A low cost, reliable method for quantifying coloration in Carcinus maenas (Linnaeus, 1758) (Decapoda, Brachyura)

Green or shore crabs, Carcinus maenas, exhibit a range of coloration from green through orange to red. Crabs turn red when they delay molting. This phenomenon is hypothesized to be part of a strategy in which crabs delay molting in order to allocate resources to reproduction. Thus a more complete understanding of the biology of C. maenas must take coloration into account. In part because of the inconsistency inherent in using subjective descriptions of color, in most studies, crabs which are not clearly "green" or "red" are either ignored or forced into one or the other category. Information about the intermediate individuals is lost. In an attempt to find a reliable, inexpensive, easy-to-use method of quantifying color variation in the field, a color index was developed using paint swatches. The index has been field tested for inter-observer reliability and found to be consistent between both trained and untrained observers, to distinguish quantitatively between subjective color groupings, and to be useful across geographically distinct populations. This kind of index allows, for the first time, a way to quantify color in C. maenas.