Eumelanin and pheomelanin pigmentation in mollusc shells may be less common than expected: insights from mass spectrometry

Background: The geometric patterns that adorn the shells of many
phylogenetically disparate molluscan species are comprised of pigments
that span the visible spectrum. Although early chemical studies implicated
melanin as a commonly employed pigment, surprisingly little evidence
generated with more recent and sensitive techniques exists to support
these observations. Results: Here we present the first mass
spectrometric investigations for the presence of eumelanin and pheomelanin
in 13 different molluscan species from three conchiferan classes:
Bivalvia, Cephalopoda and Gastropoda. In the bivalve Mytilus
edulis we demonstrate that eumelanin mainly occurs in the
outermost, non-mineralised and highly pigmented layer of the shell (often
referred to as the periostracum). We also identified eumelanin in the
shells of the cephalopod Nautilus pompilius and the
marine gastropods Clanculus
pharaonius and Steromphala adriatica. In the terrestrial
gastropod Cepaea nemoralis we verify the presence of
pheomelanin in a mollusc shell for the first time. Surprisingly, in a
large number of brown/black coloured shells we did not find any evidence
for either type of melanin. Conclusions: We recommend methods
such as high-performance liquid chromatography with mass spectrometric
detection for the analysis of complex biological samples to avoid
potential false-positive identification of melanin. Our results imply that
many molluscan species employ as yet unidentified pigments to pattern
their shells. This has implications for our understanding of how molluscs
evolved the ability to pigment and pattern their shells, and for the
identification of the molecular mechanisms that regulate these processes.