Why do Univalve Shells of Gastropods Coil so Tightly? A Head-Foot Guidance Model of Shell Growth and its Implication on Developmental Constraints

Summary

Tight coiling with whorl overlap is the most frequent mode of shell coiling in gastropods. A new model of shell growth, the “head-foot guidance model”, implies that the contact of the head-foot mass with the mantle edge plays the main role in producing tight coiling. Computer simulation of the model suggests that the head-foot guidance mechanism imposes developmental constraints on the evolution of gastropods; 1.) bilaterally symmetric coiling is associated with bilaterally symmetric musculature of the foot mass; 2) tight coiling is associated with the contact of the head-foot mass and the mantle edge; 3) insertion of the foot muscle occurs in gastropods with an uncoiled shell.