Surveys in the Dominican Republic showed that the penshell Pinna carnea is consistently found in seagrass beds, but is absent in adjacent sandflats. Population densities were low (1-7/100 m2) and the size structure was skewed towards large individuals. Experiments indicated that penshell larvae do not sélect a particular substratum to settle on, that post-settlement mortality decreases with increasing size, and that mortality is less in seagrass than sandflat habitats. P. carnea is likely restricted to seagrass beds because this habitat provides some protection from predators. Also, the Consolidated sédiments in seagrass beds provide protection from physical disturbances. Growth measurements showed that juvéniles grow very rapidly. This has likely evolved as a strategy for reducing the period of high vulnerability to predators. The dominant mode of large individuals in populations likely represents the accumulation of successive recruitment events caused by the slowing of growth when individuals attain >150 mm in length.