Introduction


    The rocky intertidal community of New England offers countless opportunities to study ecological patterns and processes.  In the Gulf of Maine, cold, nutrient rich water, flows counter clock-wise allowing for abundant marine life.  The littoral region of the New England rocky intertidal, is the area of the shore that lies within the lowest low tide and the highest high tide.  Organisms found in this region must contend with a vast array of harsh environmental factors.  With the changing tides comes alternating periods of submergence and air exposure.  During submergence, organisms encounter increased salinity levels and must endure wave stress including hydrostatic pressure, drag and lift, and physical damage from debris.  Conversely during low tides and air exposure, temperatures are more extreme and desiccation can occur.  In addition to the physical changes brought by tides, organisms in the littoral zone must also avoid predators, find food sources, reproduce, and compete for space with other organisms.
    It is in this seemingly inhospitable environment that we find the marine snail, Littorina obtusata.  Smaller than the more prominent limpet, Littorina littorea, L. obtusata offers equal wonder for anyone who encounters them.  Round and pudgy obtusata are distinguished by their various shell color morphologies. 

Image Description: Collection of Littorina obtusata shells.  Image width ca 6 cm. Image: Sue Daly (published on the MarLIN Web site)

Observations
On September 6th, 2002 our class began collecting preliminary data on organisms in the intertidal at Nahant.  I couldn't help but notice the color morphs of L. obtusata, that is, when I noticed the snails at all.  It appeared as though their coloration or shape or some combination of the two concealed them within the Ascophyllum.  There were some on the rocks too, but the majority were on the macroalgae.  In addition, darker shelled individuals seemed to be larger while lighter-morphed individuals appeared smaller.  I decided to collect data to see if these observations were correct.
Questions/Hypotheses
My general questions were: Are there differences in the color distributions of L. obtusata on algal and rock substrata?  Is there a relationship between color morphology and size?
A study of L. obtusata by Wilbur and Steneck (1999) examined color morphology and size distributions at two sites in Maine with different colored Ascophyllum fronds.  Their results show that at the wave protected site where Ascophyllum had more yellow fronds present, yellow snails were more common than at the more exposed site with darker algal fronds.  The study also showed dark morphologies as predominantly larger at both sites than lighter, yellow morphs.  Wilbur and Steneck (1999) suggest that the snail color distributions are due to predation effects.  Those more camouflaged with their environment are less likely to fall victim to visual predators, although the extent to which predators view color or contrasting light/dark values is not clear.  If shell color is important in avoiding predation, then it can be expected that darker shelled individuals would be more common on dark rock substrate and lighter shelled individuals would persist predominantly on Ascophyllum nodosum fronds.  Further evidence for this hypothesis could be accumulated if snails, especially lighter colored snails,  were discovered to migrate from Ascophyllum to rock substrate but not persist there.  Hoagland (1977) suggested that color polymorphism is correlated with visual predation on a heterogeneous substrate and with low adult mobility in the gastropod Crepidula convexa. 
Wilbur and Steneck (1999) did not discuss possible explanations for why yellow color morphs were found to be smaller than darker morphs.  However, it may be that since Littorina obtusata appears to mimic the appearance of algal fronds and is known for causing holes by grazing in Ascophyllum nodosum air bladders and then hiding inside the bladders (Pavia and Toth 2000, personal observation), there may be a point at which yellow individuals of a larger size are no longer as effective in avoiding predation as a less contrasting color morph of the same size.  Therefore, yellow individuals overall would be a smaller size than dark individuals.