Experimental Design

The rocky intertidal region of Nahant in all of it's slippery glory (taken 29 August, 2004)

         Northeastern’s Marine Science Center in the peninsula of Nahant, Massachusetts offers an ideal study site for intertidal organisms since it is easily accessible and has a low wave exposure.  The eastern side of Canoe Beach was visited on 15 October 2004, 1 November 2004, and 12 November 2004.  On the first two dates, ten 0.25 x 0.25 quadrats were taken while on 12 November, fifteen were taken in the same area in the mid intertidal region.  Snails were broken down into four different categories; dark L. obtusata on a dark background (rock), dark L. obtusata on a light background (algae), light L. obtusata on a dark background, and light L. obtusata on a light background.  The light color morphology delineation consisted of the L. obtusata with similar colors to the algae; yellow, green, and red, while the dark morphology consisted of the L. obtusata with similar colors to the rocks; dark and light grey.  On 1 November, organisms not included in the quadrats were taken back to the lab for further analysis.  Although carapace color morphology was only separated into dark and light, paint strips obtained at a local hardware store were used to further standardize the colors.  While in the field, several quadrats were photographed with the organisms next to the paint strips for color standardization.

Examples of the various color morphologies:

Dark green        Green-yellow    Red                Yellow                Dark gray        Light gray

  (Light)               (Light)              (Light)              (Light)                (Dark)             (Dark)

        The two most accurate colors “rapture” and “safari” (simulating the dark green and green yellow morphologies were taken into the lab and scanned into photo editor where they the two colors were arraigned into a checkerboard pattern with 2.5 inch blocks of the two colors.  These checkerboards were then fixed on the outside of a ten gallon fish tank with one gallon of seawater and an aerator. 

        For the first seven days, only a cover was placed on the tank.  Individuals found on the cover or along the black rims at the top and bottom of the tank were excluded in the counts, although virtually none were ever found along the bottom rim.  Individuals on the top rim were moved and placed in the water after the count was preformed.  Counts were preformed either once or twice a day, depending on scheduling conflicts.  Nine measurements were taken for both treatments.  A slightly different color classification was used in the lab as in the field; the yellowish color was considered light and the green was considered dark.  After the ninth day, a black tarp was placed over the tank to estimate the variable of light, which was thought to influence the number of L. obtusata along the top rim.  This treatment was continued for six days. 

        Results were separated into three groups; field results, lab results without the black cover over the tank, and results with the black cover on the tank.  These data were then analyzed using a one-tailed Chi-Square Contingency test. 

Introduction        Methods        Results        Discussion

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