A marine biologist investigates whether localized seawater acidity affects the shell development of the common dogwhelk (Nucella lapillus). She collects mature dogwhelks from two distinct rocky shores: Site X, which is located near an industrial estuary with high localized acidity (low pH), and Site Y, a pristine marine protected area with normal seawater pH.
Describe how the biologist should collect and process data from these dogwhelks to determine whether there is a significant difference in the mean shell thickness of the dogwhelks between the two sites.