Or at least covered over. The picture above seems to show a limpet (Patella vulgata) completely covered with an encrusting corraline alga, on which it feeds. However revenge isn't what it used to be as the alga is actually growing on an empty shell. Yet the picture does illustrate the often tenacious growing habits of corraline algae.
Corralines are red algae that encase their cells in calcium carbonate, which increases resistance to damage and herbivory, but reduces photosynthetic capacity. However there are reports of animals grazing almost exclusively on corralines, e.g. the limpet Acmaea testudinalis grazing on Clathromorphum circumscriptum (Steneck, 1982 Ecology 63 pp. 507-522). They have two distinct morphological habits: erect (as can be seen in Corallina officinalis below) and encrusting.
The encrusting species grow as radiating sheet-like patches over hard substrata such as shells (above) but more frequently on rocks. The coralline growing on the rock in the picture below is quite bleached, a result of photoinhibition (Littler, 1973 Journal of Experimental Marine Biology and Ecology 11 pp. 103–120).
Encrusting coralline alga on red sandstone rock in an intertidal rock pool
Corallines dominate the surfaces of subtidal zones in the North Western Atlantic (Adey and Macintyre, 1973 The Geological Society of America Bulletin 84 pp. 883-904). They are also common, if secondary, components of coral reefs (Riding et al., 1991 Sedimentology 38 pp. 799–818).