Histological Development of Pearl-Sac Formation in Thai Freshwater Mussels

Seven species of freshwater pearl mussels collected from three different basins in Thailand were mantle-transplanted and returned to culture in their natural habitats. The microscopic structure of the forming pearl sacs on days 7, 15, 30, and 60 was described. The outer mantle epithelium became a continuous sheet of complete pearl sac, having a small amount of material at day 7. An increasing amount of secreted materials was seen on day 15 and had developed into layers of dense materials by day 30, when the pearl sac epithelium changed from a columnar to squamous epithelium. At day 60, calcium was first seen at the central part of the pearl and later in peripheral parts. Pearls inside the sac were variable in shape. Occasionally, several sacs formed from a single mantle-transplanted piece. Four species of pearl mussel giving superior pearl characters and containing calcium deposits by day 60 were Hyriopsis (Limnoscapha) myersiana, Chamberlainia hainesiana, H. (L.) desowitzi and Pseudodon sp.