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Morphologic Variation in the Common Periwinkle

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For years I tramped up and down the Maine shore, paying little heed to the creatures under the seaweed. It was whales that fascinated me when I was little. I was sure I wanted to grow up to research them, and I couldn't wait to get started. I had hopes that some day orcas would swim into my clam-flat cove—yet I soon learned that that was highly unlikely to happen in Maine. With a lack of whales at my beck and call, I soon turned to seals. They were almost accessible; however, they were still too large to fit into a tank in my backyard. With seals and orcas off the list, I became needy for my research fix. I racked my brain and took a walk to my little cove. No orcas, no seals. The tide was low, and the clam flat's odor filled the autumn air. The blue sky was crystal clear, and the rocks were still warm from the noon sun. I heaved a sigh and kicked off my sneakers. The ledge was warm and felt perfect under my toes. I ran barefoot along the rocks, jumping on stepping-stones across the mud flat to my island. I bent low and picked up one of my periwinkles. I hummed to it as I filled my lungs with the fresh air. Then the circuit was completed. The electricity ran as excitement through my veins. I scrambled back to ask my mom. After some careful rounds of negotiation, the proposed periwinkle laboratory was approved for immediate construction in my basement. It wasn't whales, but my research had begun.

 

Background Information 
Marine snails in general are part of the family Gastropoda and vary in size. Snail species live in a variety of habitats. They are found in oceans all over the world. One species lives in Florida's mangroves, some live in New England's clam flats, and many inhabit various intertidal zones around the globe. Numerous species have been studied in depth over the last 100 years, while a few still remain less studied. Many of these invertebrate mollusks are herbivores, some are scavengers, and a few select species are carnivores (Morris, 1951).On the northeastern coast of the United States and Canada there are several species of marine snails. Among the most studied are the rough periwinkle (L. saxatilis) and the smooth periwinkle (L. obtusata), but there are few studies in the Gulf of Maine on the common periwinkle (L. littorea). All these species can be found on the rocky intertidal shores of the Gulf of Maine and are easily identified by specific characteristics (Morris, 1951).

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The common periwinkle shell (Littorina littorea).


The common periwinkle (L. littorea) is one of the most abundant marine gastropods on the North Atlantic coast. It is an exotic species from Europe that was introduced accidentally to Nova Scotia around 1857. Its range has slowly spread south, displacing some other small snails. The current range of the common periwinkle is from the Bay of Fundy to New Jersey. Its range does not extend further south, likely due to the warmer waters and sandy substrate of those shores. The common periwinkle is approximately one-half inch to one inch in height, and is gray to black in color. The shell is fairly heavy, solid, and stout, with about six to seven whorls. It has a thick outer lip, is black on the inside of the aperture, and the tip of the spire is often white (Chenoweth and McGowan, 1993).

Common periwinkles live in the intertidal zone (the area of the shore that has alternating periods of exposure to air and then water). Periwinkles distribute themselves in different positions on the shore. There is dramatic variation in the zonation of periwinkles. The distribution is not uniform along the intertidal zone and varies from site to site (Williams, 1964).

Larval periwinkles disperse on the tide and float in the water column for several weeks. Gradually they grow a shell, transform into tiny periwinkles, and settle to the bottom of the subtidal zone. According to the literature, periwinkles then move up into the intertidal zone and become sexually mature within 18 months. During these months periwinkles can grow to be 18 millimeters in shell height. Most periwinkles live only two years, while some can live up to 10 years and grow to 37 millimeters (Chenoweth and McGowan, 1993). 

The common periwinkle plays a significant role in the ecology of the Maine shore. Periwinkles help to control the abundance of the algae community. Foraging by dense populations of periwinkles removes large quantities of green algae and loosens sediment from the substrate. This causes increased water flow over the shore bottom, washing sediment away and leaving exposed rocky shores (Chenoweth and McGowan, 1993).

There is a periwinkle fishery in the state of Maine that in 2002 shipped 136,171 pounds of periwinkle meat to ethnic and foreign markets, worth a total of $359,665. According to the Maine Department of Marine Resources, the easily accessible harvesting sites have been depleted in recent years. As a result, the fishery has moved to the offshore islands and ledges where periwinkles are still abundant. There are no population or abundance estimates, nor any management plan for this species (Chenoweth and McGowan, 1993).

