Home scar, sweet home scar

A perfect May day on the coast with spring flowers framing a sparkling sea... but I can never resist examining the intertidal marine life on the rocks. Here we see some limpets, large and small, and on the left a good example of what's known as a home scar, the limpet-shaped depression to which the limpet returns after its nocturnal wanderings to graze on algae. Each limpet engraves its own scar by grinding the edge of its shell into the rock. The indentation forms a perfect fit and becomes a receptacle for moisture which ensures the animal doesn't dry out while it is at rest. Limpets travel several feet each night and find their way home by following their own mucus trail back to their personal scar. Scientists discovered this when they experimented by cunningly substituting a portion of the rock with another piece carrying the trail of a different limpet: the original limpet would not follow another's trail and wandered off faintly singing this song.
Note that each limpet looks like a "tiny hill".