Early Larval Shell Development in Mytilid Bivalve Mytilus galloprovincialis

Abstract

The early shell morphogenesis of a mytilid bivalve Mytilus galloprovincialis Lamarck was studied by scanning electron microscopy. Larvae were taken at 1-1.5 hour intervals after hatching until the early veliger stage. Trochophore larvae appeared within 16 hours after fertilization at incubating temperatures between 15.0-18.2.DEG.C.. They have the same pattern in the mode of formation of the prodissoconch I(Pd I). The Pd I began to build in the middle trochophore stage. It was initially made of an oval-shaped, fully organic univalved shell (pellicle). The univalved organic shell developed further into a bivalved shape with a median hinge line in the late trochophore stage. Calcification of the larval shell occurred in the early veliger stage within 70 hours after fertilization. At this stage of development, the shell gland on the dorsal side secreted irregularly oriented, needle-like spherulites underneath the pellicle. An oval or low, cap-shaped, univalved shell observed in the early veliger larvae of the Bivalvia is known to occur in the early stages of other molluscan groups such as the Monoplacophora, Cephalopoda and Gastropoda, suggesting its synapomorphic nature amongst the Conchifera.