Organization of Coloration in Nemerteans: Insights from Light and Electron Microscopy

Although pigment systems have been extensively studied in vertebrates and cephalopods, those of many invertebrate groups, including nemerteans (Nemertea), remain poorly understood. Nevertheless, nemerteans represent a valuable model for evolutionary studies. Here, we investigated the tissue and cellular organization underlying body coloration in nine nemertean species from three classes (Palaeonemertea, Hoplonemertea, and Pilidiophora) using light and transmission electron microscopy. Our results show that coloration in nemerteans is mediated not only by specialized pigment cells (chromatophores) but also by ciliated and glandular epithelial cells. The distribution of pigment-containing structures varies considerably among taxa, with the greatest diversity observed in heteronemerteans. In Tubulanus punctatus and Kulikovia torquata, pigment is localized in ciliated epithelial cells, where pigment granules form from modified mitochondria. In contrast, in chromatophores, granule formation involves the endoplasmic reticulum or Golgi apparatus. Leucophores, responsible for white spot formation in Kulikovia species, are identified in nemerteans for the first time. These findings reveal high evolutionary lability of nemertean pigment systems and expand our understanding of cellular diversity in spiralian pigmentation mechanisms. This study fills a significant gap in comparative chromatophore cytology and provides a foundation for future investigations into the chemical nature and physiological functions of nemertean pigments.