Marine bivalves of the family Mytilidae have achieved an impressive ability to dominate rocky shores. Mussel populations usually form highly dense, overcrowded and multilayered matrices or beds, where intraspecific competition is intense and so, self-thinning processes are expected to occur. In this work, I review the literature on self-thinning in mussels with special reference to the development of new models. I suggest that more experimental and theoretical work is needed to properly understand space- and food-driven self-thinning in overcrowded and multilayered mollusks. I conclude that the development of new kinds of 3D models that include explicitly the crowding or layering effects on self-thinning would open new theoretical and experimental avenues to explore the relationship between density and size, not only in mussels but also in gregarious animals in general