Periostracal mineralization in the gastrochaenid bivalve Spengleria

We investigated the spikes on the outer shell surface of the endolithic gastrochaenid bivalve genus Spengleria with a view to understand the mechanism by which they form and evaluate their homology with spikes in other heterodont and palaeoheterodont bivalves. We discovered that spike formation varied in mechanism between different parts of the valve. In the posterior region, spikes form within the translucent layer of the periostracum but separated from the calcareous part of the shell. By contrast those spikes in the anterior and ventral region, despite also forming within the translucent periostracal layer, become incorporated into the outer shell layer. Spikes in the posterior area of Spengleria mytiloides form only on the outer surface of the periostracum and are therefore, not encased in periostracal material. Despite differences in construction between these gastrochaenid spikes and those of other heterodont and palaeoheterodont bivalves, all involve calcification of the inner translucent periostracal layer which may indicate a deeper homology.