Carapace growth of the turtle Chrysemys scripta: a longitudinal study of shape using Fourier analysis

Because the majority of morphological structures encountered in Biology are irregular in form, conventional metrics composed of distances, angles and ratios, are inefficient shape descriptors. To circumvent this drawback in the application of morphometrics to describe 2 dimensional shapes, an alternative procedure based on Fourier analysis was developed and applied to the turtle carapace. Once size differences were controlled for, the presence of shape changes with age could be demonstrated. This study showed that small systematic differences in the phase angle are associated with increase in carapace asymmetry. The differences were not visually apparent in the original data. These results reinforce the need for precise shape descriptors that are capable of measuring a large percentage of the informational content that is present in all biological forms.