In previous studies, periwinkles have been shown to have morphologic variations on micro- and macro-geographic scales. These studies have examined the growth of periwinkles in comparison to multiple abiotic and biotic factors. Very little work has been done to catalogue morphologic variations according to intertidal position and substrate (the specific terrain of the shore: the ledge, crevice, tide pool, sand, or mud on which each individual snail is located). 

According to studies by Newell (1956), the distribution of periwinkles appears to have no relation to light, tidal level, or beach slope; periwinkles can be found in clusters, especially in crevices and tide pools, or alone. Periwinkles have been observed to remain inactive or stationary during the majority of each tide, even when covered with water. On feeding excursions, periwinkles appear to make U-shaped tracks that return to specific rocks or areas. About 95 percent of studied tracks consist of these two journeys: outward and homeward. Foraging periwinkles will return to a similar, but not exactly the same, location after each excursion.

It has been found through observations of marked and released snails that periwinkles that were displaced from their original places returned to approximately the same intertidal positions (Gowanloch and Hayes, 1926). In studies that marked and released snails to analyze growth over long periods of time, periwinkles were re-collected near the original collection position (Moore, 1936). According to another marking and displacement experiment, the exchange of periwinkles among intertidal levels was strictly limited. These studies suggest that individuals maintain a specific position on the shore, and that random clumping would not explain trends in periwinkle size. It would appear, however, that each periwinkle has a preference about its position on the shore (Smith and Newell, 1955). 

 

My Previous Research 
I have always had a curiosity for how creatures survive the cold winters of Maine. After some preliminary observations in 2001, I designed an experiment to test periwinkle activity according to water temperature. The experiment was composed of a set of three temperature environments that were maintained at specific temperatures. After observing periwinkles in my basement laboratory for 30 days in 2001, I determined a trend in periwinkle behavior according to the temperature of the water. Periwinkles in cold water were less active than in warm water, and there was an upward turn in periwinkle activity at 4 degrees C. This helped answer a few of my early questions but led to more. By my second year in high school, I was completely enthralled, and I had moved away from the lab to work with periwinkles on the shore. My next questions focused on the physiological differences in populations from different water temperatures. My results showed periwinkles from a cold water temperature site to be smaller (on average) than those from a warmer site. Another trend that I was not expecting to see appeared in my data. It showed that common periwinkles near the low tide line were larger than those near the high tide line. This trend was consistent at all three of the sites I had sampled and led to a completely new focus in my research. I designed a study of morphologic variation in periwinkles as a function of intertidal position. For my study site I chose an exposed island (Jenny Island) in Harpswell, Maine.

My previous studies suggest that in order for morphologic variation among sites to be effectively compared, the periwnkles' intertidal position must be taken into account. The purpose of the recent study was to further explore the influence of intertidal position and substrate on periwinkle morphology. I formulated a research hypothesis that stated that periwinkle shell height would vary according to intertidal position and substrate. Specifically, periwinkles on ledges near the high tide would be, on average, smaller than those on ledges near the low tide, and periwinkles in tide pools would be, on average, the smallest.

 

Methods and Materials 
A total of 398 periwinkles were sampled from five transects on Jenny Island, one mile offshore of Harpswell, on Casco Bay. The transects were spaced around the circumference of the island and ran vertically up the shore, perpendicular to the water. All data collections were made at low tide during the week of July 4, 2003. There were no drastic changes in temperature or any extreme weather conditions during the collection period.

I estimated the Extreme High Tide (EHT) using the line of dried seaweed across the top of the shore. A string was tied to a rock near the EHT and was run down the shore perpendicular to the water. A transit was set up and leveled on the EHT, running parallel to the transect string.

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Marking the shore transect in feet below extreme high tide (ft-BEHT).


Using the transit, the transect was marked off in vertical foot increments below the EHT position (BEHT). These served as benchmarks of intertidal position during data collection. Starting at the lowest possible point on the shore (greatest distance BEHT), all common periwinkles within one inch of the right-hand side of the Measuring Periwinkle Shell Height from the Base of the Shell to the Tip of the Spiretransect string were collected and measured. Their intertidal position was estimated to the nearest half-foot using stone markers. Periwinkles were measured for shell height in millimeters (base of the shell to the tip of the spire) using digital calipers (tests showed this procedure had a repeatability of /- 0.17-mm). 

