Snail Grazer Abundance & Periwinkle Snails

Abundance of periwinkle snails (Littorina scutulata) at middle elevation rocky sites, 1989-1996. The large increase in oiled/washed snails in 1991 and 1992 was due to increased numbers of juveniles.

This graph above shows mean abundance per 0.25 square meter quadrat of periwinkle snails from 1989 to 1996. Abundance is plotted for 3 categories: oiled/washed, unoiled, and oiled/unwashed. The data show a large increase in oiled/washed snails in 1991 and 1992. Mean abundance of oiled/washed snails rose from near-zero in 1989 to a peak of about 270 snails per 0.25 square meter quadrat in 1992 (mostly due to the appearance of many juvenile snails). Abundance then dropped in 1993, since then fluctuating at levels of somewhat more than 100 snails per quadrat through 1996. During the same period, the abundance of unoiled and oiled/unwashed snails fluctuated somewhat, but remained below 100 snails per quadrat.

Closeup photograph of periwinkle snails (Littorina scutulata) and a newly-laid egg mass on a boulder in Prince William Sound.

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