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Corals and their bivalve borers: the evolution of a symbiosis
Morton, B. (1990). Corals and their bivalve borers: the evolution of a symbiosis, in: Morton, B. (Ed.) The Bivalvia: Proceedings of a Memorial Symposium in honour of Sir Charles Maurice Yonge (1899-1986) at the 9th International Malacological Congress, 1986, Edinburgh, Scotland, UK. pp. 11-46
In: Morton, B. (Ed.) (1990). The Bivalvia: Proceedings of a Memorial Symposium in honour of Sir Charles Maurice Yonge (1899-1986) at the 9th International Malacological Congress, 1986, Edinburgh, Scotland, UK. Hong Kong University Press: Hong Kong. ISBN 962-209-273-X. 355 pp., more

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    Marine

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  • Morton, B.

Abstract
    The evolution of hermatypic scleractinian coral reefs in the Mesozoic was an event which prevised important phases of molluscan adaptive radiation. The bivalve borers of coral skeletons constitute an excellent example of such a radiation. Representatives of six families are known to be coral borers: i.e., Lithophaginae Mytilidae), Gastrochaenidae, Petricolidae, Pholadidae, Clavagellidae and Tridacnidae. Representative of the Lithophaginae and Gastrochaenidae are particularly important agents in the ecology and bioerosion of reefs. Until comparatively recently such bivalves were considered borers of dead coral bases and reef limestones only and one important group - the Gastrochaenidae - exploit this habitat exclusively, as do the Petricolidae and less specialized members of the Lithophaginae. Such borers are pan-tropical. Recent studies of the Lithophaginae, Pholadidae and Tridacnidae, however, have exposed more intimate relationships with living corals. Probably all are chemical borers, this being by way of secretions derived from mantle folds reflected to different degrees over the shell. In less specialized, dead coral-boring members of the Lithophaginae, calcium carbonate is used to smooth and fill the boring anteriorly. In more specialized live coral borers calcium is used in two ways, to smooth and fill the boring posteriorly and to form secondary extensions to the shell for predator defence. Most specialized members of the Lithophaginae and Pholadidae are either in inquilinistic, endocommensal or, perhaps, mutualistic relationships with either the inhabited coral or its cryptobion resulting in a spherical form and other remarkable adaptations hitherto little understood. All coral-boring bivalve phylogenies can be derived from nestling and, ultimately, infaunal ancestors. But exploitation of living corals as a habitat was facilitated by the evolution of larval adaptations to penetrate the living coral tissue and thus of mechanisms to ,overcome the coral's defences. There has subsequently been a modification of growth directions to keep pace with an enlarging (as opposed to a diminishing) substrate, with appropriate modifications to shell form, but also the development of anti -predator equipment to cope with similarly rapidly co-evolution neogastropods and other predators and their simultaneous exploitation of the reef environment. Cretaceous reefs were characterized by tropical conservative cosmopolitanism. Extant dead coral boring bivalves, handed down from this age, still reflect this. The effect of the Mesozoic Revolution in predation, however, was powerfully felt-effectively driving the bivalve borers into greater intimacy with living corals and, with the separation of the Pacific from the Atlantic in the Miocene by final closure of the Tethys, corals and their borers in these realms evolved separately heralding a new era of specialization and provincialism.

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