The distribution of polyenes in the shell of Arctica islandica from North Atlantic localities: a confocal Raman microscopy study

Many of the pigments that are widely found in coloured parts of mollusc shells are polyenes, i.e. molecules with a central polyenic chain (carbon-carbon single and double bonds). Due to a resonant coupling of these molecules at wavelengths typically used in Raman spectroscopy, this method is well suited to investigate their occurrence in biogenic materials. Here we use confocal Raman microscopy to map the spatial distribution of polyenes within the shell of the bivalve Arctica islandica and to determine their chemical characteristics (chain length). Polyene chain length does not differ between shells from different localities (off Iceland, Baltic Sea and North Sea). We also show that the pigment polyenes are not only located at the outside of the shell, but also within the shell, developing the same layered pattern typical for growth bands. This finding raises the question as to whether polyenes may play a role in the biomineralization process itself.