Not in shells, but some ants (Formicid species) can spray formic acid, and some
millipedes can exude a cyanide substance through pores in their side (not an
acid,but I always think that's an interesting tidbit).

Nora Bryan
Calgary, Alberta
CANADA

[log in to unmask] wrote:

> Some time ago, James Cheshire asked whether cowries really can emit
> sulfuric acid, as reported. I don't know, but sulfuric acid, a very
> corrosive substance, is not unknown in nature as a defense. In "Marine
> Algae of California" (Stanford University Press, Stanford, California,
> 1976, p. 220), I. A. Abbott and G. J. Hollenberg state that the seaweed
> Desmarestia is "remarkable in the field owing to large amounts of acids
> produced when plants are collected, bleaching nearby plants and producing
> an acrid odor." I have elsewhere seen it suggested that sulfuric acid is in
> this mix.
>
> It would be very remarkable if any shelled mollusk used any acid as a
> defense, considering the harm that might occur to the shell, but nature is
> remarkably inventive. Does anyone know of any examples?
>
> Andrew K. Rindsberg
> Geological Survey of Alabama