What is torsion in gastropod development?

Torsion is a developmental process unique to gastropods that occurs during the veliger larval stage. It involves a 180° twisting of the animal's body, which shifts the posterior portion of the body to an anterior position located behind the head.1

While this pronounced twisting characterizes the development of most gastropods, there are exceptions. Among opisthobranchs, for instance, torsion is often limited, or the individuals may undergo a process called "detortion" during metamorphosis. Consequently, in many opisthobranchs, the mantle cavity is positioned at the side or rear of the animal rather than at the front.2