Nonoptimal shell forms as overlapping points in functional and theoretical morphospaces

Structural inefficiency for snail shells has been measured as the ratio of the volume of shell material: volume of space enclosed) (Vshell: Vspace); however, whether structural inefficiency would be affected to a greater extent by varying Vshell or Vspace (e.g., by modifying whorl overlap) has remained uninvestigated. Structural inefficiencies for a variety of whorl overlaps were decomposed algebraically into V shell and Vspace and used to construct plots of whorl overlap and volume for 4 species of marine snails. Inflection points in those plots represent critical whorl overlaps beyond which space reduction starts to exceed significantly material conservation. Whorl overlaps that were obtained from measuring real specimens were less than whorl overlaps that were associated with inflection points, which indicates that living space might have predominated over shell material as a constraining factor during morphological evolution.