The ability to cement to a hard substratum has evolved repeatedly in the bivalved molluscs. Twenty clades of cementing bivalves have been identified from a broad range of bivalve taxa. Such polyphyletic acquisition of the habit raises a number of questions concerning the preadaptations which allow bivalves to cement and the selection pressures which favour this mode of attachment.
Detailed examination of cementation in members of the Ostreidae reveals that a calcareous extra-periostracal cement is responsible for attachment. Spherulitic growth of cement, whose composition is identical to that of the shell layers, results in structures reminiscent of diagenetic cements. These observations suggest that extrapallial fluid leaks' through a permeable periostracum; resulting in precipitation of calcium carbonate between the periostracum and the substratum. Similar observations are made for most other living shell cementing bivalves, implying a convergence of mechanism. The only exception are the chamids which appear to utilise a weaker organic bond. Less than 10% of fossil Chamacea are preserved attached to their substrata, suggesting that this weaker mode of attachment has been employed in this superfamily throughout its evolutionary history.
Having established a common mechanism by which most bivalves cement it is possible to suggest the possible preadaptations. Principal requirements are the possession of a thin and 'leaky' periostracum, a highly extensible mantle edge and a suitable life orientation. Indeed, most cementing bivalves have evolved from pleurothetic byssate stock. The Pectinidae, in which the cemented habit has evolved at least four times, are shown to possess these key preadaptations.
Mytiloids have never cemented despite having members which have dwelt on hard substrates since the Ordovician. Examination of modem Mytilacea suggests that the group lacks all the fundamental preadaptations for cementation.
There is little evidence to support the traditional view that cementation is an adaptation to life in a high energy environment. The independent appearance of many of the cementing clades during the early Mesozoic coincides with the increased predation pressure which has been documented for that time. A causal link is implied by experimental work: predators display a significant preference for more easily manipulated byssate prey
