Discussion

        The data from the field proved highly significant with p<0.0001 (1.10689E-31) meaning there is a strong relationship between snail carapace color and substrate color.  Percent cover for each quadrat was calculated, but was not included since considerations were made in the way the data were collected (substrate color).  The majority of the quadrats (26 of 35) had at least 70 percent alga cover.  The high numbers of dark L. obtusata found in these quadrats were not found on bare rock, but under the canopy of Ascophyllum nodosum and Fucus vesiculosus.  Many of the light L. obtusata on dark substrates were also found in this way.  Only one was found on an uncovered site where it would be easy for predators to find.  Three of the quadrats were completely rock and had a total of seventeen dark snails on a dark substrate, although three light colored snails were also found in these quadrats.  The greatest variation in the data occurred in the quadrats with complete alga cover.  Of the fourteen quadrats with complete alga cover, all but three had at least one dark snail on a dark background (rock) under the alga.  One quadrat had two dark L. obtusata on light substrates and seven light L. obtusata on dark backgrounds under the alga canopy.  The photography of snails for color comparison was only done for a few sites due to inhospitable weather conditions on the days the site was visited. 

Here are two examples of dark L. obtusata on dark substrates in the field

        As table 1 and figure 3 show, there is a highly statically significant relationship between L. obtusata carapace color morphology and substrate composition.  These data are supported with the findings of Wilbur and Steneck (1999) and Ellen Machala (unpublished data).  The reasoning behind the habitat selection seems to be mitigated by prey.  The laboratory component of this study involved taking L. obtusata into the lab where they were offered different colored substrate options.  The two colors chosen aptly matched two of the alga colored L. obtusata.  Very few of the gray morphologies were taken into the lab since not many were found.  For the lab experiment, the dark morphology was the “safari,” which accurately matched the dark green morphology.  During the first seven days of the experiment, several individuals were found to be on the black top rim or on the cover of the tank.  After a count was preformed, they were removed and placed in the bottom of the tank.  As figure two demonstrates, there was an increase in number of L. obtusata on the sides and bottom of the tank when the dark treatment was started.  The treatment was initiated in an effort to decrease the numbers on the rims since they were presumably there to avoid light.  This difference was not statistically significant as tables two and three show.  Both p values are very close to be significant (0.063).  The possibility that the snails can recognize their substrate color is possible, but unlikely.  Errors may occur in these data since the color classification was changed for the laboratory portion of this study.  Since the dark green phenotype was considered dark, this study showed that when given a choice of substrate, the chance of them landing on a substrate color that matches their phenotype is not significant. 

Here are light L. obtusata on a light substrate in the field

        The carapace of L. obtusata seems to exhibit high phenotypic plasticity.  Snails raised in the laboratory in the absence of predators exhibited thinner shells than those on intertidal sites with predation (Trussell 1996).  Phenotypic plasticity of carapace thickness is common phenomena in many intertidal creatures (Trussell 1996).  Ultimately this plasticity could be assumed for carapace phenotype, but his highly unlikely.  Museum specimens collected in the Gulf of Maine before the invasion of their primary predator in 1900, C. maenas show a much thinner shell than those collected from similar locations in the mid-1980s (Trussell 2000).  This is presumably the case at Nahant since C. maenas invaded several years after the initial introduction (Burtness 1999).  The cause for this range expansion is still unknown, but the most plausible hypothesis is increases in mean annual sea surface temperatures over the last 100 years in the Gulf of Maine facilitated movement (Trussell 2000).  Phenotypic plasticity in L. obtusata has been also shown to occur with foot size.  Studies preformed looking at L. obtusata on protected areas versus those on non-protected areas shows that the foot size in protected snails is plastic (Trussell 1997).  Here, individuals raised on the wave-exposed shore developed a larger foot than those raised on their native shores (Trussell 1997).  Trussell discovered that larger foot sizes are induced by increased hydrodynamic stresses associated with increased water velocity.  A possible reason for the multiple phenotypes is multiple paternity where multiple males inseminate a single female (Paterson et al. 2001).  Here, the female undergoes copulations with several males, store the sperm, and uses only a small amount to fertilize each batch of eggs she lays.  This is possible, because the females have two internal storage areas for sperm.  The longer the male guards the female after copulation occurs, the more likely his sperm is to enter the seminal receptacle, which is protected because it sits further inside the snail’s body.  The other (more temporary) sac is called the bursa copulatrix and is described as a blind-ended sac (Paterson et al. 2001).  If carapace phenotype and morphology are more genetically linked than phenotypically plastic, one female periwinkle has the potential to have offspring of several different phenotypes if multiple males inseminated her.  If the carapace morphology were phenotypically plastic, it would presumably change over time, which it does not appear to do.  Ultimately, carapace morphology is predator mitigated and is most likely genetically linked.  Since phenotypic plasticity is common in these periwinkles, it is still a plausible explanation for carapace morphology, although unlikely.  Additional studies need to be preformed to further validate this statement.

Introduction        Life History        Methods        Results

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