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Measuring periwinkle shell height from the base of the shell to the tip of the spire.


The intertidal position (ft-BEHT) and the specific substrate of each periwinkle's location were also recorded (tide pool, ledge, crevice, mud, sand, seaweed type, and barnacle cover on the rocks). After measurement, each periwinkle was placed to the left-hand side of the transect string. This eliminated the possibility of re-measuring the same periwinkles. When other organisms were detected in the area, their presence was also noted.

 


Plot of shell height vs. intertidal position (includes all five transects). (Click to enlarge.)


Results 
The chart displays the size of each periwinkle against its intertidal position. This plot includes the data from all five of the study transects. Each point represents one of the 398 periwinkles collected. On the x-axis is intertidal position as distance below the extreme high tide. The y-axis plots periwinkle shell height in millimeters. Each blue data point represents a periwinkle from a ledge. Each red point represents a periwinkle collected from a tide pool, while each green point represents a periwinkle found in a crevice. A visible trend in periwinkle shell height is noticeable according to intertidal position and type of substrate.
 

 

Intertidal Position Results 
To test for significant differences according to intertidal position, my periwinkle data was complied into three groups. Periwinkles collected from the area of 0 to 3.5 feet BEHT were considered members of the Near High Tide intertidal group. Likewise, periwinkles from 4.0 to 7.0 feet BEHT were considered part of the Mid Tide group, and periwinkles from any height greater than or equal to 7.5 feet BEHT were considered part of the Near Low Tide group. Periwinkle morphology varied according to intertidal position. Periwinkles from the Near Low Tide group were on average the largest, and periwinkles from the Near High Tide group were on average the smallest. This supported my original hypothesis.

I performed an Analysis of Variance (ANOVA) calculation on the periwinkle shell height data from the three intertidal position groups. I was testing the null hypothesis that all three samples could have come from the same population and that sample variations were due to random factors. The results of the calculation showed the probability of the null hypothesis being true to be less than one in 10,000 (p<0.0001). This indicates that I can reject the null hypothesis and say the populations are different among the three intertidal positions.

 

Substrate Results 
My previous observations made me question differences in periwinkle shell height according to substrate type. In this study I found that periwinkle size does vary according to substrate. The data showed that, on average, periwinkles from the ledges were larger than periwinkles from crevices or tide pools, and periwinkles from tide pools were the smallest. This trend also supported my hypothesis and led to further questions about periwinkle morphology.

I used an Analysis of Variance (ANOVA) calculation to test for significant differences among the three substrates: crevice, ledge, and tide pool. The results of the calculation showed the probability that "the differences among groups could represent random factors" is less than one in 10,000 (p<0.0001). This indicates that the differences among the three groups represent a non-random sorting of periwinkles. 

Plot of shell height vs. intertidal position for only ledge substrate periwinkles. (Click to enlarge.)


This chart shows the trend in periwinkle size displayed by the ledge substrate periwinkles. This plot demonstrates how eliminating the effect of small periwinkles from tide pools and crevices does not change the intertidal position trend. The periwinkles near the low tide are still on average larger than those from near the high tide. A mathematically fitted trend line is shown to help illustrate this relationship. Both the other substrate groups (crevice and tide pool) displayed similar trends.

 

Discussion 
The data displays a clear trend in periwinkle morphology according to intertidal position and substrate. This trend was also found in my previous research. While this is the second time this trend has been found, it is not represented in the literature.

My findings are important because they suggest an alternative to a common methodology of periwinkle sampling. In some studies periwinkles are sampled through a process of random collection. This method often includes a set frame that is thrown haphazardly in the intertidal zone. The location where the frame lands is designated for periwinkle collection. This method may not control for intertidal position and substrate. 

Another example of the importance of these findings is that it raises questions about the conclusions of my own previous research. My previous comparison of periwinkle morphology among temperature sites did not control for substrate. The small average size found at the cold site may have been due to the greater sampling from tide pools at that site. In light of my new study, my previous conclusions about temperature effects need to be revisited. 

