The phylum Mollusca is one of the largest, most diverse and important groups in
the animal kingdom. There are more than 50,000 described species of which around
30,000 are marine. Part of this phylum is the class Bivalvia with about 7500 species,
many of which are commercially important, such as mussels, clams, scallops and
oysters (Gosling 2003). Some bivalves adopt a burrowing lifestyle; another large group
live by attaching themselves to a variety of surfaces. The attachment can either be
provided by byssus threads or, like Spondylus, by a cement-like substance that fi xes one
of the valves to a substrate (Gosling 2003).