Shell morphometry in barnacles: quantification of shape and shape change in Balanus

Geometric design in the barnacle genus Balanus has been studied in relation to variation in adult shell form, that includes differences among species, and size‐related changes in shape. The genus comprises 40 Recent species, and as a group these display a more or less constant morphology over an extraordinary size range (10 to 200 mm in basal length). Linear and volumetric measurements were collected from 232 adult individuals of 14 species representing the variation in size, shell form and shell design thought to occur in the genus. Specimens were chosen to represent the size ranges of the species. Only isolated individuals growing on planar surfaces were used; shells were complete, undamaged and undistorted. Shell form differs among taxa, and no two species scale alike; intraspecific variation for five ratio variables shows strong allometry over the adult size range of each species. As size increases, there is a trend for the basis and orifice to maintain their shapes or to become slightly more elliptical, and for shells to become more conical and proportionately taller. Throughout their size ranges, species can be described by these geometries: paraboloid (6 species), frustum of an ellipsoidal cone (5 species), frustum of a cone (2 species) and a cone (1 species). Shell geometry is not a function of species size, but there does appear to be a correlation between shell geometry and shell volume. Species with relatively small shell volumes are described by a frustum of an ellipsoidal cone, or by a cone, while those with a relatively large shell volume are described by a paraboloid, or by the frustum of a cone.