The large sample size used in this study provides clear indication of statistically significant differences among groups. Physically, these differences are dramatic. The average periwinkle size more than doubles from 8 millimeters near the high tide line to 19 millimeters near the low tide line. These variations may have some practical significance for fisheries management, shore ecology, and environmental modeling. This study of an exposed site supported my previous observations and suggested further investigation of this trend. Does wave exposure affect this distribution trend? This study has led me to initiate an ongoing further investigation in a protected cove using the same methodology.

 
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On-site measurement.


Conclusions
The results of this Jenny Island study show that periwinkle morphology varies significantly according to intertidal position and substrate. These results support my research hypothesis and my previous findings. The periwinkles near the high tide line are on average smaller than those near the low tide line. Periwinkles from tide pool substrates were significantly smaller than periwinkles from any of the other substrates.

This trend is not reported in the literature, and my findings suggest that in order for periwinkle morphology to be effectively compared among sites, the periwinkles' intertidal position and substrate must be taken into account. The experimental design developed in this study seems to provide adequate sample size and control for intertidal position and substrate. It seems to allow periwinkle morphology to be investigated effectively, and I intend to further test this methodology in a follow-up investigation of periwinkle distribution trends.

 

References

Books
Chenoweth, Stanley, and Jay McGowan. Periwinkles in Maine: Fisheries and Biology. Boothbay Harbor, ME: Department of Marine Resources, 1993.

Kingsbury, John M. Analyses of hydrozoa, gastropods, and ectoprocts. New York: Cornell University, 1976.

Morris, Perry A. Field Guide to the Shell. Boston: Houghton, 1951.

Tupper, E.J. "Temperature Effects on Periwinkle Activity: Behavior and Metabolism." (2002): Unpublished.

Tupper, E.J. "The Intersite and Intrasite Variations in Common Periwinkle Populations for Three Temperature Sites in Casco Bay, Maine." (2003): Unpublished.

 

Journal Articles
Gowanloch, J.N. and F.R. Hayes. "Contributions to the study of marine gastropods. Part I. The physical factors, behavior and intertidal life of Littorina." Contr Canad Biol, N.S. 3 (1926): 135-165.

Kelaher, et al. "Foraging by the mud snail Ilyanassa obsoleta modulates spatial variation in benthic community structure."  J Exp Mar Bio Ecol  292 (2003): 139-157.

Moore, H.B. "The biology of Littorina littorea. Part I. Growth of the shell and tissues, length of life and mortality."  J Mar Biol Ass U.K.  21 (1936): 721-42.

Newell, G.E. "Animal zones on the North Kent Coast." S East Nat 59 (1954): 34-56.

Petraitis, Peter S. "Effects of intraspecific competition and scavenging on growth of the periwinkle Littorina littorea." Mar Ecol Prog Ser  236 (2002): 179-187.

Smith, J.E.and G.E. Newell. "The dynamics of the zonation of the common periwinkle Littorina littorea (L.) on a stony beach." J Anim Ecol  24 (1955): 35-56.

Trussell, Geoffrey C. "Evidence of counter gradient variation in the growth of an intertidal snail in response to water velocity." Mar Ecol Prog Ser  243 (2002): 123-31.

"Phenotypic selection in an intertidal snail: effects of a catastrophic storm." Mar Ecol Prog Ser 151 (1997): 73-79.

"Phenotypic clines, plasticity, and morphological tradeoffs in an intertidal snail." Evolution 54 (2000): 151-66.

Trussell, Geoffrey C, Patrick J. Ewnchuk, and Mark D. Bertness. "Trait-mediated effects in rocky intertidal food chains: predator risk cues alter prey feeding rates." Ecology 84 (2003): 629-40.

Williams, E.E. "The growth and distribution of Littorina littorea (L.) on a rocky shore in Wales." J Anim Ecol 337 (1964): 413-432.

 

Web Sites 
Marine Resources in Maine—Commercial Landings for 2002. Department of Marine Resources. Retrieved from the World Wide Web on 9 December 2003. http://www.maine.gov/dmr/commericalfishing/2002/landingsbyspecies.htm

 

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