Quantifying two-dimensional dichromatic patterns using a photographic technique:: case study on the shore crab (Carcinus maenas L.)

Contrasting patterns of pigmentation, such as those associated with crypsis and aposematism, are common in many taxa. In order to determine why patterning, varies among individuals or populations, it is important to quantify how these patches of pigment are arranged. Here we present a simple technique for measuring areas of pigmentation as well as their spatial distribution, and demonstrate its application to the study of substrate-associated patterning in shore crabs (Carcinus maenas L.). The results, based on a virtual grid laid over digital images of crab carapaces, allow for correlations to be made among sample populations. The technique, or variations of it, can be applied to any Situation where two-dimensional dichromatic patterns need to be quantified.