David Campbell wrote,

> Acid or chelating agents may be used by animals to bore into rock and
shell, but I don't remember full details offhand.

A complicated subject with a vast scientific literature! Some gastropods
have odd minerals armoring their radulae, such as magnetite, which is much
harder than the two calcium carbonate minerals (aragonite and calcite).
Aragonite is harder than calcite, too. The upshot is that most borings made
in carbonate substrates are done chemically, but most borings made in shale,
sandstone, or other substrates are done mechanically. With a huge number of
exceptions. For example, some sandstones are only weakly cemented by
calcite. Some bivalves dissolve the calcite cement, which loosens the quartz
sand grains enough to move them out of the boring.

The problem for a shelly organism of using acid is, of course, the danger of
corroding one's own shell. And where do you hold the acid after you've made
it? All this can be a highly specialized and metabolically expensive
endeavor.

Andrew K. Rindsberg
Geological Survey of Alabama

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