FORMATION AND HOMOLOGY OF RADULAR TEETH; A CASE STUDY USING COLUMBELLID GASTROPODS (NEOGASTROPODA: COLUMBELLIDAE)

Radular morphology has contributed prominently to molluscan systematic classification, especially that of the Gastropoda. Inferences of homology have been based on the shape, number and position of radular teeth. In this study, we demonstrate the significance of the mode of radular formation in assessing hypotheses of homology. The columbellid radula, considered representative of the stenoglossate type, is used as a case study. The columbellid middle tooth is acuspate, spanning part of a wide membrane between two relatively large, complex lateral teeth. We examined radulae from several species with acuspate or multicuspate middle teeth, using histological and whole mount techniques, to compare the modes of formation. Our results show that acuspate rachidian structures in the species we examined are not secreted by odontoblastic fields as has been shown for radular teeth in other taxa, but are instead produced independently by other secretory cell types. The structure can not, therefore, be considered homologous to rachidian teeth, illustrating how different formational mechanisms can produce positionally equivalent structures. We suggest not using the term 'rachidian' to refer to the central plate in the columbellid radulae.