Hi there. I think the first thing you need to quantify, or at least take into account, is the tidal range. Are you looking strictly at terrestrial Littorina, or can they be covered by the tidal range. I would also look for similar patterns of mollusk habitat...i.e. lower branches, upper or entire tree? But looking for these patterns it might be easier to estimate total 'range' and therefore density.
I did something similiar for echinoderms - the difficulty lies in not necessarily measuring the total surface area of the tree, but rather the surface area that is colonized.