Space competition is documented in lithophages, off the Istrian coast (North Adriatic Sea). Due to high population-density, borehole crossings occur. Spatial relationships between competing bivalves determine which one will be successful, or whether competition remains undecided and “stenomorphs” (Bartsch, 1923, as quoted by Turner, 1955) are formed. There is no spatial competition in gastrochaenes in the area investigated, but probably in petricoles. In areas best suited to its own living requirements, e.g. steep and overhanging “declivium”, Lithophaga lithophaga represses Gastrochaena dubia and Petricola lithophaga.

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