Hinge development and functional morphology in the larvae and early postlarvae of the boring bivalve, Barnea manilensis (Pholadidae)

Morphological features of the hinge during larval and early postlarval development of Barnea manilensis (Philippi), especially during metamorphosis, are described using optical and scanning electron microscopy. After the beginning of the metamorphosis, B. manilensis begins to rotate the two valves around the dorso-ventral axis, which is emphasized as effective boring motion. Relationships between the hinge development and the dorsoventral rotation are discussed. The dorsal pivot of the dorso-ventral rotation is initiated by the first fibrous ligament. In the early postlarvae, antero-ventral margins, separately opened by the rotation, become wider with overlapping the postero-dorsal parts to slide the left valve inside the right. In the hinge area, the overlapping is adjusted by left chondrophore, which is located on the left valve inside the right. In the subsequent early postlarval stage, the first fibrous ligament reduces while the dorsal condyle develops into a knob. The dorsal pivot of the dorso-ventral rotation secondarily shifts from the first fibrous ligament to a point touching two dorsal condyles.