Pigments and microstructure of the colour polymorphic shells of Polymita picta and P. muscarum (Gastropoda: Cepolidae), with observations on a new light-transmitting shell spot system

Colour polymorphism in the Cuban painted snails Polymita picta and P. muscarum is striking, yet the pigmentary and structural bases remain unclear. We combined spectrophotometric screening, Raman micro-spectroscopy, scanning electron microscopy (SEM) and LED transillumination to link pigments, ultrastructure and optics across shell morphs. Melanin standard (Sepia officinalis) yielded a robust linear calibration used to quantify total melanin pigments at 215 nm in pooled extracts. Melanin was detected in all samples with predominance in darker morphs. Raman spectra (785 nm) confirmed aragonite mineral organization and revealed carotenoid bands, consistent with a mixed-pigment model in which carotenoids contribute to ground and band colours and melanins underlie darker elements. SEM showed a canonical crossed-lamellar wall with alternating transverse and co-marginal tiers. At "spot" domains surfaces were cribose; fracture exposed locally disordered, more porous mineral arrangement enriched in organic matrix, bounded basally by an organic layer. We understand these as a photo-transmissive system in terrestrial gastropods probably overlooked. Under transillumination, spots acted as discrete light-transmitting windows, abundant in P. muscarum and sparse in P. picta. We propose a pigment-structure-optics framework, in which pigments and microstructural packing jointly play potential roles in photoprotection and behavioural thermoregulation. These results provide a mechanistic context for colour polymorphism in Polymita and suggest testable links to thermal ecology and conservation.