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

Abstract

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. A Sepia officinalis melanin standard 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 layer with alternating transverse and co-marginal layers. At shell spot areas, shell surfaces were cribose; fracturing shells near such spots showed a locally disordered, more porous mineral arrangement enriched in organic matrix, bounded basally by an organic layer. Under transillumination, these spots acted as discrete light-transmitting windows, abundantly in P. muscarum and also sparsely in P. picta; as the spots appear only as pigmented dots but also transmit light, we refer to them as cryptotransmissive domains, which is apparently a novel finding in terrestrial gastropods. We speculate that these may represent 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.