Article: Sectorial-expansion analysis of irregularity coiled shells; application to the Recent gastropod Distorsio

In a sectorial-expansion analysis the shell can be described as a series of independent helicospirals joining homologous points on the shell's surface. In actual shells, these helicospirals are longitudinal ornaments. Therefore, the aperture is considered as a set of points, obtained at its intersection with these ornaments. Any apertural segment (between two consecutive points) is potentially able to expand or contract independently during growth. With the aim of quantifying this change in length in any coiled shell, a differential parameter (Sc) has been devised. Sc measures the rate at which any apertural chord of length D varies with regard to a particular spiral length (Lm). This morphometric analysis has been performed on the gastropod Distorsio reticulata. The ontogenetic distribution of S leads us to conclude that the distorted, periodically bulging spiral of this species is constructed by alternatively expanding and contracting the apertural mantle. This striking morphology might allow the animal to retract the body deeply within the shell as a passive defence and improves its stability on the substrate.