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

Cailocardia hungerfordi (Veneridae: Pitarinae) lives in subtidal muds (-20 to -40m C.D.) and is covered by a dense mat of mud that, effectively, camou flages 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 lever develo p numerous, delicate (0.5 mu m in diameter), calcified, periostracal needle s. These penetrate the outer periostracum. Mucus produced from sub-epitheli al glands in the inner surface of the mantle, slides over the cuticle-cover ed epithelium of the inner and outer surfaces of the inner fold and the inn er surface of the middle mantle fold to coat the outer surface of the perio stracum and its calcified needles. Increased production at some times produ ces solidified strands of mucus which bind mud and detrital material into t heir fabric to create the shell camouflage. Calcified periostracal needles have been identified in other venerids, incl uding some members of the Pitarinae, but how they are secreted and how the covering they attract is produced and, thus, how the whole structure functi ons, has not been explained.