Mollusca

Mollusks have a well-deserved reputation for being expert mineralizers based only on their much-admired shell-making abilities. Table 6.1 shows that the reputation is deserved 10-fold as shell formation is just one of many different processes that these animals perform in which biogenic minerals are utilized. The table lists no less than 21 different minerals and about 17 different functions! The list contains both amorphous minerals (amorphous fluorite, calcium carbonate, calcium phosphate, calcium pyrophosphate, and silica) and many crystalline ones, including rather uncommon ones such as weddelite, calcium fluorite, barite, magnetite, lepidocrocite, and goethite. Weddelite, for example, is a calcium oxalate mineral frequently formed pathologically in vertebrates. Certain gastropods use the rather soft weddelite nonpathologically to cap pestlelike objects (gizzard plates) in their stomachs (Lowenstam 1968), which they use for crushing shelled prey. One mollusk, the chambered Nautilus, forms no less than five different minerals. An individual tooth of a chiton contains three different mature minerals that are products of two other transient minerals. In addition to the more familiar functions of mineralized tissues, mollusks use biogenic minerals as buoyancy devices, trap doors, egg shells, and love darts. The varieties of crystal shapes, sizes, organizational arrays, and tissue sites present a picture of overwhelming diversity all within one phylum. It is illustrative to compare the mollusks with the echinoderms. The echinoderms also use minerals for a wide variety of functions, but in contrast to the mollusks they use essentially the same “building material” for many different purposes. Thus, understanding how one echinoderm mineralized tissue forms provides insight into how most of the others form. This is not so with mollusks. It seems futile to expect that they too have adapted one basic process to form all their mineralized tissues. It seems just as futile to look for a different explanation for each type of mineralized product. The mollusks force us to seek a level of understanding of mineralization that identifies common approaches, strategies, and principles and, at the same time, appears to dispel any “dreams” about discovering the mechanism of mineralization. The mollusk phylum contains seven different taxonomic classes.