at the anterior end. The head is composed of two tentacles, with one eye next to each tentacle, and a snout that houses a mouth at the end. The only difference in morphology that I could determine between my periwinkles was that, in general, the features of the specimens taken from Graves Island were larger in size, though they generally looked the same as the North West Arm specimens.

The sole of the foot acts as a suction and allows the periwinkle to hold on to rocks and other materials

When a specimen in the tank died, I took the opportunity to perform a dissection on it. I removed the shell using nutcrackers. I found that behind the head on the dorsal side is a depression known as the mantle cavity; this is where the gills are located. In healthy specimens, the gill structure has a faint white pigmentation. However, in the specimen I examined, the pigmentation of the gills was light brown. Within the mantle cavity is the kidney, which is described as pinkish-brown. During my dissection, I discovered the kidney to be gray. I was able to locate the stomach among the intestines, but I found it difficult to follow the path of the intestine to the rectum. I also closely examined the heart, which appeared to be a healthy brown color. I also located the ventricle and the atrium. I was able to determine the atrium from the ventricle because the aorta exits from the ventricle.

After thoroughly examining the specimens I had collected, I devised a series of experiments to test the similarities and differences between gastropods from an environment polluted with fecal wastes, metals, petroleum, and a host of other chemicals, and those collected from a relatively pristine environment.

The first experiment I conducted tested the strength of the shell, or how much force the shell could endure without becoming permanently disfigured (i.e. cracked). I deemed a strong shell to be integral to the survival of a periwinkle, as it is their main defense against predators. To test this, I used a ruler, five Newton scales, four periwinkle shells from the control site, and four periwinkle shells from the North West Arm site. I attached each periwinkle shell to a hook at the end of a single spring scale. If the periwinkle did not demonstrate cracking after I exerted the scale's maximum force of 30 Newtons (N), I attached a second spring scale and continued to pull until the shell cracked or that scale also reached its maximum force. The results were as follows:Comparison of Shell Strength

North West Arm Graves Island
Thickness of Shell (in mm) Force (N) Thickness of Shell (in mm) Force (N)
0.8 48 1.8 102
1.2 65 1.9 93
1.8 81 2.1 121
1.0 51 2.6 Over 150

The results of this experiment indicate that the shells of Littorina littorea inhabiting the North West Arm are weaker than those in the healthier Graves Island environment. This feature compromises the ability of the animal to confront attackers and survive.I was also interested to see if pollutants in an environment affect a periwinkle's sensitivity to light. I reasoned that pollutants may slow an organism's response to various stimuli, such as light and touch. To test light sensitivity, I used a variety of lamps, each with a different bulb wattage; a petri dish; four periwinkles from the North West Arm; and four periwinkles from the Graves Island control site. The experiment was conducted in a darkened room while the specimen had its head and foot exposed. Comparison of Periwinkles' Sensitivity to Light

  North West Arm Graves Island Control Site
Light Intensity NW1 NW2 NW3 NW4 GI1 GI2 GI3 GI4
40W E E E E E E E E
60W E E E E E E E E
80W E P P E E E E E
100W P P C E E P E P
Flashlight C P C P P E E P
Semidarkness E E E E E E E E
  P E P P E E E E

E=no response (fully exposed); C = strong response (concealed inside shell); P = mild response (partially exposed)


Although I had hypothesized that pollutants would slow an organism's response to light, the results of this experiment indicated otherwise. The experiment showed that periwinkles exposed to fecal bacteria and high levels of metals, petroleum, and phosphates, may have an increased sensitivity to light. I reasoned that perhaps the stimulus of mild light has a greater effect on an animal that lives in a dark environment as a result of high water turbidity.

I also wanted to find a way to test the concentration of heavy metals not only in the water of the North West Arm, but also in the specimens themselves. To test for heavy metals, I recalled a flame color experiment that indicated the presence of various metal ions by the color of a burning alcohol flame. To carry out this experiment, I used 15 milliliters of rubbing alcohol, a 50-milliliter ceramic bowl, hydrochloric acid, matches, and nichrome wire. To determine if the heavy metals already documented to be in the North West Arm were also present in the body of a periwinkle from that area, I used the liquids I extracted during my dissection. I accumulated about one milliliter of fluid. I then mixed this with one milliliter of hydrochloric acid. I dipped the nichrome wire in the mixture, held the wire over the alcohol flame, and examined the color. The flame was not significantly affected in color by the presence of the mixture. This indicates there were not sufficient amounts of metal present to be detected through this method. Through experimentation, I found that at least two milliliters of a particular metal had to be present to produce a conclusive effect on the flame.

Examining this species has given me insight into the ability of creatures to adapt to even the most hostile of environments. In an environment where many creatures can no longer survive, the common periwinkle has demonstrated that chemical changes alone in its environment will not eradicate it, as the species will find ways to adapt.


The periwinkles living in the area of the North West Arm are typically smaller in shell diameter than those found in unpolluted water, indicating malnutrition or a decreased metabolic rate. However, features pertaining to appearance and function appeared to be the same as those in periwinkles from more stable ecosystems. The ability of the shell to absorb force in both sets of periwinkles was proportional to the shell's thickness. However, the shells from the North West Arm site were weaker than those from the Graves Island control site. This indicates that the material in their shells has been altered by the materials present in the polluted water.

The darker color of the shells found in the North West Arm may simply relate to water turbidity, as light is not able to diffuse through water as readily in this area as at Graves Island. Increased light may simply bleach the shells. The lower turbidity at Graves Island may also account for these specimens' lack of response to light stimuli.

I also discovered that periwinkles can survive in a polluted environment because their main source of food is algae, and in a nitrogen- and phosphorus-rich aquatic environment, algae is plentiful. The ability of periwinkles to survive in an environment with high amounts of fecal waste may indicate a high resistance to bacteria such as fecal streptococci.I cannot account for all my findings, however, as I could not find conclusive reasons why periwinkles are able to survive in a habitat with lower-than-normal levels of oxygen, as is the case in the North West Arm, as cold-blooded animals need large amounts of oxygen. To further understand this, I would like to conduct tests on the levels of oxygen a periwinkle needs to survive, and how this compares to other aquatic animals.

As a result of this research I have a greater appreciation for the survival abilities of all creatures, especially periwinkles.



Davis, D.S. "Periwinkles." Nova Scotia Museum of Natural History. Retrieved from the World Wide Web:

"Environmental Protection Agency." Encarta Encyclopedia 98. Microsoft, 1998.

Fox, Richard. "Littorina littorea." Lander University, Department of Biology. 1994. Retrieved from the World Wide Web on May 7, 2002:

"Important Water Quality Factors," Hach Company. September 5, 2002. Retrieved from the World Wide Web on July 26, 2002:

Kruge, Michael. "Organic Contaminants in Halifax Harbor: Sediments." Southern Illinois University, Department of Geology. December 22, 1997. Retrieved from the World Wide Web on June 16, 2002:

"Periwinkle." Columbia Encyclopedia, Sixth Edition. 2002. Retrieved from the World Wide Web on March 17, 2002: http://www.encyclopedia. com/printable.asp?url=/ssi/p1/periwink2.html.

"Periwinkle (Gastropod)." Encarta Encyclopedia 98. Microsoft, 1998.Raymond, Joanne. "Halifax Harbor Annual Report." Halifax Harbor Cleanup Inc. "Shellfish and Water Quality." Environment Canada. April 26, 2001. Retrieved from the World Wide Web on June 17, 2002:

Stewart, C., K. Legrow, and D. Leffin. "Is the North West Arm Safe for Sailing Lessons?" Queen Elizabeth II Health Sciences Center, 2002.

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