The anatomy of Callocardia hungerfordi (Bivalvia: Veneridae) and the origin of its shell camouflage

Abstract

Callocardia hungerfordi (Veneridae: Pitarinae) lives in subtidal muds (-20 to -40m C.D.) and is covered by a dense mat of mud that, effectively, camouflages the shell. The periostracum is two layered. The inner layer is thick and pleated, the outer thin and perforated. From the outer surface of the inner layer develop numerous, delicate (0.5 μm in diameter), calcified, periostracal needles. These penetrate the outer periostracum. Mucus produced from sub-epithelial glands in the inner surface of the mantle, slides over the cuticle-covered epithelium of the inner and outer surfaces of the inner fold and the inner surface of the middle mantle fold to coat the outer surface of the periostracum and its calcified needles. Increased production at some times produces solidified strands of mucus which bind mud and detrital material into their fabric to create the shell camouflage. Calcified periostracal needles have been identified in other venerids, including some members of the Pitarinae, but how they are secreted and how the covering they attract is produced and, thus, how the whole structure functions, has not been explained.