Histomorphology of the hypobranchial gland in Dicathais orbita (Gmelin, 1791) (Neogastropoda: Muricidae)

Tyrian purple is a dye used in antiquity and is a natural product of marine molluscs of the family Muricidae. Bioactive precursors of Tyrian purple occur in the hypobranchial gland, reproductive glands and egg masses of muricids, such as Dicathais orbita. Histomorphological examination of the hypobranchial–gonoduct complex (rectum and hypobranchial, capsule, albumen and rectal glands) was conducted to provide the first description of the hypobranchial gland in D. orbita and to determine a mechanism for the transfer of Tyrian purple precursors to the gonoduct and ultimately to the egg masses. Seven secretory cell types were identified in the hypobranchial epithelium of D. orbita, which can be broadly classified into cells containing mucoproteins and acidic sulphated mucopolysaccharides. Three secretory cells new to the Muricidae were identified, along with two cell types that appear to be associated with synthesis of Tyrian purple. A subepithelial vascular sinus surrounding the rectum and rectal gland occurs between the hypobranchial gland and gonoduct. Examination of this region failed to reveal a direct anatomical mechanism for the transfer of precursors to the gonoduct. However, biochemical similarities in secretions from the hypobranchial, capsule and albumen glands suggest that the synthesis of precursors within the gonoduct may be possible.