Experiments
Experiments
In the
experiment comparing color morphology to substrate, the colors were combined
from 6 color categories into 3 for statistical analysis. This method
helped to find relationships between more general color groupings with
substrata but also may have concealed any possible patterns present between
all the various color groupings. Though this method was used in Wilbur
and Steneck (1999), my definition of dark ("dark complex" according to
Wilbur Steneck 1999), green, and yellow groups probably differed. I
grouped red brown and light brown along with dark brown into the "dark"
category. The red brown color morphs I encountered in Nahant were of a
dark rusty or maroon red and some were a mix of red and brown. They
seemed best suited to the "dark" group. I did not encounter many light
brown color morphs. Their appearance seemed to vary between brown and
grey with green, so some light brown morphs may have been categorized as
dark brown or olive green, depending upon lighting conditions. The
"green" category was simply olive green color morphs. These were very
abundant and were more distinguishable than the ambiguous "dark" groups.
The "yellow" category included greenish yellow morphs because, while they
may not have been pure yellow, they were much lighter in color than the
other groups with some having patches of both yellow and light
green.
Color assessment can be problematic due its subjectivity. Wilbur and
Steneck (1999) used a color chart for visual analyses as well as an
additional computer imaging program to help standardize color
determinations. Some type of color chart in the field may help
ameliorate problems with subjective color identification. Also, color
appearance can differ depending on whether the shell is wet or dry and
should probably be sprayed with water before sorting.
Since L. obtusata are famous for their camouflage, finding them
during sampling can be difficult. The longer the search, the more may
be found. Especially if information on snail density is desired, a
standard search time should be set for each sample area.
The snail mobility experiment may not have resulted
in conclusive data, but its failure leads to some interesting
considerations. Because it was raining on the day the experiment was
conducted, the nail enamel used to mark the snails may not have dried before
being smeared off by handling or movement of the snail. Indeed, this
was observed during the process. Second, the experiment can only
show whether or not snails remain upon the same alga. It does not
show if snails moved between algal and rock substrata or from alga to alga,
nor if they do this by choice or are tossed about by wave action.
Standardizing snail search time may also be
important for dimensional data. If the search is short, larger
individuals may be found while smaller individuals are overlooked.
Different dimensions may also reflect the age of the snail instead of
ecological factors determining their size.
Results
Results
for shell color verses substrata indicate a greater proportion of dark
morphs upon rock surfaces with proportionately lighter morphs on
Ascophyllum. In Wilbur and Steneck (1999), differential color
morph distributions were seen relative to exposed and protected sites where
the color of Ascophyllum also differed. Here, I attempted a similar
comparison using different substrata instead of multiple sites.
Differential shell coloration may be due to many factors. One
hypothesis states that shell color variation is maintained by selection from
predation (Wilbur and Steneck 1999). This seems probable from strong
evidence that the introduction of the green crab over that last century has
caused a significant increase in snail shell thickness (Bertness 2001).
However, an increase in shell thickness due to phenotypic plasticity instead
of selection on genes is thought to also contribute to the morphological
differences (Trussell 1996). However, trade offs have been shown to
exist between shell thickness and body size (Trussell 2000, Trussell and
Nicklin 2002) but may be worth the expense to avoid predation.
Although it is possible that these inducible defenses have evolved as
opposed to genetically based phenotypes because of trade offs with body size
(Trussell and Smith 2000).
Another hypothesis is varying color morphs are due to food consumed by the
snail. This may be possible to some extent, especially in altering the
degree of color intensity, but most findings seem to indicate a majority of
genetically based cases (Etter 1988). Overall, there were more snails
on Ascophyllum than on rock substrate (88 vs. 28 individuals).
This could be caused by greater food quantities available on Ascophyllum.
Shell color may also be due to closely linked character traits with other
traits that selection acts upon.
Etter (1988) was able to show that brown color morphs of Nucella lapillus
were selected upon more heavily by the physiological stress of desiccation
at protected sites than white morphs.
Finally, color morphology may have evolved locally because of the low
dispersal ability of L. obtusata's direct development of offspring.
In Wilbur and Steneck (1999), yellow morphs of L. obtusata were shown
to be smaller than darker morphs. Additionally, at the exposed site
where Ascophyllum was said to be darker in color, yellow morphs were less
abundant. There is little evidence that the size a shell
attains changes their adhesive ability since a larger foot size is more
useful for adhesion (Trussell 1997). This suggests that dark color
morphologies may have no advantage in that respect if the color morphs were
found to be different sizes on average.
Yet, the dimension data of this study shows no significant difference in
dimensions between the color morphs. Thus, the hypothesis that
larger, light-colored morphs might experience increased predation was not
supported. Differences in color morph size as found in Wilbur and
Steneck (1999) may have reflected the age of the snails sampled or other
temporal factors such as time of year.
Future
Study
Possible
future studies may include comparing L. obtusata morphologies at
protected and exposed sites. Since I focused my study in one area,
there may be differences if more area is examined. Additional studies
could also address snail size verses substrate which was not explored in
this study. Also, a more cleverly designed snail mobility experiment
could be performed- one allowing nail polish to dry thoroughly and
perhaps marking snails found on rock substrate with a different color.
Still, few individuals may be recovered if wave action greatly upsets the
snails.