Shellfish habitat—whether it is a living assemblage or an accumulation of dead shells—
provides hard substrate for the attachment of many species that would not be present in areas
consisting only, or mainly, of soft sediments (Figure 1). The overall ecological result is greatly
enhanced biodiversity in shellfish habitat compared to surrounding areas of the seabed. For
example, in his classic study on eastern oyster reefs, Wells (1961) found over 300 species of
invertebrates that were largely restricted to the reef structure or other hard-bottom habitats, and
thus did not typically occur in adjacent non-reef habitat.
Shellfish habitat is also characterized by a greater amount of vertical relief in comparison
to the surrounding seabed. This enhanced vertical relief is of major importance, with
implications for assessing habitat value for managed species and for development of
management policy. For example, an oyster or mussel reef protruding only several centimeters
above the bottom represents, in terms of fluid mechanics, a "roughness element." Roughness