Background

Littorina littorea
lit

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Taxonomy

Kingdom: Animalia

Phylum: Mollusca

Class: Gastropoda

Order: Mesogastropoda

Family: Littorinidae

Genus: Littorina

Species: littorea

Many Littorina species show greater abundance of smaller individuals in their lower distribution limits and greater abundance of larger individuals in their higher distribution limits (Saier, 2000). Littorina littorea, however, often show a highly variable size distribution pattern (Saier, 2000). Early studies of Littorina planaxis and Littorina scutulata by Wheeler (1954) found that environmental factors, particularly wave action, caused size distributions. More recently, Eschweile, et al (2008) found that L. littorrea shell size distribution patterns vary across different environments. I wanted to see if the tidal zones at Nahant would differer physically and/or biologically enough to produce similar results. Because of these differences, I hypothesized that the low, middle and high tidal zones would favor different sizes of L. littorea. The variation in physical pressures, temperature, and wave action, as well as biological pressures, such as predation and competition might select against the survival of different size groups of L. littorea. Such selection would result in different size groups of snails in the three zones.

Littorina littorea is a relatively large herbivorous intertidal snail, that can grow up to 52mm in size (Jackson, 2002), although none of the L. littorea I found in Nahant were near that size. This species was formerly believed to be invasive, possibly brought over from Europe, but new molecular evidence shows that this is not true (Bertness, 2007). The species has actually been in North America for thousands of years, survived the last ice age by inhabiting northern Candian water, and then extended their range south in the mid 1800s (Bertness, 2007).