Larval Settlement and Ontogenetic Development of Hippuritella vasseuri (DOUVILLÉ) (Hippuritoidea, Bivalvia)

Abstract

An entire bouquet of some 250 specimens of Hippuritella vasseuri (DOUVILLE) was mapped three dimensionally, based on ascending serial sections spaced at 1 mm, in order to evaluate the earliest
ontogenetic development in hippuritid rudists. This method supplied more than 5000 cross-cuts through rudist specimens of different ontogenetic stages beginning from <1 mm larval spat, up to fully grown “senior” rudists. Based on near-commissure cross-sections of all ontogenetic stages, seven characteristic growth stages are distinguished, which show an increasing complexity of morphological features and a change in shell material composition. The complete transformation from a larval settling stage to adult-like morphology happened within the first 8–10 mm of vertical growth and comprised five stages. (1) The larval stage is defined by an undifferentiated aragonitic shell of about 0.3 mm diameter. (2) The “baby teeth” stage is still aragonitic but had a first hinge system. (3) The “first calcite” stage is characterized by the first occurrence of low magnesium calcite. (4) The “first pillar” stage shows weakly developed pillars and (5), the juvenile stage is of adult-like morphology but shows salient ornamentation and rapid diameter–size increase. Subsequent development is dominated by the adult stage growth progress (6) with morphological continuity prior to a short lasting senior stage (7) and demise.