Pigments, Colours and Patterns - The contribution of eumelanin and pheomelanin to molluscan shell ornamentation with a special focus on the terrestrial snail Cepaea nemoralis

Abstract

In recent years interest into molluscan pigments increased. But a lot of techniques have to be adapted to be usable with often difficult molluscan tissues and shell material. A comprehensive approach needs to encompass both pigment chemistry and molecular biology. Here an improved method for testing molluscan shells for the presence of characteristic melanin oxidation products is presented. The established method of RT-qPCR relies heavily on sufficiently tested reference genes. Comprehensive testing was carried out for both established house keeping genes and novel reference genes in the terrestrial gastropod Cepaea nemoralis. Both of these techniques were used to test for melanin in mollusc shell pigmentation. Evidence for eumelanin could be found in three conchiferan classes: Nautilus pompilius (Cephalopoda), Mytilus edulis (Bivalvia), Clanculus pharaonius and Steromphala adriatica (Gastropoda). Both eumelanin and pheomelanin were detected in the gastropod C. nemoralis. In this species genes known to be involved in melanin synthesis in insects and mammals were screened for their quantitative expression rates in shell producing mantle tissue. It was found that Tyrosinase and Tyrosinase Related Protein are well expressed all over the mantle tissue, but show no differential expression in band building mantle tissue. Together with evidence of both eumelanin and pheomelanin oxidation products throughout the shell of C. nemoralis, this finding leads to the conclusion that both types of melanin seem to be involved in shell colouration, but not band patterning, of this gastropod shell. A surprisingly large number of other bivalve and gastropod species tested for melanin show similar geometric patterns, that could not be verified as eumelanin. Future research will hopefully shed light onto this very structurally stable molluscan shell pigmentation.