By local hookah diver, Cordova, Cebu, Philippines, 83.3mm., early 2014.
The « Coral Scallop » is a strange and unique pectenid that strongly resemble spondylids (hence the binomial name), and is the only species in genus Pedum. It is a common obligate scleractinian stony coral associate widely distributed throughout the entire Indo-Pacific region and lives embedded in live coral skeleton to which it is attached using byssus threads. It uses a variety of coral hosts such as genera Montipora, Porites, and Pavona in shallow water ranging between -2~20m; massive Porites appear to be the favorite. The shell is mostly surrounded by coral skeleton in adults as the coral would have grown and expanded since it settled. It has been suggested that this scallop-coral relationship may be mutualistic with the scallop providing coral enhanced water circulation for feeding and generating water jets to repel coral predator while the coral provides structural support and protection to the scallop. Both valves are usually white, sometimes carrying brown blotches or streaks; black specimens are also known but rarer. The shape is very variable. Typical shell length around 60mm., gigantic specimens may exceed 100mm.